RFC1247 - OSPF Version 2

Network Working Group J. Moy

Request for Comments: 1247 Proteon, Inc.

Obsoletes: RFC1131 July 1991

OSPF Version 2

Status of this Memo

This RFCspecifies an IAB standards track protocol for the Internet

community, and requests discussion and suggestions for improvements.

Please refer to the current edition of the ``IAB Official Protocol

Standards'' for the standardization state and status of this protocol.

Distribution of this memo is unlimited.

Abstract

This memo documents version 2 of the OSPF protocol. OSPF is a link-

state based routing protocol. It is designed to be run internal to a

single Autonomous System. Each OSPF router maintains an identical

database describing the Autonomous System's topology. From this

database, a routing table is calculated by constrUCting a shortest-path

tree.

OSPF recalculates routes quickly in the face of topological changes,

utilizing a minimum of routing protocol traffic. OSPF provides support

for equal-cost multipath. Separate routes can be calculated for each IP

type of service. An area routing capability is provided, enabling an

additional level of routing protection and a reduction in routing

protocol traffic. In addition, all OSPF routing protocol exchanges are

authenticated.

Version 1 of the OSPF protocol was documented in RFC1131. The

differences between the two versions are eXPlained in Appendix F.

Please send comments to ospf@trantor.umd.edu.

1. Introduction

This document is a specification of the Open Shortest Path First (OSPF)

internet routing protocol. OSPF is classified as an Internal Gateway

Protocol (IGP). This means that it distributes routing information

between routers belonging to a single Autonomous System. The OSPF

protocol is based on SPF or link-state technology. This is a departure

from the Bellman-Ford base used by traditional internet routing

protocols.

The OSPF protocol was developed by the OSPF working group of the

Internet Engineering Task Force. It has been designed expressly for the

internet environment, including explicit support for IP subnetting,

TOS-based routing and the tagging of externally-derived routing

information. OSPF also provides for the authentication of routing

updates, and utilizes IP multicast when sending/receiving the updates.

In addition, much work has been done to produce a protocol that responds

quickly to topology changes, yet involves small amounts of routing

protocol traffic.

The author would like to thank Rob Coltun, Milo Medin, Mike Petry and

the rest of the OSPF working group for the ideas and support they have

given to this project.

1.1 Protocol overview

OSPF routes IP packets based solely on the destination IP address and IP

Type of Service found in the IP packet header. IP packets are routed

"as is" -- they are not encapsulated in any further protocol headers as

they transit the Autonomous System. OSPF is a dynamic routing protocol.

It quickly detects topological changes in the AS (such as router

interface failures) and calculates new loop-free routes after a period

of convergence. This period of convergence is short and involves a

minimum of routing traffic.

In an SPF-based routing protocol, each router maintains a database

describing the Autonomous System's topology. Each participating router

has an identical database. Each individual piece of this database is a

particular router's local state (e.g., the router's usable interfaces

and reachable neighbors). The router distributes its local state

throughout the Autonomous System by flooding.

All routers run the exact same algorithm, in parallel. From the

topological database, each router constructs a tree of shortest paths

with itself as root. This shortest-path tree gives the route to each

destination in the Autonomous System. Externally derived routing

information appears on the tree as leaves.

OSPF calculates separate routes for each Type of Service (TOS). When

several equal-cost routes to a destination exist, traffic is distributed

equally among them. The cost of a route is described by a single

dimensionless metric.

OSPF allows sets of networks to be grouped together. Such a grouping is

called an area. The topology of an area is hidden from the rest of the

Autonomous System. This information hiding enables a significant

reduction in routing traffic. Also, routing within the area is

determined only by the area's own topology, lending the area protection

from bad routing data. An area is a generalization of an IP subnetted

network.

OSPF enables the flexible configuration of IP subnets. Each route

distributed by OSPF has a destination and mask. Two different subnets

of the same IP network number may have different sizes (i.e., different

masks). This is commonly referred to as variable length subnets. A

packet is routed to the best (i.e., longest or most specific) match.

Host routes are considered to be subnets whose masks are "all ones"

(0xffffffff).

All OSPF protocol exchanges are authenticated. This means that only

trusted routers can participate in the Autonomous System's routing. A

variety of authentication schemes can be used; a single authentication

scheme is configured for each area. This enables some areas to use much

stricter authentication than others.

Externally derived routing data (e.g., routes learned from the Exterior

Gateway Protocol (EGP)) is passed transparently throughout the

Autonomous System. This externally derived data is kept separate from

the OSPF protocol's link state data. Each external route can also be

tagged by the advertising router, enabling the passing of additional

information between routers on the boundaries of the Autonomous System.

1.2 Definitions of commonly used terms

Here is a collection of definitions for terms that have a specific

meaning to the protocol and that are used throughout the text. The

reader unfamiliar with the Internet Protocol Suite is referred to [RS-

85-153] for an introduction to IP.

Router

A level three Internet Protocol packet switch. Formerly called a

gateway in much of the IP literature.

Autonomous System

A group of routers exchanging routing information via a common

routing protocol. Abbreviated as AS.

Internal Gateway Protocol

The routing protocol spoken by the routers belonging to an

Autonomous system. Abbreviated as IGP. Each Autonomous System has

a single IGP. Different Autonomous Systems may be running different

IGPs.

Router ID

A 32-bit number assigned to each router running the OSPF protocol.

This number uniquely identifies the router within an Autonomous

System.

Network

In this paper, an IP network or subnet. It is possible for one

physical network to be assigned multiple IP network/subnet numbers.

We consider these to be separate networks. Point-to-point physical

networks are an exception - they are considered a single network no

matter how many (if any at all) IP network/subnet numbers are

assigned to them.

Network mask

A 32-bit number indicating the range of IP addresses residing on a

single IP network/subnet. This specification displays network masks

as hexadecimal numbers. For example, the network mask for a class C

IP network is displayed as 0xffffff00. Such a mask is often

displayed elsewhere in the literature as 255.255.255.0.

Multi-Access networks

Those physical networks that support the attachment of multiple

(more than two) routers. Each pair of routers on such a network is

assumed to be able to communicate directly (e.g., multi-drop

networks are excluded).

Interface

The connection between a router and one of its attached networks.

An interface has state information associated with it, which is

oBTained from the underlying lower level protocols and the routing

protocol itself. An interface to a network has associated with it a

single IP address and mask (unless the network is an unnumbered

point-to-point network). An interface is sometimes also referred to

as a link.

Neighboring routers

Two routers that have interfaces to a common network. On multi-

access networks, neighbors are dynamically discovered by OSPF's

Hello Protocol.

Adjacency

A relationship formed between selected neighboring routers for the

purpose of exchanging routing information. Not every pair of

neighboring routers become adjacent.

Link state advertisement

Describes to the local state of a router or network. This includes

the state of the router's interfaces and adjacencies. Each link

state advertisement is flooded throughout the routing domain. The

collected link state advertisements of all routers and networks

forms the protocol's topological database.

Hello protocol

The part of the OSPF protocol used to establish and maintain

neighbor relationships. On multi-access networks the Hello protocol

can also dynamically discover neighboring routers.

Designated Router

Each multi-access network that has at least two attached routers has

a Designated Router. The Designated Router generates a link state

advertisement for the multi-access network and has other special

responsibilities in the running of the protocol. The Designated

Router is elected by the Hello Protocol.

The Designated Router concept enables a reduction in the number of

adjacencies required on a multi-access network. This in turn

reduces the amount of routing protocol traffic and the size of the

topological database.

Lower-level protocols

The underlying network access protocols that provide services to the

Internet Protocol and in turn the OSPF protocol. Examples of these

are the X.25 packet and frame levels for PDNs, and the ethernet data

link layer for ethernets.

1.3 Brief history of SPF-based routing technology

OSPF is an SPF-based routing protocol. Such protocols are also referred

to in the literature as link-state or distributed-database protocols.

This section gives a brief description of the developments in SPF-based

technology that have influenced the OSPF protocol.

The first SPF-based routing protocol was developed for use in the

ARPANET packet switching network. This protocol is described in

[McQuillan]. It has formed the starting point for all other SPF-based

protocols. The homogeneous Arpanet environment, i.e., single-vendor

packet switches connected by synchronous serial lines, simplified the

design and implementation of the original protocol.

Modifications to this protocol were proposed in [Perlman]. These

modifications dealt with increasing the fault tolerance of the routing

protocol through, among other things, adding a checksum to the link

state advertisements (thereby detecting database corruption). The paper

also included means for reducing the routing traffic overhead in an

SPF-based protocol. This was accomplished by introducing mechanisms

which enabled the interval between link state advertisements to be

increased by an order of magnitude.

An SPF-based algorithm has also been proposed for use as an ISO IS-IS

routing protocol. This protocol is described in [DEC]. The protocol

includes methods for data and routing traffic reduction when operating

over broadcast networks. This is accomplished by election of a

Designated Router for each broadcast network, which then originates a

link state advertisement for the network.

The OSPF subcommittee of the IETF has extended this work in developing

the OSPF protocol. The Designated Router concept has been greatly

enhanced to further reduce the amount of routing traffic required.

Multicast capabilities are utilized for additional routing bandwidth

reduction. An area routing scheme has been developed enabling

information hiding/protection/reduction. Finally, the algorithm has

been modified for efficient operation in the internet environment.

1.4 Organization of this document

The first three sections of this specification give a general overview

of the protocol's capabilities and functions. Sections 4-16 explain the

protocol's mechanisms in detail. Packet formats, protocol constants,

configuration items and required management statistics are specified in

the appendices.

Labels such as HelloInterval encountered in the text refer to protocol

constants. They may or may not be configurable. The architectural

constants are explained in Appendix B. The configurable constants are

explained in Appendix C.

The detailed specification of the protocol is presented in terms of data

structures. This is done in order to make the explanation more precise.

Implementations of the protocol are required to support the

functionality described, but need not use the precise data structures

that appear in this paper.

2. The Topological Database

The database of the Autonomous System's topology describes a directed

graph. The vertices of the graph consist of routers and networks. A

graph edge connects two routers when they are attached via a physical

point-to-point network. An edge connecting a router to a network

indicates that the router has an interface on the network.

The vertices of the graph can be further typed according to function.

Only some of these types carry transit data traffic; that is, traffic

that is neither locally originated nor locally destined. Vertices that

can carry transit traffic are indicated on the graph by having both

incoming and outgoing edges.

Vertex type Vertex name Transit?

_____________________________________

1 Router yes

2 Network yes

3 Stub network no

Table 1: OSPF vertex types.

OSPF supports the following types of physical networks:

Point-to-point networks

A network that joins a single pair of routers. A 56Kb serial line

is an example of a point-to-point network.

Broadcast networks

Networks supporting many (more than two) attached routers, together

with the capability to address a single physical message to all of

the attached routers (broadcast). Neighboring routers are

discovered dynamically on these nets using OSPF's Hello Protocol.

The Hello Protocol itself takes advantage of the broadcast

capability. The protocol makes further use of multicast

capabilities, if they exist. An ethernet is an example of a

broadcast network.

Non-broadcast networks

Networks supporting many (more than two) routers, but having no

broadcast capability. Neighboring routers are also discovered on

these nets using OSPF's Hello Protocol. However, due to the lack of

broadcast capability, some configuration information is necessary

for the correct operation of the Hello Protocol. On these networks,

OSPF protocol packets that are normally multicast need to be sent to

each neighboring router, in turn. An X.25 Public Data Network (PDN)

is an example of a non-broadcast network.

The neighborhood of each network node in the graph depends on whether

the network has multi-access capabilities (either broadcast or non-

broadcast) and, if so, the number of routers having an interface to the

network. The three cases are depicted in Figure 1. Rectangles indicate

routers. Circles and oblongs indicate multi-access networks. Router

names are prefixed with the letters RT and network names with N. Router

interface names are prefixed by I. Lines between routers indicate

point-to-point networks. The left side of the figure shows a network

with its connected routers, with the resulting graph shown on the right.

Two routers joined by a point-to-point network are represented in the

directed graph as being directly connected by a pair of edges, one in

each direction. Interfaces to physical point-to-point networks need not

be assigned IP addresses. Such a point-to-point network is called

unnumbered. The graphical representation of point-to-point networks is

designed so that unnumbered networks can be supported naturally. When

interface addresses exist, they are modelled as stub routes. Note that

each router would then have a stub connection to the other router's

interface address (see Figure 1).

When multiple routers are attached to a multi-access network, the

directed graph shows all routers bidirectionally connected to the

network vertex (again, see Figure 1). If only a single router is

attached to a multi-access network, the network will appear in the

directed graph as a stub connection.

Each network (stub or transit) in the graph has an IP address and

associated network mask. The mask indicates the number of nodes on the

network. Hosts attached directly to routers (referred to as host

routes) appear on the graph as stub networks. The network mask for a

host route is always 0xffffffff, which indicates the presence of a

single node.

Figure 2 shows a sample map of an Autonomous System. The rectangle

labelled H1 indicates a host, which has a SLIP connection to router

RT12. Router RT12 is therefore advertising a host route. Lines between

______________________________________

(Figure not included in text version.)

Figure 1: Network map components

______________________________________

routers indicate physical point-to-point networks. The only point-to-

point network that has been assigned interface addresses is the one

joining routers RT6 and RT10. Routers RT5 and RT7 have EGP connections

to other Autonomous Systems. A set of EGP-learned routes have been

displayed for both of these routers.

A cost is associated with the output side of each router interface.

This cost is configurable by the system administrator. The lower the

cost, the more likely the interface is to be used to forward data

traffic. Costs are also associated with the externally derived routing

data (e.g., the EGP-learned routes).

The directed graph resulting from the map in Figure 2 is depicted in

Figure 3. Arcs are labelled with the cost of the corresponding router

output interface. Arcs having no labelled cost have a cost of 0. Note

that arcs leading from networks to routers always have cost 0; they are

significant nonetheless. Note also that the externally derived routing

data appears on the graph as stubs.

The topological database (or what has been referred to above as the

directed graph) is pieced together from link state advertisements

generated by the routers. The neighborhood of each transit vertex is

represented in a single, separate link state advertisement. Figure 4

shows graphically the link state representation of the two kinds of

transit vertices: routers and multi-access networks. Router RT12 has an

______________________________________

(Figure not included in text version.)

Figure 2: A sample Autonomous System

______________________________________

__________________________________________

(Figures not included in text version.)

Figure 3: The resulting directed graph

Figure 4: Individual link state components

__________________________________________

interface to two broadcast networks and a SLIP line to a host. Network

N6 is a broadcast network with three attached routers. The cost of all

links from network N6 to its attached routers is 0. Note that the link

state advertisement for network N6 is actually generated by one of the

attached routers: the router that has been elected Designated Router for

the network.

2.1 The shortest-path tree

When no OSPF areas are configured, each router in the Autonomous System

has an identical topological database, leading to an identical graphical

representation. A router generates its routing table from this graph by

calculating a tree of shortest paths with the router itself as root.

Obviously, the shortest-path tree depends on the router doing the

calculation. The shortest-path tree for router RT6 in our example is

depicted in Figure 5.

The tree gives the entire route to any destination network or host.

However, only the next hop to the destination is used in the forwarding

process. Note also that the best route to any router has also been

calculated. For the processing of external data, we note the next hop

and distance to any router advertising external routes. The resulting

routing table for router RT6 is pictured in Table 2. Note that there is

a separate route for each end of a numbered serial line (in this case,

the serial line between routers RT6 and RT10).

Routes to networks belonging to other AS'es (such as N12) appear as

dashed lines on the shortest path tree in Figure 5. Use of this

externally derived routing information is considered in the next

section.

______________________________________

(Figure not included in text version.)

Figure 5: The SPF tree for router RT6

______________________________________

Destination Next Hop Distance

__________________________________

N1 RT3 10

N2 RT3 10

N3 RT3 7

N4 RT3 8

Ib * 7

Ia RT10 12

N6 RT10 8

N7 RT10 12

N8 RT10 10

N9 RT10 11

N10 RT10 13

N11 RT10 14

H1 RT10 21

__________________________________

RT5 RT5 6

RT7 RT10 8

Table 2: The portion of router RT6's routing table listing local

destinations.

2.2 Use of external routing information

After the tree is created the external routing information is examined.

This external routing information may originate from another routing

protocol such as EGP, or be statically configured (static routes).

Default routes can also be included as part of the Autonomous System's

external routing information.

External routing information is flooded unaltered throughout the AS. In

our example, all the routers in the Autonomous System know that router

RT7 has two external routes, with metrics 2 and 9.

OSPF supports two types of external metrics. Type 1 external metrics

are equivalent to the link state metric. Type 2 external metrics are

greater than the cost of any path internal to the AS. Use of Type 2

external metrics assumes that routing between AS'es is the major cost of

routing a packet, and eliminates the need for conversion of external

costs to internal link state metrics.

Here is an example of Type 1 external metric processing. Suppose that

the routers RT7 and RT5 in Figure 2 are advertising Type 1 external

metrics. For each external route, the distance from Router RT6 is

calculated as the sum of the external route's cost and the distance from

Router RT6 to the advertising router. For every external destination,

the router advertising the shortest route is discovered, and the next

hop to the advertising router becomes the next hop to the destination.

Both Router RT5 and RT7 are advertising an external route to destination

network N12. Router RT7 is preferred since it is advertising N12 at a

distance of 10 (8+2) to Router RT6, which is better than router RT5's 14

(6+8). Table 3 shows the entries that are added to the routing table

when external routes are examined:

Destination Next Hop Distance

__________________________________

N12 RT10 10

N13 RT5 14

N14 RT5 14

N15 RT10 17

Table 3: The portion of router RT6's routing table listing external

destinations.

Processing of Type 2 external metrics is simpler. The AS boundary

router advertising the smallest external metric is chosen, regardless of

the internal distance to the AS boundary router. Suppose in our example

both router RT5 and router RT7 were advertising Type 2 external routes.

Then all traffic destined for network N12 would be forwarded to router

RT7, since 2 < 8. When several equal-cost Type 2 routes exist, the

internal distance to the advertising routers is used to break the tie.

Both Type 1 and Type 2 external metrics can be present in the AS at the

same time. In that event, Type 1 external metrics always take

precedence.

This section has assumed that packets destined for external destinations

are always routed through the advertising AS boundary router. This is

not always desirable. For example, suppose in Figure 2 there is an

additional router attached to network N6, called Router RTX. Suppose

further that RTX does not participate in OSPF routing, but does exchange

EGP information with the AS boundary router RT7. Then, router RT7 would

end up advertising OSPF external routes for all destinations that should

be routed to RTX. An extra hop will sometimes be introduced if packets

for these destinations need always be routed first to router RT7 (the

advertising router).

To deal with this situation, the OSPF protocol allows an AS boundary

router to specify a "forwarding address" in its external advertisements.

In the above example, Router RT7 would specify RTX's IP address as the

"forwarding address" for all those destinations whose packets should be

routed directly to RTX.

The "forwarding address" has one other application. It enables routers

in the Autonomous System's interior to function as "route servers". For

example, in Figure 2 the router RT6 could become a route server, gaining

external routing information through a combination of static

configuration and external routing protocols. RT6 would then start

advertising itself as an AS boundary router, and would originate a

collection of OSPF external advertisements. In each external

advertisement, router RT6 would specify the correct Autonomous System

exit point to use for the destination through appropriate setting of the

advertisement's "forwarding address" field.

2.3 Equal-cost multipath

The above discussion has been simplified by considering only a single

route to any destination. In reality, if multiple equal-cost routes to

a destination exist, they are all discovered and used. This requires no

conceptual changes to the algorithm, and its discussion is postponed

until we consider the tree-building process in more detail.

With equal cost multipath, a router potentially has several available

next hops towards any given destination.

2.4 TOS-based routing

OSPF can calculate a separate set of routes for each IP Type of Service.

The IP TOS values are represented in OSPF exactly as they appear in the

IP packet header. This means that, for any destination, there can

potentially be multiple routing table entries, one for each IP TOS.

Up to this point, all examples shown have assumed that routes do not

vary on TOS. In order to differentiate routes based on TOS, separate

interface costs can be configured for each TOS. For example, in Figure

2 there could be multiple costs (one for each TOS) listed for each

interface. A cost for TOS 0 must always be specified.

When interface costs vary based on TOS, a separate shortest path tree is

calculated for each TOS (see Section 2.1). In addition, external costs

can vary based on TOS. For example, in Figure 2 router RT7 could

advertise a separate type 1 external metric for each TOS. Then, when

calculating the TOS X distance to network N15 the cost of the shortest

TOS X path to RT7 would be added to the TOS X cost advertised by RT7

(see Section 2.2).

All OSPF implementations must be capable of calculating routes based on

TOS. However, OSPF routers can be configured to route all packets on

the TOS 0 path (see Appendix C), eliminating the need to calculate non-

zero TOS paths. This can be used to conserve routing table space and

processing resources in the router. These TOS-0-only routers can be

mixed with routers that do route based on TOS. TOS-0-only routers will

be avoided as much as possible when forwarding traffic requesting a

non-zero TOS.

It may be the case that no path exists for some non-zero TOS, even if

the router is calculating non-zero TOS paths. In that case, packets

requesting that non-zero TOS are routed along the TOS 0 path (see

Section 11.1).

3. Splitting the AS into Areas

OSPF allows collections of contiguous networks and hosts to be grouped

together. Such a group, together with the routers having interfaces to

any one of the included networks, is called an area. Each area runs a

separate copy of the basic SPF routing algorithm. This means that each

area has its own topological database and corresponding graph, as

explained in the previous section.

The topology of an area is invisible from the outside of the area.

Conversely, routers internal to a given area know nothing of the

detailed topology external to the area. This isolation of knowledge

enables the protocol to effect a marked reduction in routing traffic as

compared to treating the entire Autonomous System as a single SPF

domain.

With the introduction of areas, it is no longer true that all routers in

the AS have an identical topological database. A router actually has a

separate topological database for each area it is connected to.

(Routers connected to multiple areas are called area border routers).

Two routers belonging to the same area have, for that area, identical

area topological databases.

Routing in the Autonomous System takes place on two levels, depending on

whether the source and destination of a packet reside in the same area

(intra-area routing is used) or different areas (inter-area routing is

used). In intra-area routing, the packet is routed solely on

information obtained within the area; no routing information obtained

from outside the area can be used. This protects intra-area routing

from the injection of bad routing information. We discuss inter-area

routing in Section 3.2.

3.1 The backbone of the Autonomous System

The backbone consists of those networks not contained in any area, their

attached routers, and those routers that belong to multiple areas. The

backbone must be contiguous.

It is possible to define areas in such a way that the backbone is no

longer contiguous. In this case the system administrator must restore

backbone connectivity by configuring virtual links.

Virtual links can be configured between any two backbone routers that

have an interface to a common non-backbone area. Virtual links belong

to the backbone. The protocol treats two routers joined by a virtual

link as if they were connected by an unnumbered point-to-point network.

On the graph of the backbone, two such routers are joined by arcs whose

costs are the intra-area distances between the two routers. The routing

protocol traffic that flows along the virtual link uses intra-area

routing only.

The backbone is responsible for distributing routing information between

areas. The backbone itself has all of the properties of an area. The

topology of the backbone is invisible to each of the areas, while the

backbone itself knows nothing of the topology of the areas.

3.2 Inter-area routing

When routing a packet between two areas the backbone is used. The path

that the packet will travel can be broken up into three contiguous

pieces: an intra-area path from the source to an area border router, a

backbone path between the source and destination areas, and then another

intra-area path to the destination. The algorithm finds the set of such

paths that have the smallest cost.

Looking at this another way, inter-area routing can be pictured as

forcing a star configuration on the Autonomous System, with the backbone

as hub and and each of the areas as spokes.

The topology of the backbone dictates the backbone paths used between

areas. The topology of the backbone can be enhanced by adding virtual

links. This gives the system administrator some control over the routes

taken by inter-area traffic.

The correct area border router to use as the packet exits the source

area is chosen in exactly the same way routers advertising external

routes are chosen. Each area border router in an area summarizes for

the area its cost to all networks external to the area. After the SPF

tree is calculated for the area, routes to all other networks are

calculated by examining the summaries of the area border routers.

3.3 Classification of routers

Before the introduction of areas, the only OSPF routers having a

specialized function were those advertising external routing

information, such as router RT5 in Figure 2. When the AS is split into

OSPF areas, the routers are further divided according to function into

the following four overlapping categories:

Internal routers

A router with all directly connected networks belonging to the same

area. Routers with only backbone interfaces also belong to this

category. These routers run a single copy of the basic routing

algorithm.

Area border routers

A router that attaches to multiple areas. Area border routers run

multiple copies of the basic algorithm, one copy for each attached

area and an additional copy for the backbone. Area border routers

condense the topological information of their attached areas for

distribution to the backbone. The backbone in turn distributes the

information to the other areas.

Backbone routers

A router that has an interface to the backbone. This includes all

routers that interface to more than one area (i.e., area border

routers). However, backbone routers do not have to be area border

routers. Routers with all interfaces connected to the backbone are

considered to be internal routers.

AS boundary routers

A router that exchanges routing information with routers belonging

to other Autonomous Systems. Such a router has AS external routes

that are advertised throughout the Autonomous System. The path to

each AS boundary router is known by every router in the AS. This

classification is completely independent of the previous

classifications: AS boundary routers may be internal or area border

routers, and may or may not participate in the backbone.

3.4 A sample area configuration

Figure 6 shows a sample area configuration. The first area consists of

networks N1-N4, along with their attached routers RT1-RT4. The second

area consists of networks N6-N8, along with their attached routers RT7,

RT8, RT10, RT11. The third area consists of networks N9-N11 and host

H1, along with their attached routers RT9, RT11, RT12. The third area

has been configured so that networks N9-N11 and host H1 will all be

grouped into a single route, when advertised external to the area (see

Section 3.5 for more details).

In Figure 6, routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are internal

routers. Routers RT3, RT4, RT7, RT10 and RT11 are area border routers.

Finally as before, routers RT5 and RT7 are AS boundary routers.

Figure 7 shows the resulting topological database for the Area 1. The

figure completely describes that area's intra-area routing. It also

shows the complete view of the internet for the two internal routers RT1

and RT2. It is the job of the area border routers, RT3 and RT4, to

advertise into Area 1 the distances to all destinations external to the

area. These are indicated in Figure 7 by the dashed stub routes. Also,

RT3 and RT4 must advertise into Area 1 the location of the AS boundary

routers RT5 and RT7. Finally, external advertisements from RT5 and RT7

are flooded throughout the entire AS, and in particular throughout Area

1. These advertisements are included in Area 1's database, and yield

routes to networks N12-N15.

Routers RT3 and RT4 must also summarize Area 1's topology for

distribution to the backbone. Their backbone advertisements are shown

in Table 4. These summaries show which networks are contained in Area 1

(i.e., networks N1-N4), and the distance to these networks from the

routers RT3 and RT4 respectively.

The topological database for the backbone is shown in Figure 8. The set

of routers pictured are the backbone routers. Router RT11 is a backbone

router because it belongs to two areas. In order to make the backbone

connected, a virtual link has been configured between routers R10 and

R11.

__________________________________________

(Figure not included in text version.)

Figure 6: A sample OSPF area configuration

__________________________________________

Network RT3 adv. RT4 adv.

_____________________________

N1 4 4

N2 4 4

N3 1 1

N4 2 3

Table 4: Networks advertised to the backbone by routers RT3 and RT4.

______________________________________

(Figure not included in text version.)

Figure 7: Area 1's Database

Figure 8: The backbone database

______________________________________

Again, routers RT3, RT4, RT7, RT10 and RT11 are area border routers. As

routers RT3 and RT4 did above, they have condensed the routing

information of their attached areas for distribution via the backbone;

these are the dashed stubs that appear in Figure 8. Remember that the

third area has been configured to condense networks N9-N11 and Host H1

into a single route. This yields a single dashed line for networks N9-

N11 and Host H1 in Figure 8. Routers RT5 and RT7 are AS boundary

routers; their externally derived information also appears on the graph

in Figure 8 as stubs.

The backbone enables the exchange of summary information between area

border routers. Every area border router hears the area summaries from

all other area border routers. It then forms a picture of the distance

to all networks outside of its area by examining the collected

advertisements, and adding in the backbone distance to each advertising

router.

Again using routers RT3 and RT4 as an example, the procedure goes as

follows: They first calculate the SPF tree for the backbone. This gives

the distances to all other area border routers. Also noted are the

distances to networks (Ia and Ib) and AS boundary routers (RT5 and RT7)

that belong to the backbone. This calculation is shown in Table 5.

Next, by looking at the area summaries from these area border routers,

RT3 and RT4 can determine the distance to all networks outside their

Area border dist from dist from

router RT3 RT4

______________________________________

to RT3 * 21

to RT4 22 *

to RT7 20 14

to RT10 15 22

to RT11 18 25

______________________________________

to Ia 20 27

to Ib 15 22

______________________________________

to RT5 14 8

to RT7 20 14

Table 5: Backbone distances calculated by routers RT3 and RT4.

area. These distances are then advertised internally to the area by RT3

and RT4. The advertisements that router RT3 and RT4 will make into Area

1 are shown in Table 6. Note that Table 6 assumes that an area range

has been configured for the backbone which groups I5 and I6 into a

single advertisement.

The information imported into Area 1 by routers RT3 and RT4 enables an

internal router, such as RT1, to choose an area border router

intelligently. Router RT1 would use RT4 for traffic to network N6, RT3

for traffic to network N10, and would load share between the two for

Destination RT3 adv. RT4 adv.

_________________________________

Ia,Ib 15 22

N6 16 15

N7 20 19

N8 18 18

N9-N11,H1 19 26

_________________________________

RT5 14 8

RT7 20 14

Table 6: Destinations advertised into Area 1 by routers RT3 and RT4.

traffic to network N8.

Router RT1 can also determine in this manner the shortest path to the AS

boundary routers RT5 and RT7. Then, by looking at RT5 and RT7's

external advertisements, router RT1 can decide between RT5 or RT7 when

sending to a destination in another Autonomous System (one of the

networks N12-N15).

Note that a failure of the line between routers RT6 and RT10 will cause

the backbone to become disconnected. Configuring another virtual link

between routers RT7 and RT10 will give the backbone more connectivity

and more resistance to such failures. Also, a virtual link between RT7

and RT10 would allow a much shorter path between the third area

(containing N9) and the router RT7, which is advertising a good route to

external network N12.

3.5 IP subnetting support

OSPF attaches an IP address mask to each advertised route. The mask

indicates the range of addresses being described by the particular

route. For example, a summary advertisement for the destination

128.185.0.0 with a mask of 0xffff0000 actually is describing a single

route to the collection of destinations 128.185.0.0 - 128.185.255.255.

Similarly, host routes are always advertised with a mask of 0xffffffff,

indicating the presence of only a single destination.

Including the mask with each advertised destination enables the

implementation of what is commonly referred to as variable-length subnet

masks. This means that a single IP class A, B, or C network number can

be broken up into many subnets of various sizes. For example, the

network 128.185.0.0 could be broken up into 64 variable-sized subnets:

16 subnets of size 4K, 16 subnets of size 256, and 32 subnets of size 8.

Table 7 shows some of the resulting network addresses together with

their masks:

Network address IP address mask Subnet size

_______________________________________________

128.185.16.0 0xfffff000 4K

128.185.1.0 0xffffff00 256

128.185.0.8 0xfffffff8 8

Table 7: Some sample subnet sizes.

There are many possible ways of dividing up a class A, B, and C network

into variable sized subnets. The precise procedure for doing so is

beyond the scope of this specification. The specification however

establishes the following guideline: When an IP packet is forwarded, it

is always forwarded to the network that is the best match for the

packet's destination. Here best match is synonymous with the longest or

most specific match. For example, the default route with destination of

0.0.0.0 and mask 0x00000000 is always a match for every IP destination.

Yet it is always less specific than any other match. Subnet masks must

be assigned so that the best match for any IP destination is

unambiguous.

The OSPF area concept is modelled after an IP subnetted network. OSPF

areas have been loosely defined to be a collection of networks. In

actuality, an OSPF area is specified to be a list of address ranges (see

Section C.2 for more details). Each address range is defined as an

[address,mask] pair. Many separate networks may then be contained in a

single address range, just as a subnetted network is composed of many

separate subnets. Area border routers then summarize the area contents

(for distribution to the backbone) by advertising a single route for

each address range. The cost of the route is the minimum cost to any of

the networks falling in the specified range.

For example, an IP subnetted network can be configured as a single OSPF

area. In that case, the area would be defined as a single address

range: a class A, B, or C network number along with its natural IP mask.

Inside the area, any number of variable sized subnets could be defined.

External to the area, a single route for the entire subnetted network

would be distributed, hiding even the fact that the network is subnetted

at all. The cost of this route is the minimum of the set of costs to

the component subnets.

3.6 Supporting stub areas

In some Autonomous Systems, the majority of the topological database may

consist of external advertisements. An OSPF external advertisement is

usually flooded throughout the entire AS. However, OSPF allows certain

areas to be configured as "stub areas". External advertisements are not

flooded into/throughout stub areas; routing to AS external destinations

in these areas is based on a (per-area) default only. This reduces the

topological database size, and therefore the memory requirements, for a

stub area's internal routers.

In order to take advantage of the OSPF stub area support, default

routing must be used in the stub area. This is accomplished as follows.

One or more of the stub area's area border routers must advertise a

default route into the stub area via summary advertisements. These

summary defaults are flooded throughout the stub area, but no further.

(For this reason these defaults pertain only to the particular stub

area). These summary default routes will match any destination that is

not explicitly reachable by an intra-area or inter-area path (i.e., AS

external destinations).

An area can be configured as stub when there is a single exit point from

the area, or when the choice of exit point need not be made on a per-

external-destination basis. For example, area 3 in Figure 6 could be

configured as a stub area, because all external traffic must travel

though its single area border router RT11. If area 3 were configured as

a stub, router RT11 would advertise a default route for distribution

inside area 3 (in a summary advertisement), instead of flooding the

external advertisements for networks N12-N15 into/throughout the area.

The OSPF protocol ensures that all routers belonging to an area agree on

whether the area has been configured as a stub. This guarantees that no

confusion will arise in the flooding of external advertisements.

There are a couple of restrictions on the use of stub areas. Virtual

links cannot be configured through stub areas. In addition, AS boundary

routers cannot be placed internal to stub areas.

3.7 Partitions of areas

OSPF does not actively attempt to repair area partitions. When an area

becomes partitioned, each component simply becomes a separate area. The

backbone then performs routing between the new areas. Some destinations

reachable via intra-area routing before the partition will now require

inter-area routing.

In the previous section, an area was described as a list of address

ranges. Any particular address range must still be completely contained

in a single component of the area partition. This has to do with the

way the area contents are summarized to the backbone. Also, the

backbone itself must not partition. If it does, parts of the Autonomous

System will become unreachable. Backbone partitions can be repaired by

configuring virtual links (see Section 15).

Another way to think about area partitions is to look at the Autonomous

System graph that was introduced in Section 2. Area IDs can be viewed

as colors for the graph's edges.[1] Each edge of the graph connects to a

network, or is itself a point-to-point network. In either case, the

edge is colored with the network's Area ID.

A group of edges, all having the same color, and interconnected by

vertices, represents an area. If the topology of the Autonomous System

is intact, the graph will have several regions of color, each color

being a distinct Area ID.

When the AS topology changes, one of the areas may become partitioned.

The graph of the AS will then have multiple regions of the same color

(Area ID). The routing in the Autonomous System will continue to

function as long as these regions of same color are connected by the

single backbone region.

4. Functional Summary

A separate copy of OSPF's basic routing algorithm runs in each area.

Routers having interfaces to multiple areas run multiple copies of the

algorithm. A brief summary of the routing algorithm follows.

When a router starts, it first initializes the routing protocol data

structures. The router then waits for indications from the lower-level

protocols that its interfaces are functional.

A router then uses the OSPF's Hello Protocol to acquire neighbors. The

router sends Hello packets to its neighbors, and in turn receives their

Hello packets. On broadcast and point-to-point networks, the router

dynamically detects its neighboring routers by sending its Hello packets

to the multicast address AllSPFRouters. On non-broadcast networks, some

configuration information is necessary in order to discover neighbors.

On all multi-access networks (broadcast or non-broadcast), the Hello

Protocol also elects a Designated router for the network.

The router will attempt to form adjacencies with some of its newly

acquired neighbors. Topological databases are synchronized between

pairs of adjacent routers. On multi-access networks, the Designated

Router determines which routers should become adjacent.

Adjacencies control the distribution of routing protocol packets.

Routing protocol packets are sent and received only on adjacencies. In

particular, distribution of topological database updates proceeds along

adjacencies.

A router periodically advertises its state, which is also called link

state. Link state is also advertised when a router's state changes. A

router's adjacencies are reflected in the contents of its link state

advertisements. This relationship between adjacencies and link state

allows the protocol to detect dead routers in a timely fashion.

Link state advertisements are flooded throughout the area. The flooding

algorithm is reliable, ensuring that all routers in an area have exactly

the same topological database. This database consists of the collection

of link state advertisements received from each router belonging to the

area. From this database each router calculates a shortest-path tree,

with itself as root. This shortest-path tree in turn yields a routing

table for the protocol.

4.1 Inter-area routing

The previous section described the operation of the protocol within a

single area. For intra-area routing, no other routing information is

pertinent. In order to be able to route to destinations outside of the

area, the area border routers inject additional routing information into

the area. This additional information is a distillation of the rest of

the Autonomous System's topology.

This distillation is accomplished as follows: Each area border router is

by definition connected to the backbone. Each area border router

summarizes the topology of its attached areas for transmission on the

backbone, and hence to all other area border routers. A area border

router then has complete topological information concerning the

backbone, and the area summaries from each of the other area border

routers. From this information, the router calculates paths to all

destinations not contained in its attached areas. The router then

advertises these paths to its attached areas. This enables the area's

internal routers to pick the best exit router when forwarding traffic to

destinations in other areas.

4.2 AS external routes

Routers that have information regarding other Autonomous Systems can

flood this information throughout the AS. This external routing

information is distributed verbatim to every participating router.

There is one exception: external routing information is not flooded into

"stub" areas (see Section 3.6).

To utilize external routing information, the path to all routers

advertising external information must be known throughout the AS

(excepting the stub areas). For that reason, the locations of these AS

boundary routers are summarized by the (non-stub) area border routers.

4.3 Routing protocol packets

The OSPF protocol runs directly over IP, using IP protocol 89. OSPF

does not provide any explicit fragmentation/reassembly support. When

fragmentation is necessary, IP fragmentation/reassembly is used. OSPF

protocol packets have been designed so that large protocol packets can

generally be split into several smaller protocol packets. This practice

is recommended; IP fragmentation should be avoided whenever possible.

Routing protocol packets should always be sent with the IP TOS field set

to 0. If at all possible, routing protocol packets should be given

preference over regular IP data traffic, both when being sent and

received. As an aid to accomplishing this, OSPF protocol packets should

have their IP precedence field set to the value Internetwork Control

(see [RFC791]).

All OSPF protocol packets share a common protocol header that is

described in Appendix A. The OSPF packet types are listed below in

Table 8. Their formats are also described in Appendix A.

Type Packet name Protocol function

__________________________________________________________

1 Hello Discover/maintain neighbors

2 Database Description Summarize database contents

3 Link State Request Database download

4 Link State Update Database update

5 Link State Ack Flooding acknowledgment

Table 8: OSPF packet types.

OSPF's Hello protocol uses Hello packets to discover and maintain

neighbor relationships. The Database Description and Link State Request

packets are used in the forming of adjacencies. OSPF's reliable update

mechanism is implemented by the Link State Update and Link State

Acknowledgment packets.

Each Link State Update packet carries a set of new link state

advertisements one hop further away from their point of origination. A

single Link State Update packet may contain the link state

advertisements of several routers. Each advertisement is tagged with

the ID of the originating router and a checksum of its link state

contents. The five different types of OSPF link state advertisements

are listed below in Table 9.

LS Advertisement Advertisement description

type name

____________________________________________________________________________

1 Router links advs. Originated by all routers. This

advs. advertisement describes the collected

states of the router's interfaces to an

area. Flooded throughout a single area

only.

____________________________________________________________________________

2 Network links Originated for multi-access networks by

advs. the Designated Router. This

advertisement contains the list of

routers connected to the network.

Flooded throughout a single area only.

LS Advertisement Advertisement description

type name

____________________________________________________________________________

____________________________________________________________________________

3,4 Summary link Originated by area border routers, and

advs. flooded throughout their associated

area. Each summary link advertisement

describes a route to a destination

outside the area, yet still inside the

AS (i.e., an inter-area route). Type 3

advertisements describe routes to

networks. Type 4 advertisements

describe routes to AS boundary routers.

____________________________________________________________________________

5 AS external Originated by AS boundary routers, and

link advs. flooded throughout the AS. Each external

advertisement describes a route to a

destination in another Autonomous

System. Default routes for the AS can

also be described by AS external advertisements.

Table 9: OSPF link state advertisements.

As mentioned above, OSPF routing packets (with the exception of Hellos)

are sent only over adjacencies. Note that this means that all protocol

packets travel a single IP hop, except those that are sent over virtual

adjacencies. The IP source address of an OSPF protocol packet is one

end of a router adjacency, and the IP destination address is either the

other end of the adjacency or an IP multicast address.

4.4 Basic implementation requirements

An implementation of OSPF requires the following pieces of system

support:

Timers

Two different kind of timers are required. The first kind, called

single shot timers, fire once and cause a protocol event to be

processed. The second kind, called interval timers, fire at

continuous intervals. These are used for the sending of packets at

regular intervals. A good example of this is the regular broadcast

of Hello packets (on broadcast networks). The granularity of both

kinds of timers is one second.

Interval timers should be implemented to avoid drift. In some

router implementations, packet processing can affect timer

execution. When multiple routers are attached to a single network,

all doing broadcasts, this can lead to the synchronization of

routing packets (which should be avoided). If timers cannot be

implemented to avoid drift, small random amounts should be added

to/subtracted from the timer interval at each firing.

IP multicast

Certain OSPF packets use IP multicast. Support for receiving and

sending IP multicasts, along with the appropriate lower-level

protocol support, is required. These IP multicast packets never

travel more than one hop. For information on IP multicast, see [RFC

1112].

Lower-level protocol support

The lower level protocols referred to here are the network access

protocols, such as the Ethernet data link layer. Indications must

be passed from from these protocols to OSPF as the network interface

goes up and down. For example, on an ethernet it would be valuable

to know when the ethernet transceiver cable becomes unplugged.

Non-broadcast lower-level protocol support

Remember that non-broadcast networks are multi-access networks such

as a X.25 PDN. On these networks, the Hello Protocol can be aided

by providing an indication to OSPF when an attempt is made to send a

packet to a dead or non-existent router. For example, on a PDN a

dead router may be indicated by the reception of a X.25 clear with

an appropriate cause and diagnostic, and this information would be

passed to OSPF.

List manipulation primitives

Much of the OSPF functionality is described in terms of its

operation on lists of link state advertisements. For example, the

advertisements that will be retransmitted to an adjacent router

until acknowledged are described as a list. Any particular

advertisement may be on many such lists. An OSPF implementation

needs to be able to manipulate these lists, adding and deleting

constituent advertisements as necessary.

TaSKINg support

Certain procedures described in this specification invoke other

procedures. At times, these other procedures should be executed

in-line, that is, before the current procedure is finished. This is

indicated in the text by instructions to execute a procedure. At

other times, the other procedures are to be executed only when the

current procedure has finished. This is indicated by instructions

to schedule a task.

4.5 Optional OSPF capabilities

The OSPF protocol defines several optional capabilities. A router

indicates the optional capabilities that it supports in its OSPF Hello

packets, Database Description packets and in its link state

advertisements. This enables routers supporting a mix of optional

capabilities to coexist in a single Autonomous System.

Some capabilities must be supported by all routers attached to a

specific area. In this case, a router will not accept a neighbor's

Hello unless there is a match in reported capabilities (i.e., a

capability mismatch prevents a neighbor relationship from forming). An

example of this is the external routing capability (see below).

Other capabilities can be negotiated during the database synchronization

process. This is accomplished by specifying the optional capabilities

in Database Description packets. A capability mismatch with a neighbor

is this case will result in only a subset of link state advertisements

being exchanged between the two neighbors.

The routing table build process can also be affected by the

presence/absence of optional capabilities. For example, since the

optional capabilities are reported in link state advertisements, routers

incapable of certain functions can be avoided when building the shortest

path tree. An example of this is the TOS routing capability (see

below).

The current OSPF optional capabilities are listed below. See Section

A.2 for more information.

External routing capability

Entire OSPF areas can be configured as "stubs" (see Section 3.6).

AS external advertisements will not be flooded into stub areas.

This capability is represented by the E-bit in the OSPF options

field (see Section A.2). In order to ensure consistent

configuration of stub areas, all routers interfacing to such an area

must have the E-bit clear in their Hello packets (see Sections 9.5

and 10.5).

TOS capability

All OSPF implementations must be able to calculate separate routes

based on IP Type of Service. However, to save routing table space

and processing resources, an OSPF router can be configured to ignore

TOS when forwarding packets. In this case, the router calculates

routes for TOS 0 only. This capability is represented by the T-bit

in the OSPF options field (see Section A.2). TOS-capable routers

will attempt to avoid non-TOS-capable routers when calculating non-

zero TOS paths.

5. Protocol Data Structures

The OSPF protocol is described in this specification in terms of its

operation on various protocol data structures. The following list

comprises the top-level OSPF data structures. Any initialization that

needs to be done is noted. Areas, OSPF interfaces and neighbors also

have associated data structures that are described later in this

specification.

Router ID

a 32-bit number that uniquely identifies this router in the AS. One

possible implementation strategy would be to use the smallest IP

interface address belonging to the router.

Pointers to area structures

Each one of the areas to which the router is connected has its own

data structure. This data structure describes the working of the

basic algorithm. Remember that each area runs a separate copy of

the basic algorithm.

Pointer to the backbone structure

The basic algorithm operates on the backbone as if it were an area.

For this reason the backbone is represented as an area structure.

Virtual links configured

The virtual links configured with this router as one endpoint. In

order to have configured virtual links, the router itself must be an

area border router. Virtual links are identified by the Router ID

of the other endpoint -- which is another area border router. These

two endpoint routers must be attached to a common area, called the

virtual link's transit area. Virtual links are part of the

backbone, and behave as if they were unnumbered point-to-point

networks between the two routers. A virtual link uses the intra-

area routing of its transit area to forward packets. Virtual links

are brought up and down through the building of the shortest-path

trees for the transit area.

List of external routes

These are routes to destinations external to the Autonomous System,

that have been gained either through direct experience with another

routing protocol (such as EGP), or through configuration

information, or through a combination of the two (e.g., dynamic

external info. to be advertised by OSPF with configured metric).

Any router having these external routes is called an AS boundary

router. These routes are advertised by the router to the entire AS

through AS external link advertisements.

List of AS external link advertisements

Part of the topological database. These have have originated from

the AS boundary routers. They comprise routes to destinations

external to the Autonomous System. Note that, if the router is

itself an AS boundary router, some of these AS external link

advertisements have been self originated.

The routing table

Derived from the topological database. Each destination that the

router can forward to is represented by a cost and a set of paths.

A path is described by its type and next hop. For more information,

see Section 11.

TOS capability

This item indicates whether the router will calculate separate

routes based on TOS. This is a configurable parameter. For more

information, see Sections 4.5 and 16.9.

Figure 9 shows the collection of data structures present in a typical

router. The router pictured is RT10, from the map in Figure 6. Note

that router RT10 has a virtual link configured to router RT11, with Area

2 as the link's transit area. This is indicated by the dashed line in

Figure 9. When the virtual link becomes active, through the building of

the shortest path tree for Area 2, it becomes an interface to the

backbone (see the two backbone interfaces depicted in Figure 9).

6. The Area Data Structure

The area data structure contains all the information used to run the

basic routing algorithm. Remember that each area maintains its own

topological database. Router interfaces and adjacencies belong to a

_______________________________________

(Figure not included in text version.)

Figure 9: Router RT10's Data Structures

_______________________________________

single area.

The backbone has all the properties of an area. For that reason it is

also represented by an area data structure. Note that some items in the

structure apply differently to the backbone than to areas.

The area topological (or link state) database consists of the collection

of router links, network links and summary links advertisements that

have originated from the area's routers. This information is flooded

throughout a single area only. The list of AS external advertisements

is also considered to be part of each area's topological database.

Area ID

A 32-bit number identifying the area. 0 is reserved for the area ID

of the backbone. If assigning subnetted networks as separate areas,

the IP network number could be used as the Area ID.

List of component address ranges

The address ranges that define the area. Each address range is

specified by an [address,mask] pair. Each network is then assigned

to an area depending on the address range that it falls into

(specified address ranges are not allowed to overlap). As an

example, if an IP subnetted network is to be its own separate OSPF

area, the area is defined to consist of a single address range - an

IP network number with its natural (class A, B or C) mask.

Associated router interfaces

This router's interfaces connecting to the area. A router interface

belongs to one and only one area (or the backbone). For the

backbone structure this list includes all the virtual adjacencies.

A virtual adjacency is identified by the router ID of its other

endpoint; its cost is the cost of the shortest intra-area path that

exists between the two routers.

List of router links advertisements

A router links advertisement is generated by each router in the

area. It describes the state of the router's interfaces to the

area.

List of network links advertisements

One network links advertisement is generated for each transit

multi-access network in the area. It describes the set of routers

currently connected to the network.

List of summary links advertisements

Summary link advertisements originate from the area's area border

routers. They describe routes to destinations internal to the

Autonomous System, yet external to the area.

Shortest-path tree

The shortest-path tree for the area, with this router itself as

root. Derived from the collected router links and network links

advertisements by the Dijkstra algorithm.

Authentication type

The type of authentication used for this area. Authentication types

are defined in Appendix E. All OSPF packet exchanges are

authenticated. Different authentication schemes may be used in

different areas.

External routing capability

Whether AS external advertisements will be flooded into/throughout

the area. This is a configurable parameter. If AS external

advertisements are excluded from the area, the area is called a

"stub". Internal to stub areas, routing to external destinations

will be based solely on a default summary route. The backbone

cannot be configured as a stub area. Also, virtual links cannot be

configured through stub areas. For more information, see Section

3.6.

StubDefaultCost

If the area has been configured as a stub area, and the router

itself is an area border router, then the StubDefaultCost indicates

the cost of the default summary link that the router should

advertise into the area. There can be a separate cost configured

for each IP TOS. See Section 12.4.3 for more information.

Unless otherwise specified, the remaining sections of this document

refer to the operation of the protocol in a single area.

7. Bringing Up Adjacencies

OSPF creates adjacencies between neighboring routers for the purpose of

exchanging routing information. Not every two neighboring routers will

become adjacent. This section covers the generalities involved in

creating adjacencies. For further details consult Section 10.

7.1 The Hello Protocol

The Hello Protocol is responsible for establishing and maintaining

neighbor relationships. It also ensures that communication between

neighbors is bidirectional. Hello packets are sent periodically out all

router interfaces. Bidirectional communication is indicated when the

router sees itself listed in the neighbor's Hello Packet.

On multi-access networks, the Hello Protocol elects a Designated Router

for the network. Among other things, the Designated Router controls

what adjacencies will be formed over the network (see below).

The Hello Protocol works differently on broadcast networks, as compared

to non-broadcast networks. On broadcast networks, each router

advertises itself by periodically multicasting Hello Packets. This

allows neighbors to be discovered dynamically. These Hello Packets

contain the router's view of the Designated Router's identity, and the

list of routers whose Hellos have been seen recently.

On non-broadcast networks some configuration information is necessary

for the operation of the Hello Protocol. Each router that may

potentially become Designated Router has a list of all other routers

attached to the network. A router, having Designated Router potential,

sends hellos to all other potential Designated Routers when its

interface to the non-broadcast network first becomes operational. This

is an attempt to find the Designated Router for the network. If the

router itself is elected Designated Router, it begins sending hellos to

all other routers attached to the network.

After a neighbor has been discovered, bidirectional communication

ensured, and (if on a multi-access network) a Designated Router elected,

a decision is made regarding whether or not an adjacency should be

formed with the neighbor (see Section 10.4). An attempt is always made

to establish adjacencies over point-to-point networks and virtual links.

The first step in bringing up an adjacency is to synchronize the

neighbors' topological databases. This is covered in the next section.

7.2 The Synchronization of Databases

In an SPF-based routing algorithm, it is very important for all routers'

topological databases to stay synchronized. OSPF simplifies this by

requiring only adjacent routers to remain synchronized. The

synchronization process begins as soon as the routers attempt to bring

up the adjacency. Each router describes its database by sending a

sequence of Database Description packets to its neighbor. Each Database

Description Packet describes a set of link state advertisements

belonging to the database. When the neighbor sees a link state

advertisement that is more recent than its own database copy, it makes a

note that this newer advertisement should be requested.

This sending and receiving of Database Description packets is called the

"Database Exchange Process". During this process, the two routers form

a master/slave relationship. Each Database Description Packet has a

sequence number. Database Description Packets sent by the master

(polls) are acknowledged by the slave through echoing of the sequence

number. Both polls and their responses contain summaries of link state

data. The master is the only one allowed to retransmit Database

Description Packets. It does so only at fixed intervals, the length of

which is the configured constant RxmtInterval.

Each Database Description contains an indication that there are more

packets to follow --- the M-bit. The Database Exchange Process is over

when a router has received and sent Database Description Packets with

the M-bit off.

During and after the Database Exchange Process, each router has a list

of those link state advertisements for which the neighbor has more up-

to-date instances. These advertisements are requested in Link State

Request Packets. Link State Request packets that are not satisfied are

retransmitted at fixed intervals of time RxmtInterval. When the

Database Description Process has completed and all Link State Requests

have been satisfied, the databases are deemed synchronized and the

routers are marked fully adjacent. At this time the adjacency is fully

functional and is advertised in the two routers' link state

advertisements.

The adjacency is used by the flooding procedure as soon as the Database

Exchange Process begins. This simplifies database synchronization, and

guarantees that it finishes in a predictable period of time.

7.3 The Designated Router

Every multi-access network has a Designated Router. The Designated

Router performs two main functions for the routing protocol:

o The Designated Router originates a network links advertisement on

behalf of the network. This advertisement lists the set of routers

(including the Designated Router itself) currently attached to the

network. The Link State ID for this advertisement (see Section

12.1.4) is the IP interface address of the Designated Router. The

IP network number can then be obtained by using the subnet/network

mask.

o The Designated router becomes adjacent to all other routers on the

network. Since the link state databases are synchronized across

adjacencies (through adjacency bring-up and then the flooding

procedure), the Designated Router plays a central part in the

synchronization process.

The Designated Router is elected by the Hello Protocol. A router's

Hello Packet contains its Router Priority, which is configurable on a

per-interface basis. In general, when a router's interface to a network

first becomes functional, it checks to see whether there is currently a

Designated Router for the network. If there is, it accepts that

Designated Router, regardless of its Router Priority. (This makes it

harder to predict the identity of the Designated Router, but ensures

that the Designated Router changes less often. See below.) Otherwise,

the router itself becomes Designated Router if it has the highest Router

Priority on the network. A more detailed (and more accurate)

description of Designated Router election is presented in Section 9.4.

The Designated Router is the endpoint of many adjacencies. In order to

optimize the flooding procedure on broadcast networks, the Designated

Router multicasts its Link State Update Packets to the address

AllSPFRouters, rather than sending separate packets over each adjacency.

Section 2 of this document discusses the directed graph representation

of an area. Router nodes are labelled with their Router ID. Broadcast

network nodes are actually labelled with the IP address of their

Designated Router. It follows that when the Designated Router changes,

it appears as if the network node on the graph is replaced by an

entirely new node. This will cause the network and all its attached

routers to originate new link state advertisements. Until the

topological databases again converge, some temporary loss of

connectivity may result. This may result in ICMP unreachable messages

being sent in response to data traffic. For that reason, the Designated

Router should change only infrequently. Router Priorities should be

configured so that the most dependable router on a network eventually

becomes Designated Router.

7.4 The Backup Designated Router

In order to make the transition to a new Designated Router smoother,

there is a Backup Designated Router for each multi-access network. The

Backup Designated Router is also adjacent to all routers on the network,

and becomes Designated Router when the previous Designated Router fails.

If there were no Backup Designated Router, when a new Designated Router

became necessary, new adjacencies would have to be formed between the

router and all other routers attached to the network. Part of the

adjacency forming process is the synchronizing of topological databases,

which can potentially take quite a long time. During this time, the

network would not be available for transit data traffic. The Backup

Designated obviates the need to form these adjacencies, since they

already exist. This means the period of disruption in transit traffic

lasts only as long as it take to flood the new link state advertisements

(which announce the new Designated Router).

The Backup Designated Router does not generate a network links

advertisement for the network. (If it did, the transition to a new

Designated Router would be even faster. However, this is a tradeoff

between database size and speed of convergence when the Designated

Router disappears.)

The Backup Designated Router is also elected by the Hello Protocol.

Each Hello Packet has a field that specifies the Backup Designated

Router for the network.

In some steps of the flooding procedure, the Backup Designated Router

plays a passive role, letting the Designated Router do more of the work.

This cuts down on the amount of local routing traffic. See Section 13.3

for more information.

7.5 The graph of adjacencies

An adjacency is bound to the network that the two routers have in

common. If two routers have multiple networks in common, they may have

multiple adjacencies between them.

One can picture the collection of adjacencies on a network as forming an

undirected graph. The vertices consist of routers, with an edge joining

two routers if they are adjacent. The graph of adjacencies describes

the flow of routing protocol packets, and in particular Link State

Updates, through the Autonomous System.

Two graphs are possible, depending on whether the common network is

multi-access. On physical point-to-point networks (and virtual links),

the two routers joined by the network will be adjacent after their

databases have been synchronized. On multi-access networks, both the

Designated Router and the Backup Designated Router are adjacent to all

other routers attached to the network, and these account for all

adjacencies.

These graphs are shown in Figure 10. It is assumed that router RT7 has

become the Designated Router, and router RT3 the Backup Designated

Router, for the network N2. The Backup Designated Router performs a

lesser function during the flooding procedure than the Designated Router

(see Section 13.3). This is the reason for the dashed lines connecting

the Backup Designated Router RT3.

8. Protocol Packet Processing

This section discusses the general processing of routing protocol

packets. It is very important that the router topological databases

remain synchronized. For this reason, routing protocol packets should

get preferential treatment over ordinary data packets, both in sending

and receiving.

Routing protocol packets are sent along adjacencies only (with the

exception of Hello packets, which are used to discover the adjacencies).

This means that all protocol packets travel a single IP hop, except

those sent over virtual links.

All routing protocol packets begin with a standard header. The sections

below give the details on how to fill in and verify this standard

header. Then, for each packet type, the section is listed that gives

more details on that particular packet type's processing.

8.1 Sending protocol packets

When a router sends a routing protocol packet, it fills in the fields of

that standard header as follows. For more details on the header format

consult Section A.3.1:

Version #

Set to 2, the version number of the protocol as documented in this

specification.

Packet type

The type of OSPF packet, such as Link state Update or Hello Packet.

Packet length

The length of the entire OSPF packet in bytes, including the

standard header.

Router ID

The identity of the router itself (who is originating the packet).

______________________________________

(Figure not included in text version.)

Figure 10: The graph of adjacencies

Figure 11: Interface state changes

______________________________________

Area ID

The area that the packet is being sent into.

Checksum

The standard IP 16-bit one's complement checksum of the entire OSPF

packet, excluding the 64-bit authentication field. This checksum

should be calculated before handing the packet to the appropriate

authentication procedure.

Autype and Authentication

Each OSPF packet exchange is authenticated. Authentication types

are assigned by the protocol and documented in Appendix E. A

different authentication scheme can be used for each OSPF area. The

64-bit authentication field is set by the appropriate authentication

procedure (determined by Autype). This procedure should be the last

called when forming the packet to be sent. The setting of the

authentication field is determined by the packet contents and the

authentication key (which is configurable on a per-interface basis).

The IP destination address for the packet is selected as follows. On

physical point-to-point networks, the IP destination is always set to

the the address AllSPFRouters. On all other network types (including

virtual links), the majority of OSPF packets are sent as unicasts, i.e.,

sent directly to the other end of the adjacency. In this case, the IP

destination is just the neighbor IP address associated with the other

end of the adjacency (see Section 10). The only packets not sent as

unicasts are on broadcast networks; on these networks Hello packets are

sent to the multicast destination AllSPFRouters, the Designated Router

and its Backup send both Link State Update Packets and Link State

Acknowledgment Packets to the multicast address AllSPFRouters, while all

other routers send both their Link State Update and Link State

Acknowledgment Packets to the multicast address AllDRouters.

Retransmissions of Link State Update packets are ALWAYS sent as

unicasts.

The IP source address should be set to the IP address of the sending

interface. Interfaces to unnumbered point-to-point networks have no

associated IP address. On these interfaces, the IP source should be set

to any of the other IP addresses belonging to the router. For this

reason, there must be at least one IP address assigned to the router.[2]

Note that, for most purposes, virtual links act precisely the same as

unnumbered point-to-point networks. However, each virtual link does

have an interface IP address (discovered during the routing table build

process) which is used as the IP source when sending packets over the

virtual link.

For more information on the format of specific packet types, consult the

sections listed in Table 10.

Type Packet name detailed section (transmit)

_________________________________________________________

1 Hello Section 9.5

2 Database description Section 10.8

3 Link state request Section 10.9

4 Link state update Section 13.3

5 Link state ack Section 13.5

Table 10: Sections describing packet transmission.

8.2 Receiving protocol packets

Whenever a protocol packet is received by the router it is marked with

the interface it was received on. For routers that have virtual links

configured, it may not be immediately obvious which interface to

associate the packet with. For example, consider the router RT11

depicted in Figure 6. If RT11 receives an OSPF protocol packet on its

interface to network N8, it may want to associate the packet with the

interface to area 2, or with the virtual link to router RT10 (which is

part of the backbone). In the following, we assume that the packet is

initially associated with the non-virtual link.[3]

In order for the packet to be accepted at the IP level, it must pass a

number of tests, even before the packet is passed to OSPF for

processing:

o The IP checksum must be correct.

o The packet's IP destination address must be the IP address of the

receiving interface, or one of the IP multicast addresses

AllSPFRouters or AllDRouters.

o The IP protocol specified must be OSPF (89).

o Locally originated packets should not be passed on to OSPF. That

is, the source IP address should be examined to make sure this is

not a multicast packet that the router itself generated.

Next, the OSPF packet header is verified. The fields specified in the

header must match those configured for the receiving interface. If they

do not, the packet should be discarded:

o The version number field must specify protocol version 2.

o The 16-bit checksum of the OSPF packet's contents must be verified.

Remember that the 64-bit authentication field must be excluded from

the checksum calculation.

o The Area ID found in the OSPF header must be verified. If both of

the following cases fail, the packet should be discarded. The Area

ID specified in the header must either:

(1) Match the Area ID of the receiving interface. In this case, the

packet has been sent over a single hop. Therefore, the packet's

IP source address must be on the same network as the receiving

interface. This can be determined by comparing the packet's IP

source address to the interface's IP address, after masking both

addresses with the interface mask.

(2) Indicate the backbone. In this case, the packet has been sent

over a virtual link. The receiving router must be an area

border router, and the router ID specified in the packet (the

source router) must be the other end of a configured virtual

link. The receiving interface must also attach to the virtual

link's configured transit area. If all of these checks succeed,

the packet is accepted and is from now on associated with the

virtual link (and the backbone area).

o Packets whose IP destination is AllDRouters should only be accepted

if the state of the receiving interface is DR or Backup (see Section

9.1).

o The Authentication type specified must match the authentication type

specified for the associated area.

Next, the packet must be authenticated. This depends on the

authentication type specified (see Appendix E). The authentication

procedure may use an Authentication key, which can be configured on a

per-interface basis. If the authentication fails, the packet should be

discarded.

If the packet type is Hello, it should then be further processed by the

Hello Protocol (see Section 10.5). All other packet types are

sent/received only on adjacencies. This means that the packet must have

been sent by one of the router's active neighbors. If the receiving

interface is a multi-access network (either broadcast or non-broadcast)

the sender is identified by the IP source address found in the packet's

IP header. If the receiving interface is a point-to-point link or a

virtual link, the sender is identified by the Router ID (source router)

found in the packet's OSPF header. The data structure associated with

the receiving interface contains the list of active neighbors. Packets

not matching any active neighbor are discarded.

At this point all received protocol packets are associated with an

active neighbor. For the further input processing of specific packet

types, consult the sections listed in Table 11.

Type Packet name detailed section (receive)

________________________________________________________

1 Hello Section 10.5

2 Database description Section 10.6

3 Link state request Section 10.7

4 Link state update Section 13

5 Link state ack Section 13.7

Table 11: Sections describing packet reception.

9. The Interface Data Structure

An OSPF interface is the connection between a router and a network.

There is a single OSPF interface structure for each attached network;

each interface structure has at most one IP interface address (see

below). The support for multiple addresses on a single network is a

matter for future consideration.

An OSPF interface can be considered to belong to the area that contains

the attached network. All routing protocol packets originated by the

router over this interface are labelled with the interface's Area ID.

One or more router adjacencies may develop over an interface. A

router's link state advertisements reflect the state of its interfaces

and their associated adjacencies.

The following data items are associated with an interface. Note that a

number of these items are actually configuration for the attached

network; those items must be the same for all routers connected to the

network.

Type

The kind of network to which the interface attaches. Its value is

either broadcast, non-broadcast yet still multi-access, point-to-

point or virtual link.

State

The functional level of an interface. State determines whether or

not full adjacencies are allowed to form over the interface. State

is also reflected in the router's link state advertisements.

IP interface address

The IP address associated with the interface. This appears as the

IP source address in all routing protocol packets originated over

this interface. Interfaces to unnumbered point-to-point networks do

not have an associated IP address.

IP interface mask

This indicates the portion of the IP interface address that

identifies the attached network. This is often referred to as the

subnet mask. Masking the IP interface address with this value

yields the IP network number of the attached network.

Area ID

The Area ID to which the attached network belongs. All routing

protocol packets originating from the interface are labelled with

this Area ID.

HelloInterval

The length of time, in seconds, between the Hello packets that the

router sends on the interface. Advertised in Hello packets sent out

this interface.

RouterDeadInterval

The number of seconds before the router's neighbors will declare it

down, when they stop hearing the router's hellos. Advertised in

Hello packets sent out this interface.

InfTransDelay

The estimated number of seconds it takes to transmit a Link State

Update Packet over this interface. Link state advertisements

contained in the update packet will have their age incremented by

this amount before transmission. This value should take into

account transmission and propagation delays; it must be greater than

zero.

Router Priority

An 8-bit unsigned integer. When two routers attached to a network

both attempt to become Designated Router, the one with the highest

Router Priority takes precedence. A router whose Router Priority is

set to 0 is ineligible to become Designated Router on the attached

network. Advertised in Hello packets sent out this interface.

Hello Timer

An interval timer that causes the interface to send a Hello packet.

This timer fires every HelloInterval seconds. Note that on non-

broadcast networks a separate Hello packet is sent to each qualified

neighbor.

Wait Timer

A single shot timer that causes the interface to exit the Waiting

state, and as a consequence select a Designated Router on the

network. The length of the timer is RouterDeadInterval seconds.

List of neighboring routers

The other routers attached to this network. On multi-access

networks, this list is formed by the Hello Protocol. Adjacencies

will be formed to some of these neighbors. The set of adjacent

neighbors can be determined by an examination of all of the

neighbors' states.

Designated Router

The Designated Router selected for the attached network. The

Designated Router is selected on all multi-access networks by the

Hello Protocol. Two pieces of identification are kept for the

Designated Router: its Router ID and its interface IP address on the

network. The Designated Router advertises link state for the

network. The network link state advertisement is labelled with the

Designated Router's IP address. This item is initialized to 0,

which indicates the lack of a Designated Router.

Backup Designated Router

The Backup Designated Router is also selected on all multi-access

networks by the Hello Protocol. All routers on the attached network

become adjacent to both the Designated Router and the Backup

Designated Router. The Backup Designated Router becomes Designated

Router when the current Designated Router fails. Initialized to 0

indicating the lack of a Backup Designated Router.

Interface output cost(s)

The cost of sending a packet on the interface, expressed in the link

state metric. This is advertised as the link cost for this

interface in the router links advertisement. There may be a

separate cost for each IP Type of Service. The cost of an interface

must be greater than zero.

RxmtInterval

The number of seconds between link state advertisement

retransmissions, for adjacencies belonging to this interface. Also

used when retransmitting Database Description and Link State Request

Packets.

Authentication key

This configured data allows the authentication procedure to generate

and/or verify the authentication field in the OSPF header. The

authentication key can be configured on a per-interface basis. For

example, if the authentication type indicates simple passWord, the

authentication key would be a 64-bit password. This key would be

inserted directly into the OSPF header when originating routing

protocol packets, and there could be a separate password for each

network.

9.1 Interface states

The various states that router interface may attain is documented in

this section. The states are listed in order of progressing

functionality. For example, the inoperative state is listed first,

followed by a list of intermediate states before the final, fully

functional state is achieved. The specification makes use of this

ordering by sometimes making references such as "those interfaces in

state greater than X".

Figure 11 shows the graph of interface state changes. The arcs of the

graph are labelled with the event causing the state change. These

events are documented in Section 9.2. The interface state machine is

described in more detail in Section 9.3.

Down

This is the initial interface state. In this state, the lower-level

protocols have indicated that the interface is unusable. No

protocol traffic at all will be sent or received on such a

interface. In this state, interface parameters should be set to

their initial values. All interface timers should be disabled, and

there should be no adjacencies associated with the interface.

Loopback

In this state, the router's interface to the network is looped back.

The interface may be looped back in hardware or software. The

interface will be unavailable for regular data traffic. However, it

may still be desirable to gain information on the quality of this

interface, either through sending ICMP pings to the interface or

through something like a bit error test. For this reason, IP

packets may still be addressed to an interface in Loopback state.

To facilitate this, such interfaces are advertised in router links

advertisements as single host routes, whose destination is the IP

interface address.[4]

Waiting

In this state, the router is trying to determine the identity of the

Backup Designated Router for the network. To do this, the router

monitors the Hellos it receives. The router is not allowed to elect

a Backup Designated Router nor Designated Router until it

transitions out of Waiting state. This prevents unnecessary changes

of (Backup) Designated Router.

Point-to-point

In this state, the interface is operational, and connects either to

a physical point-to-point network or to a virtual link. Upon

entering this state, the router attempts to form an adjacency with

the neighboring router. Hellos are sent to the neighbor every

HelloInterval seconds.

DR Other

The interface is to a multi-access network on which another router

has been selected to be the Designated Router. In this state, the

router itself has not been selected Backup Designated Router either.

The router forms adjacencies to both the Designated Router and the

Backup Designated Router (if they exist).

Backup

In this state, the router itself is the Backup Designated Router on

the attached network. It will be promoted to Designated Router when

the present Designated Router fails. The router establishes

adjacencies to all other routers attached to the network. The

Backup Designated Router performs slightly different functions

during the Flooding Procedure, as compared to the Designated Router

(see Section 13.3). See Section 7.4 for more details on the

functions performed by the Backup Designated Router.

DR In this state, this router itself is the Designated Router on the

attached network. Adjacencies are established to all other routers

attached to the network. The router must also originate a network

links advertisement for the network node. The advertisement will

contain links to all routers (including the Designated Router

itself) attached to the network. See Section 7.3 for more details

on the functions performed by the Designated Router.

9.2 Events causing interface state changes

State changes can be effected by a number of events. These events are

pictured as the labelled arcs in Figure 11. The label definitions are

listed below. For a detailed explanation of the effect of these events

on OSPF protocol operation, consult Section 9.3.

Interface Up

Lower-level protocols have indicated that the network interface is

operational. This enables the interface to transition out of Down

state. On virtual links, the interface operational indication is

actually a result of the shortest path calculation (see Section

16.7).

Wait Timer

The Wait timer has fired, indicating the end of the waiting period

that is required before electing a (Backup) Designated Router.

Backup seen

The router has detected the existence or non-existence of a Backup

Designated Router for the network. This is done in one of two ways.

First, a Hello Packet may be received from a neighbor claiming to be

itself the Backup Designated Router. Alternatively, a Hello Packet

may be received from a neighbor claiming to be itself the Designated

Router, and indicating that there is no Backup. In either case

there must be bidirectional communication with the neighbor, i.e.,

the router must also appear in the neighbor's Hello Packet. This

event signals an end to the Waiting state.

Neighbor Change

There has been a change in the set of bidirectional neighbors

associated with the interface. The (Backup) Designated Router needs

to be recalculated. The following neighbor changes lead to the

Neighbor Change event. For an explanation of neighbor states, see

Section 10.1.

o Bidirectional communication has been established to a neighbor.

In other words, the state of the neighbor has transitioned to

2-Way or higher.

o There is no longer bidirectional communication with a neighbor.

In other words, the state of the neighbor has transitioned to

Init or lower.

o One of the bidirectional neighbors is newly declaring itself as

either Designated Router or Backup Designated Router. This is

detected through examination of that neighbor's Hello Packets.

o One of the bidirectional neighbors is no longer declaring itself

as Designated Router, or is no longer declaring itself as Backup

Designated Router. This is again detected through examination

of that neighbor's Hello Packets.

o The advertised Router Priority for a bidirectional neighbor has

changed. This is again detected through examination of that

neighbor's Hello Packets.

Loop Ind

An indication has been received that the interface is now looped

back to itself. This indication can be received either from network

management or from the lower level protocols.

Unloop Ind

An indication has been received that the interface is no longer

looped back. As with the Loop Ind event, this indication can be

received either from network management or from the lower level

protocols.

Interface Down

Lower-level protocols indicate that this interface is no longer

functional. No matter what the current interface state is, the new

interface state will be Down.

9.3 The Interface state machine

A detailed description of the interface state changes follows. Each

state change is invoked by an event (Section 9.2). This event may

produce different effects, depending on the current state of the

interface. For this reason, the state machine below is organized by

current interface state and received event. Each entry in the state

machine describes the resulting new interface state and the required set

of additional actions.

When an interface's state changes, it may be necessary to originate a

new router links advertisement. See Section 12.4 for more details.

Some of the required actions below involve generating events for the

neighbor state machine. For example, when an interface becomes

inoperative, all neighbor connections associated with the interface must

be destroyed. For more information on the neighbor state machine, see

Section 10.3.

State(s): Down

Event: Interface Up

New state: Depends on action routine

Action: Start the interval Hello Timer, enabling the periodic

sending of Hello packets out the interface. If the attached

network is a physical point-to-point network or virtual

link, the interface state transitions to Point-to-Point.

Else, if the router is not eligible to become Designated

Router the interface state transitions to DR other.

Otherwise, the attached network is multi-access and the

router is eligible to become Designated Router. In this

case, in an attempt to discover the attached network's

Designated Router the interface state is set to Waiting and

the single shot Wait Timer is started. If in addition the

attached network is non-broadcast, examine the configured

list of neighbors for this interface and generate the

neighbor event Start for each neighbor that is also eligible

to become Designated Router.

State(s): Waiting

Event: Backup Seen

New state: Depends upon action routine.

Action: Calculate the attached network's Backup Designated Router

and Designated Router, as shown in Section 9.4. As a result

of this calculation, the new state of the interface will be

either DR other, Backup or DR.

State(s): Waiting

Event: Wait Timer

New state: Depends upon action routine.

Action: Calculate the attached network's Backup Designated Router

and Designated Router, as shown in Section 9.4. As a result

of this calculation, the new state of the interface will be

either DR other, Backup or DR.

State(s): DR Other, Backup or DR

Event: Neighbor Change

New state: Depends upon action routine.

Action: Recalculate the attached network's Backup Designated Router

and Designated Router, as shown in Section 9.4. As a result

of this calculation, the new state of the interface will be

either DR other, Backup or DR.

State(s): Any State

Event: Interface Down

New state: Down

Action: All interface variables are reset, and interface timers

disabled. Also, all neighbor connections associated with

the interface are destroyed. This is done by generating the

event KillNbr on all associated neighbors (see Section

10.2).

State(s): Any State

Event: Loop Ind

New state: Loopback

Action: Since this interface is no longer connected to the attached

network the actions associated with the above Interface Down

event are executed.

State(s): Loopback

Event: Unloop Ind

New state: Down

Action: No actions are necessary. For example, the interface

variables have already been reset upon entering the Loopback

state. Note that reception of an Interface Up event is

necessary before the interface again becomes fully

functional.

9.4 Electing the Designated Router

This section describes the algorithm used for calculating a network's

Designated Router and Backup Designated Router. This algorithm is

invoked by the Interface state machine. The initial time a router runs

the election algorithm for a network, the network's Designated Router

and Backup Designated Router are initialized to 0.0.0.0. This indicates

the lack of both a Designated Router and a Backup Designated Router.

The Designated Router election algorithm proceeds as follows: Call the

router doing the calculation Router X. The list of neighbors attached

to the network and having established bidirectional communication with

Router X is examined. This list is precisely the collection of Router

X's neighbors (on this network) whose state is greater than or equal to

2-Way (see Section 10.1). Router X itself is also considered to be on

the list. Discard all routers from the list that are ineligible to

become Designated Router. (Routers having Router Priority of 0 are

ineligible to become Designated Router.) The following steps are then

executed, considering only those routers that remain on the list:

(1) Note the current values for the network's Designated Router and

Backup Designated Router. This is used later for comparison

purposes.

(2) Calculate the new Backup Designated Router for the network as

follows. Only those routers on the list that have not declared

themselves to be Designated Router are eligible to become Backup

Designated Router. If one or more of these routers have declared

themselves Backup Designated Router (i.e., they are currently

listing themselves as Backup Designated Router, but not as

Designated Router, in their Hello Packets) the one having highest

Router Priority is declared to be Backup Designated Router. In case

of a tie, the one having the highest Router ID is chosen. If no

routers have declared themselves Backup Designated Router, choose

the router having highest Router Priority, (again excluding those

routers who have declared themselves Designated Router), and again

use the Router ID to break ties.

(3) Calculate the new Designated Router for the network as follows. If

one or more of the routers have declared themselves Designated

Router (i.e., they are currently listing themselves as Designated

Router in their Hello Packets) the one having highest Router

Priority is declared to be Designated Router. In case of a tie, the

one having the highest Router ID is chosen. If no routers have

declared themselves Designated Router, promote the new Backup

Designated Router to Designated Router.

(4) If Router X is now newly the Designated Router or newly the Backup

Designated Router, or is now no longer the Designated Router or no

longer the Backup Designated Router, repeat steps 2 and 3, and then

proceed to step 5. For example, if Router X is now the Designated

Router, when step 2 is repeated X will no longer be eligible for

Backup Designated Router election. Among other things, this will

ensure that no router will declare itself both Backup Designated

Router and Designated Router.[5]

(5) As a result of these calculations, the router itself may now be

Designated Router or Backup Designated Router. See Sections 7.3 and

7.4 for the additional duties this would entail. The router's

interface state should be set accordingly. If the router itself is

now Designated Router, the new interface state is DR. If the router

itself is now Backup Designated Router, the new interface state is

Backup. Otherwise, the new interface state is DR Other.

(6) If the attached network is non-broadcast, and the router itself has

just become either Designated Router or Backup Designated Router, it

must start sending hellos to those neighbors that are not eligible

to become Designated Router (see Section 9.5.1). This is done by

invoking the neighbor event Start for each neighbor having a Router

Priority of 0.

(7) If the above calculations have caused the identity of either the

Designated Router or Backup Designated Router to change, the set of

adjacencies associated with this interface will need to be modified.

Some adjacencies may need to be formed, and others may need to be

broken. To accomplish this, invoke the event AdjOK? on all

neighbors whose state is at least 2-Way. This will cause their

eligibility for adjacency to be reexamined (see Sections 10.3 and

10.4).

The reason behind the election algorithm's complexity is the desire for

an orderly transition from Backup Designated Router to Designated

Router, when the current Designated Router fails. This orderly

transition is ensured through the introduction of hysteresis: no new

Backup router can be chosen until the old Backup accepts its new

Designated Router responsibilities.

If Router X is not itself eligible to become Designated Router, it is

possible that neither a Backup Designated Router nor a Designated Router

will be selected in the above procedure. Note also that if Router X is

the only attached router that is eligible to become Designated Router,

it will select itself as Designated Router and there will be no Backup

Designated Router for the network.

9.5 Sending Hello packets

Hello packets are sent out each functioning router interface. They are

used to discover and maintain neighbor relationships.[6] On multi-access

networks, hellos are also used to elect the Designated Router and Backup

Designated Router, and in that way determine what adjacencies should be

formed.

The format of a Hello packet is detailed in Section A.3.2. The Hello

Packet contains the router's Router Priority (used in choosing the

Designated Router), and the interval between Hello broadcasts

(HelloInterval). The Hello Packet also indicates how often a neighbor

must be heard from to remain active (RouterDeadInterval). Both

HelloInterval and RouterDeadInterval must be the same for all routers

attached to a common network. The Hello packet also contains the IP

address mask of the attached network (Network Mask). On unnumbered

point-to-point networks and on virtual links this field should be set to

0.

The Hello packet's Options field describes the router's optional OSPF

capabilities. There are currently two optional capabilities defined

(see Sections 4.5 and A.2). The T-bit of the Options field should be

set if the router is capable of calculating separate routes for each IP

TOS. The E-bit should be set if and only if the attached area is

capable of processing AS external advertisements (i.e., it is not a stub

area). If the E-bit is set incorrectly the neighboring routers will

refuse to accept the Hello Packet (see Section 10.5). The rest of the

Hello Packet's Options field should be set to zero.

In order to ensure two-way communication between adjacent routers, the

Hello packet contains the list of all routers from which hellos have

been seen recently. The Hello packet also contains the router's current

choice for Designated Router and Backup Designated Router. A value of 0

in these fields means that one has not yet been selected.

On broadcast networks and physical point-to-point networks, Hello

packets are sent every HelloInterval seconds to the IP multicast address

AllSPFRouters. On virtual links, Hello packets are sent as unicasts

(addressed directly to the other end of the virtual link) every

HelloInterval seconds. On non-broadcast networks, the sending of Hello

packets is more complicated. This will be covered in the next section.

9.5.1 Sending Hello packets on non-broadcast networks

Static configuration information is necessary in order for the Hello

Protocol to function on non-broadcast networks (see Section C.5). Every

attached router which is eligible to become Designated Router has a

configured list of all of its neighbors on the network. Each listed

neighbor is labelled with its Designated Router eligibility.

The interface state must be at least Waiting for any hellos to be sent.

Hellos are then sent directly (as unicasts) to some subset of a router's

neighbors. Sometimes an hello is sent periodically on a timer; at other

times it is sent as a response to a received hello. A router's hello-

sending behavior varies depending on whether the router itself is

eligible to become Designated Router.

If the router is eligible to become Designated Router, it must

periodically send hellos to all neighbors that are also eligible. In

addition, if the router is itself the Designated Router or Backup

Designated Router, it must also send periodic hellos to all other

neighbors. This means that any two eligible routers are always

exchanging hellos, which is necessary for the correct operation of the

Designated Router election algorithm. To minimize the number of hellos

sent, the number of eligible routers on a non-broadcast network should

be kept small.

If the router is not eligible to become Designated Router, it must

periodically send hellos to both the Designated Router and the Backup

Designated Router (if they exist). It must also send an hello in reply

to an hello received from any eligible neighbor (other than the current

Designated Router and Backup Designated Router). This is needed to

establish an initial bidirectional relationship with any potential

Designated Router.

When sending Hello packets periodically to any neighbor, the interval

between hellos is determined by the neighbor's state. If the neighbor

is in state Down, hellos are sent every PollInterval seconds.

Otherwise, hellos are sent every HelloInterval seconds.

10. The Neighbor Data Structure

An OSPF router converses with its neighboring routers. Each separate

conversation is described by a "neighbor data structure". Each

conversation is bound to a particular OSPF router interface, and is

identified either by the neighboring router's OSPF router ID or by its

Neighbor IP address (see below). Thus if the OSPF router and another

router have multiple attached networks in common, multiple conversations

ensue, each described by a unique neighbor data structure. Each

separate conversation is loosely referred to in the text as being a

separate "neighbor".

The neighbor data structure contains all information pertinent to the

forming or formed adjacency between the two neighbors. (However,

remember that not all neighbors become adjacent.) An adjacency can be

viewed as a highly developed conversation between two routers.

State

The functional level of the neighbor conversation. This is

described in more detail in Section 10.1.

Inactivity Timer

A single shot timer whose firing indicates that no Hello Packet has

been seen from this neighbor recently. The length of the timer is

RouterDeadInterval seconds.

Master/Slave

When the two neighbors are exchanging databases, they form a Master

Slave relationship. The Master sends the first Database Description

Packet, and is the only part that is allowed to retransmit. The

slave can only respond to the master's Database Description Packets.

The master/slave relationship is negotiated in state ExStart.

Sequence Number

A 32-bit number identifying individual Database Description packets.

When the neighbor state ExStart is entered, the sequence number

should be set to a value not previously seen by the neighboring

router. One possible scheme is to use the machine's time of day

counter. The sequence number is then incremented by the master with

each new Database Description packet sent. The slave's sequence

number indicates the last packet received from the master. Only one

packet is allowed outstanding at a time.

Neighbor ID

The OSPF Router ID of the neighboring router. The neighbor ID is

learned when Hello packets are received from the neighbor, or is

configured if this is a virtual adjacency (see Section C.4).

Neighbor priority

The Router Priority of the neighboring router. Contained in the

neighbor's Hello packets, this item is used when selecting the

Designated Router for the attached network.

Neighbor IP address

The IP address of the neighboring router's interface to the attached

network. Used as the Destination IP address when protocol packets

are sent as unicasts along this adjacency. Also used in router

links advertisements as the Link ID for the attached network if the

neighboring router is selected to be Designated Router (see Section

12.4.1). The neighbor IP address is learned when Hello packets are

received from the neighbor. For virtual links, the neighbor IP

address is learned during the routing table build process (see

Section 15).

Neighbor Options

The optional OSPF capabilities supported by the neighbor. Learned

during the Database Exchange process (see Section 10.6). The

neighbor's optional OSPF capabilities are also listed in its Hello

packets. This enables received Hellos to be rejected (i.e.,

neighbor relationships will not even start to form) if there is a

mismatch in certain crucial OSPF capabilities (see Section 10.5).

The optional OSPF capabilities are documented in Section 4.5.

Neighbor's Designated Router

The neighbor's idea of the Designated Router. If this is the

neighbor itself, this is important in the local calculation of the

Designated Router. Defined only on multi-access networks.

Neighbor's Backup Designated Router

The neighbor's idea of the Backup Designated Router. If this is the

neighbor itself, this is important in the local calculation of the

Backup Designated Router. Defined only on multi-access networks.

The next set of variables are lists of link state advertisements. These

lists describe subsets of the area topological database. There can be

five distinct types of link state advertisements in an area topological

database: router links, network links, and type 3 and 4 summary links

(all stored in the area data structure), and AS external links (stored

in the global data structure).

Link state retransmission list

The list of link state advertisements that have been flooded but not

acknowledged on this adjacency. These will be retransmitted at

intervals until they are acknowledged, or until the adjacency is

destroyed.

Database summary list

The complete list of link state advertisements that make up the area

topological database, at the moment the neighbor goes into Database

Exchange state. This list is sent to the neighbor in Database

Description packets.

Link state request list

The list of link state advertisements that need to be received from

this neighbor in order to synchronize the two neighbors' topological

databases. This list is created as Database Description packets are

received, and is then sent to the neighbor in Link State Request

packets. The list is depleted as appropriate Link State Update

packets are received.

10.1 Neighbor states

The state of a neighbor (really, the state of a conversation being held

with a neighboring router) is documented in the following sections. The

states are listed in order of progressing functionality. For example,

the inoperative state is listed first, followed by a list of

intermediate states before the final, fully functional state is

achieved. The specification makes use of this ordering by sometimes

making references such as "those neighbors/adjacencies in state greater

than X". Figures 12 and 13 show the graph of neighbor state changes.

The arcs of the graphs are labelled with the event causing the state

change. The neighbor events are documented in Section 10.2.

The graph in Figure 12 show the state changes effected by the Hello

Protocol. The Hello Protocol is responsible for neighbor acquisition

and maintenance, and for ensuring two way communication between

neighbors.

The graph in Figure 13 shows the forming of an adjacency. Not every two

neighboring routers become adjacent (see Section 10.4). The adjacency

starts to form when the neighbor is in state ExStart. After the two

routers discover their master/slave status, the state transitions to

Exchange. At this point the neighbor starts to be used in the flooding

procedure, and the two neighboring routers begin synchronizing their

databases. When this synchronization is finished, the neighbor is in

state Full and we say that the two routers are fully adjacent. At this

point the adjacency is listed in link state advertisements.

For a more detailed description of neighbor state changes, together with

the additional actions involved in each change, see Section 10.3.

_____________________________________________________

(Figures not included in text version.)

Figure 12: Neighbor state changes (Hello Protocol)

Figure 13: Neighbor state changes (Database Exchange)

_____________________________________________________

Down

This is the initial state of a neighbor conversation. It indicates

that there has been no recent information received from the

neighbor. On non-broadcast networks, Hello packets may still be

sent to "Down" neighbors, although at a reduced frequency (see

Section 9.5.1).

Attempt

This state is only valid for neighbors attached to non-broadcast

networks. It indicates that no recent information has been received

from the neighbor, but that a more concerted effort should be made

to contact the neighbor. This is done by sending the neighbor Hello

packets at intervals of HelloInterval (see Section 9.5.1).

Init

In this state, an Hello packet has recently been seen from the

neighbor. However, bidirectional communication has not yet been

established with the neighbor (i.e., the router itself did not

appear in the neighbor's Hello packet). All neighbors in this state

(or higher) are listed in the Hello packets sent from the associated

interface.

2-Way

In this state, communication between the two routers is

bidirectional. This has been assured by the operation of the Hello

Protocol. This is the most advanced state short of beginning

adjacency establishment. The (Backup) Designated Router is selected

from the set of neighbors in state 2-Way or greater.

ExStart

This is the first step in creating an adjacency between the two

neighboring routers. The goal of this step is to decide which

router is the master, and to decide upon the initial sequence

number. Neighbor conversations in this state or greater are called

adjacencies.

Exchange

In this state the router is describing its entire link state

database by sending Database Description packets to the neighbor.

Each Database Description Packet has a sequence number, and is

explicitly acknowledged. Only one Database Description Packet is

allowed outstanding at any one time. In this state, Link State

Request Packets may also be sent asking for the neighbor's more

recent advertisements. All adjacencies in Exchange state or greater

are used by the flooding procedure. In fact, these adjacencies are

fully capable of transmitting and receiving all types of OSPF

routing protocol packets.

Loading

In this state, Link State Request packets are sent to the neighbor

asking for the more recent advertisements that have been discovered

(but not yet received) in the Exchange state.

Full

In this state, the neighboring routers are fully adjacent. These

adjacencies will now appear in router links and network links

advertisements.

10.2 Events causing neighbor state changes

State changes can be effected by a number of events. These events are

shown in the labels of the arcs in Figures 12 and 13. The label

definitions are as follows:

Hello Received

A Hello packet has been received from a neighbor.

Start

This is an indication that Hello Packets should now be sent to the

neighbor at intervals of HelloInterval seconds. This event is

generated only for neighbors associated with non-broadcast networks.

2-Way Received

Bidirectional communication has been realized between the two

neighboring routers. This is indicated by this router seeing itself

in the other's Hello packet.

NegotiationDone

The Master/Slave relationship has been negotiated, and sequence

numbers have been exchanged. This signals the start of the

sending/receiving of Database Description packets. For more

information on the generation of this event, consult Section 10.8.

Exchange Done

Both routers have successfully transmitted a full sequence of

Database Description packets. Each router now knows what parts of

its link state database are out of date. For more information on

the generation of this event, consult Section 10.8.

BadLSReq

A Link State Request has been received for a link state

advertisement not contained in the database. This indicates an

error in the synchronization process.

Loading Done

Link State Updates have been received for all out-of-date portions

of the database. This is indicated by the Link state request list

becoming empty after the Database Description Process has completed.

AdjOK?

A decision must be made (again) as to whether an adjacency should be

established/maintained with the neighbor. This event will start

some adjacencies forming, and destroy others.

The following events cause well developed neighbors to revert to lesser

states. Unlike the above events, these events may occur when the

neighbor conversation is in any of a number of states.

Seq Number Mismatch

A Database Description packet has been received that either a) has

an unexpected sequence number, b) unexpectedly has the Init bit set

or c) has an Options field differing from the last Options field

received in a Database Description packet. Any of these conditions

indicate that some error has occurred during adjacency

establishment.

1-Way

An Hello packet has been received from the neighbor, in which this

router is not mentioned. This indicates that communication with the

neighbor is not bidirectional.

KillNbr

This is an indication that all communication with the

neighbor is now impossible, forcing the neighbor to revert

to Down state.

Inactivity Timer

The inactivity Timer has fired. This means that no Hello packets

have been seen recently from the neighbor. The neighbor reverts to

Down state.

LLDown

This is an indication from the lower level protocols that the

neighbor is now unreachable. For example, on an X.25 network this

could be indicated by an X.25 clear indication with appropriate

cause and diagnostic fields. This event forces the neighbor into

Down state.

10.3 The Neighbor state machine

A detailed description of the neighbor state changes follows. Each

state change is invoked by an event (Section 10.2). This event may

produce different effects, depending on the current state of the

neighbor. For this reason, the state machine below is organized by

current neighbor state and received event. Each entry in the state

machine describes the resulting new neighbor state and the required set

of additional actions.

When an neighbor's state changes, it may be necessary to rerun the

Designated Router election algorithm. This is determined by whether the

interface Neighbor Change event is generated (see Section 9.2). Also,

if the Interface is in DR state (the router is itself Designated

Router), changes in neighbor state may cause a new network links

advertisement to be originated (see Section 12.4).

When the neighbor state machine needs to invoke the interface state

machine, it should be done as a scheduled task (see Section 4.4). This

simplifies things, by ensuring that neither state machine will be

executed recursively.

State(s): Down

Event: Start

New state: Attempt

Action: Send an hello to the neighbor (this neighbor is always

associated with a non-broadcast network) and start the

inactivity timer for the neighbor. The timer's later firing

would indicate that communication with the neighbor was not

attained.

State(s): Attempt

Event: Hello Received

New state: Init

Action: Restart the inactivity timer for the neighbor, since the

neighbor has now been heard from.

State(s): Down

Event: Hello Received

New state: Init

Action: Start the inactivity timer for the neighbor. The timer's

later firing would indicate that the neighbor is dead.

State(s): Init or greater

Event: Hello Received

New state: No state change.

Action: Restart the inactivity timer for the neighbor, since the

neighbor has again been heard from.

State(s): Init

Event: 2-Way Received

New state: Depends upon action routine.

Action: Determine whether an adjacency should be established with

the neighbor (see Section 10.4). If not, the new neighbor

state is 2-Way.

Otherwise (an adjacency should be established) the neighbor

state transitions to ExStart. Upon entering this state, the

router increments the sequence number for this neighbor. If

this is the first time that an adjacency has been attempted,

the sequence number should be assigned some unique value

(like the time of day clock). It then declares itself

master (sets the master/slave bit to master), and starts

sending Database Description Packets, with the initialize

(I), more (M) and master (MS) bits set. This Database

Description Packet should be otherwise empty. This Database

Description Packet should be retransmitted at intervals of

RxmtInterval until the next state is entered (see Section

10.8).

State(s): ExStart

Event: NegDone

New state: Exchange

Action: The router must list the contents of its entire area link

state database in the neighbor Database summary list. The

area link state database consists of the router links,

network links and summary links contained in the area

structure, along with the AS external links contained in the

global structure. AS external link advertisements are

omitted from a virtual neighbor's Database summary list. AS

external advertisements are omitted from the Database

summary list if the area has been configured as a stub (see

Section 3.6). Advertisements whose age is equal to MaxAge

are instead added to the neighbor's Link state

retransmission list. A summary of the Database summary list

will be sent to the neighbor in Database Description

packets. Each Database Description Packet has a sequence

number, and is explicitly acknowledged. Only one Database

Description Packet is allowed outstanding at any one time.

For more detail on the sending and receiving of Database

Description packets, see Sections 10.8 and 10.6.

State(s): Exchange

Event: Exchange Done

New state: Depends upon action routine.

Action: If the neighbor Link state request list is empty, the new

neighbor state is Full. No other action is required. This

is an adjacency's final state.

Otherwise, the new neighbor state is Loading. Start (or

continue) sending Link State Request packets to the neighbor

(see Section 10.9). These are requests for the neighbor's

more recent advertisements (which were discovered but not

yet received in the Exchange state). These advertisements

are listed in the Link state request list associated with

the neighbor.

State(s): Loading

Event: Loading Done

New state: Full

Action: No action required. This is an adjacency's final state.

State(s): 2-Way

Event: AdjOK?

New state: Depends upon action routine.

Action: Determine whether an adjacency should be formed with the

neighboring router (see Section 10.4). If not, the neighbor

state remains at 2-Way. Otherwise, transition the neighbor

state to ExStart and perform the actions associated with the

above state machine entry for state Init and event 2-Way

Received.

State(s): ExStart or greater

Event: AdjOK?

New state: Depends upon action routine.

Action: Determine whether the neighboring router should still be

adjacent. If yes, there is no state change and no further

action is necessary.

Otherwise, the (possibly partially formed) adjacency must be

destroyed. The neighbor state transitions to 2-Way. The

Link state retransmission list, Database summary list and

Link state request list are cleared of link state

advertisements.

State(s): Exchange or greater

Event: Seq Number Mismatch

New state: ExStart

Action: The (possibly partially formed) adjacency is torn down, and

then an attempt is made at reestablishment. The neighbor

state first transitions to ExStart. The Link state

retransmission list, Database summary list and Link state

request list are cleared of link state advertisements. Then

the router increments the sequence number for this neighbor,

declares itself master (sets the master/slave bit to

master), and starts sending Database Description Packets,

with the initialize (I), more (M) and master (MS) bits set.

This Database Description Packet should be otherwise empty

(see Section 10.8).

State(s): Exchange or greater

Event: BadLSReq

New state: ExStart

Action: The action for event BadLSReq is exactly the same as for the

neighbor event SeqNumberMismatch. The (possibly partially

formed) adjacency is torn down, and then an attempt is made

at reestablishment. For more information, see the neighbor

state machine entry that is invoked when event

SeqNumberMismatch is generated in state Exchange or greater.

State(s): Any state

Event: KillNbr

New state: Down

Action: The Link state retransmission list, Database summary list

and Link state request list are cleared of link state

advertisements. Also, the inactivity timer is disabled.

State(s): Any state

Event: LLDown

New state: Down

Action: The Link state retransmission list, Database summary list

and Link state request list are cleared of link state

advertisements. Also, the inactivity timer is disabled.

State(s): Any state

Event: Inactivity Timer

New state: Down

Action: The Link state retransmission list, Database summary list

and Link state request list are cleared of link state

advertisements.

State(s): 2-Way or greater

Event: 1-Way Received

New state: Init

Action: The Link state retransmission list, Database summary list

and Link state request list are cleared of link state

advertisements.

State(s): 2-Way or greater

Event: 2-Way received

New state: No state change.

Action: No action required.

State(s): Init

Event: 1-Way received

New state: No state change.

Action: No action required.

10.4 Whether to become adjacent

Adjacencies are established with some subset of the router's neighbors.

Routers connected by point-to-point networks and virtual links always

become adjacent. On multi-access networks, all routers become adjacent

to both the Designated Router and the Backup Designated Router.

The adjacency-forming decision occurs in two places in the neighbor

state machine. First, when bidirectional communication is initially

established with the neighbor, and secondly, when the identity of the

attached network's (Backup) Designated Router changes. If the decision

is made to not attempt an adjacency, the state of the neighbor

communication stops at 2-Way.

An adjacency should be established with a (bidirectional) neighbor when

at least one of the following conditions holds:

o The underlying network type is point-to-point

o The underlying network type is virtual link

o The router itself is the Designated Router

o The router itself is the Backup Designated Router

o The neighboring router is the Designated Router

o The neighboring router is the Backup Designated Router

10.5 Receiving Hello packets

This section explains the detailed processing of a received Hello

packet. (See Section A.3.2 for the format of Hello packets.) The

generic input processing of OSPF packets will have checked the validity

of the IP header and the OSPF packet header. Next, the values of the

Network Mask, HelloInt, and DeadInt fields in the received Hello packet

must be checked against the values configured for the receiving

interface. Any mismatch causes processing to stop and the packet to be

dropped. In other words, the above fields are really describing the

attached network's configuration. Note that the value of the Network

Mask field should not be checked in Hellos received on unnumbered serial

lines or on virtual links.

The receiving interface attaches to a single OSPF area (this could be

the backbone). The setting of the E-bit found in the Hello Packet's

option field must match this area's external routing capability. If AS

external advertisements are not flooded into/throughout the area (i.e,

the area is a "stub") the E-bit must be clear in received hellos,

otherwise the E-bit must be set. A mismatch causes processing to stop

and the packet to be dropped. The setting of the rest of the bits in

the Hello Packet's option field should be ignored.

At this point, an attempt is made to match the source of the Hello

Packet to one of the receiving interface's neighbors. If the receiving

interface is a multi-access network (either broadcast or non-broadcast)

the source is identified by the IP source address found in the Hello's

IP header. If the receiving interface is a point-to-point link or a

virtual link, the source is identified by the Router ID found in the

Hello's OSPF packet header. The interface's current list of neighbors

is contained in the interface's data structure. If a matching neighbor

structure cannot be found, (i.e., this is the first time the neighbor

has been detected), one is created. The initial state of a newly

created neighbor is set to Down.

When receiving an Hello Packet from a neighbor on a multi-access network

(broadcast or non-broadcast), set the neighbor structure's Neighbor ID

equal to the Router ID found in the packet's OSPF header. When

receiving an Hello on a point-to-point network (but not on a virtual

link) set the neighbor structure's Neighbor IP address to the packet's

IP source address.

Now the rest of the Hello Packet is examined, generating events to be

given to the neighbor and interface state machines. These state

machines are specified either to be executed or scheduled (see Section

4.4). For example, by specifying below that the neighbor state machine

be executed in line, several neighbor state transitions may be effected

by a single received Hello:

o Each Hello Packet causes the neighbor state machine to be executed

with the event Hello Received.

o Then the list of neighbors contained in the Hello Packet is

examined. If the router itself appears in this list, the neighbor

state machine should be executed with the event 2-Way Received.

Otherwise, the neighbor state machine should be executed with the

event 1-Way Received, and the processing of the packet stops.

o Next, the Hello packet's Router Priority field is examined. If this

field is different than the one previously received from the

neighbor, the receiving interface's state machine is scheduled with

the event NeighborChange. In any case, the Router Priority field in

the neighbor data structure should be set accordingly.

o Next the Designated Router field in the Hello Packet is examined.

If the neighbor is both declaring itself to be Designated Router

(Designated Router field = neighbor IP address) and the Backup

Designated Router field in the packet is equal to 0.0.0.0 and the

receiving interface is in state Waiting, the receiving interface's

state machine is scheduled with the event BackupSeen. Otherwise, if

the neighbor is declaring itself to be Designated Router and it had

not previously, or the neighbor is not declaring itself Designated

Router where it had previously, the receiving interface's state

machine is scheduled with the event NeighborChange. In any case,

the Designated Router item in the neighbor structure is set

accordingly.

o Finally, the Backup Designated Router field in the Hello Packet is

examined. If the neighbor is declaring itself to be Backup

Designated Router (Backup Designated Router field = neighbor IP

address) and the receiving interface is in state Waiting, the

receiving interface's state machine is scheduled with the event

BackupSeen. Otherwise, if the neighbor is declaring itself to be

Backup Designated Router and it had not previously, or the neighbor

is not declaring itself Backup Designated Router where it had

previously, the receiving interface's state machine is scheduled

with the event NeighborChange. In any case, the Backup Designated

Router item in the neighbor structure is set accordingly.

10.6 Receiving Database Description Packets

This section explains the detailed processing of a received Database

Description packet. The incoming Database Description Packet has

already been associated with a neighbor and receiving interface by the

generic input packet processing (Section 8.2). The further processing

of the Database Description Packet depends on the neighbor state. If

the neighbor's state is Down or Attempt the packet should be ignored.

Otherwise, if the state is:

Init

The neighbor state machine should be executed with the event 2-Way

Received. This causes an immediate state change to either state 2-

Way or state Exstart. The processing of the current packet should

then continue in this new state.

2-Way

The packet should be ignored. Database description packets are used

only for the purpose of bringing up adjacencies.[7]

ExStart

If the received packet matches one of the following cases, then the

neighbor state machine should be executed with the event

NegotiationDone (causing the state to transition to Exchange), the

packet's Options field should be recorded in the neighbor

structure's Neighbor Options field and the packet should be accepted

as next in sequence and processed further (see below). Otherwise,

the packet should be ignored.

o The initialize(I), more (M) and master(MS) bits are set, the

contents of the packet are empty, and the neighbor's Router ID

is larger than the router's own. In this case the router is now

Slave. Set the master/slave bit to slave, and set the sequence

number to that specified by the master.

o The initialize(I) and master(MS) bits are off, the packet's

sequence number equals the router's own sequence number

(indicating acknowledgment) and the neighbor's Router ID is

smaller than the router's own. In this case the router is

Master.

Exchange

If the state of the MS-bit is inconsistent with the master/slave

state of the connection, generate the neighbor event Seq Number

Mismatch and stop processing the packet. Otherwise:

o If the initialize(I) bit is set, generate the neighbor event Seq

Number Mismatch and stop processing the packet.

o If the packet's Options field indicates a different set of

optional OSPF capabilities than were previously received from

the neighbor (recorded in the Neighbor Options field of the

neighbor structure), generate the neighbor event Seq Number

Mismatch and stop processing the packet.

o If the router is master, and the packet's sequence number equals

the router's own sequence number (this packet is the next in

sequence) the packet should be accepted and its contents

processed (below).

o If the router is master, and the packet's sequence number is one

less than the router's sequence number, the packet is a

duplicate. Duplicates should be discarded by the master.

o If the router is slave, and the packet's sequence number is one

more than the router's own sequence number (this packet is the

next in sequence) the packet should be accepted and its contents

processed (below).

o If the router is slave, and the packet's sequence number is

equal to the router's sequence number, the packet is a

duplicate. The slave must respond to duplicates by repeating

the last Database Description packet that it sent.

o Else, generate the neighbor event Seq Number Mismatch and stop

processing the packet.

Loading or Full

In this state, the router has sent and received an entire sequence

of Database Descriptions. The only packets received should be

duplicates (see above). In particular, the packet's Options field

should match the set of optional OSPF capabilities previously

indicated by the neighbor (stored in the neighbor structure's

neighbor Options field). Any other packets received, including the

reception of a packet with the Initialize(I) bit set, should

generate the neighbor event Seq Number Mismatch.[8] Duplicates

should be discarded by the master. The slave must respond to

duplicates by repeating the last Database Description packet that it

sent.

When the router accepts a received Database Description Packet as the

next in sequence the packet contents are processed as follows. For each

link state advertisement listed, the advertisement's LS type is checked

for validity. If the LS type is unknown (e.g., not one of the LS types

1-5 defined by this specification), or if this is a AS external

advertisement (LS type = 5) and the neighbor is associated with a stub

area, generate the neighbor event Seq Number Mismatch and stop

processing the packet. Otherwise, the router looks up the advertisement

in its database to see whether it also has an instance of the link state

advertisement. If it does not, or if the database copy is less recent

(see Section 13.1), the link state advertisement is put on the Link

state request list so that it can be requested (immediately or at some

later time) in Link State Request Packets.

When the router accepts a received Database Description Packet as the

next in sequence, it also performs the following actions, depending on

whether it is master or slave:

Master

Increments the sequence number. If the router has already sent its

entire sequence of Database Descriptions, and the just accepted

packet has the more bit (M) set to 0, the neighbor event Exchange

Done is generated. Otherwise, it should send a new Database

Description to the slave.

Slave

Sets the sequence number to the sequence number appearing in the

received packet. The slave must send a Database Description in

reply. If the received packet has the more bit (M) set to 0, and

the packet to be sent by the slave will have the M-bit set to 0

also, the neighbor event Exchange Done is generated. Note that the

slave always generates this event before the master.

10.7 Receiving Link State Request Packets

This section explains the detailed processing of received Link State

Request packets. Received Link State Request Packets specify a list of

link state advertisements that the neighbor wishes to receive. Link

state Request Packets should be accepted when the neighbor is in states

Exchange, Loading, or Full. In all other states Link State Request

Packets should be ignored.

Each link state advertisement specified in the Link State Request packet

should be located in the router's database, and copied into Link State

Update packets for transmission to the neighbor. These link state

advertisements should NOT be placed on the Link state retransmission

list for the neighbor. If a link state advertisement cannot be found in

the database, something has gone wrong with the synchronization

procedure, and neighbor event BadLSReq should be generated.

10.8 Sending Database Description Packets

This section describes how Database Description Packets are sent to a

neighbor. The router's optional OSPF capabilities (see Section 4.5) are

transmitted to the neighbor in the Options field of the Database

Description packet. The router should maintain the same set of optional

capabilities throughout the Database Exchange and flooding procedures.

If for some reason the router's optional capabilities change, the

Database Exchange procedure should be restarted by reverting to neighbor

state ExStart. There are currently two optional capabilities defined.

The T-bit should be set if and only if the router is capable of

calculating separate routes for each IP TOS. The E-bit should be set if

and only if the attached network belongs to a non-stub area. The rest

of the Options field should be set to zero.

The sending of Database Description packets depends on the neighbor's

state. In state ExStart the router sends empty Database Description

packets, with the initialize (I), more (M) and master (MS) bits set.

These packets are retransmitted every RxmtInterval seconds.

In state Exchange the Database Description Packets actually contain

summaries of the link state information contained in the router's

database. Each link state advertisement in the area's topological

database (at the time the neighbor transitions into Exchange state) is

listed in the neighbor Database summary list. When a new Database

Description Packet is to be sent, the packet's sequence number is

incremented, and the (new) top of the Database summary list is described

by the packet. Items are removed from the Database summary list when

the previous packet is acknowledged.

In state Exchange, the determination of when to send a packet depends on

whether the router is master or slave:

Master

Packets are sent when either a) the slave acknowledges the previous

packet by echoing the sequence number or b) RxmtInterval seconds

elapse without an acknowledgment, in which case the previous packet

is retransmitted.

Slave

Packets are sent only in response to packets received from the

master. If the packet received from the master is new, a new packet

is sent, otherwise the previous packet is resent.

In states Loading and Full the slave must resend its last packet in

response to duplicate packets received from the master. For this reason

the slave must wait RouterDeadInterval seconds before freeing the last

packet. Reception of a packet from the master after this interval will

generate a Seq Number Mismatch neighbor event.

10.9 Sending Link State Request Packets

In neighbor states Exchange or Loading, the Link state request list

contains a list of those link state advertisements that need to be

obtained from the neighbor. To request these advertisements, a router

sends the neighbor the beginning of the Link state request list,

packaged in a Link State Request packet.

When the neighbor responds to these requests with the proper Link State

Update packet(s), the Link state request list is truncated and a new

Link State Request packet is sent. This process continues until the

link state request list becomes empty. Unsatisfied Link State Requests

are retransmitted at intervals of RxmtInterval. There should be at most

one Link State Request packet outstanding at any one time.

When the Link state request list becomes empty, and the neighbor state

is Loading (i.e., a complete sequence of Database Description packets

has been received from the neighbor), the Loading Done neighbor event is

generated.

10.10 An Example

Figure 14 shows an example of an adjacency forming. Routers RT1 and RT2

are both connected to a broadcast network. It is assumed that RT2 is

the Designated Router for the network, and that RT2 has a higher Router

ID that router RT1.

The neighbor state changes realized by each router are listed on the

sides of the figure.

At the beginning of Figure 14, router RT1's interface to the network

becomes operational. It begins sending hellos, although it doesn't know

the identity of the Designated Router or of any other neighboring

routers. Router RT2 hears this hello (moving the neighbor to Init

state), and in its next hello indicates that it is itself the Designated

Router and that it has heard hellos from RT1. This in turn causes RT1

to go to state ExStart, as it starts to bring up the adjacency.

RT1 begins by asserting itself as the master. When it sees that RT2 is

indeed the master (because of RT2's higher Router ID), RT1 transitions

to slave state and adopts its neighbor's sequence number. Database

Description packets are then exchanged, with polls coming from the

master (RT2) and responses from the slave (RT1). This sequence of

Database Description Packets ends when both the poll and associated

response has the M-bit off.

In this example, it is assumed that RT2 has a completely up to date

database. In that case, RT2 goes immediately into Full state. RT1 will

go into Full state after updating the necessary parts of its database.

This is done by sending Link State Request Packets, and receiving Link

State Update Packets in response. Note that, while RT1 has waited until

a complete set of Database Description Packets has been received (from

RT2) before sending any Link State Request Packets, this need not be the

case. RT1 could have interleaved the sending of Link State Request

Packets with the reception of Database Description Packets.

11. The Routing Table Structure

The routing table data structure contains all the information necessary

to forward an IP data packet toward its destination. Each routing table

entry describes the collection of best paths to a particular

destination. When forwarding an IP data packet, the routing table entry

providing the best match for the packet's IP destination is located.

________________________________________

(Figure not included in text version.)

Figure 14: An adjacency bring-up example

________________________________________

The matching routing table entry then provides the next hop towards the

packet's destination. OSPF also provides for the existence of a default

route (Destination ID = DefaultDestination). When the default route

exists, it matches all IP destinations (although any other matching

entry is a better match). Finding the routing table entry that best

matches an IP destination is further described in Section 11.1.

There is a single routing table in each router. Two sample routing

tables are described in Sections 11.2 and 11.3. The building of the

routing table is discussed in Section 16.

The rest of this section defines the fields found in a routing table

entry. The first set of fields describes the routing table entry's

destination.

Destination Type

The destination can be one of three types. Only the first type,

Network, is actually used when forwarding IP data traffic. The

other destinations are used solely as intermediate steps in the

routing table build process.

Network

A range of IP addresses, to which IP data traffic may be

forwarded. This includes IP networks (class A, B, or C), IP

subnets, and single IP hosts. The default route also falls in

this category.

Area border router

Routers that are connected to multiple OSPF areas. Such routers

originate summary link advertisements. These routing table

entries are used when calculating the inter-area routes (see

Section 16.2). These routing table entries may also be

associated with configured virtual links.

AS boundary router

Routers that originate AS external link advertisements. These

routing table entries are used when calculating the AS external

routes (see Section 16.4).

Destination ID

The destination's identifier or name. This depends on the

destination's type. For networks, the identifier is their

associated IP address. For all other types, the identifier is the

OSPF Router ID.[9]

Address Mask

Only defined for networks. The network's IP address together with

its address mask defines a range of IP addresses. For IP subnets,

the address mask is referred to as the subnet mask. For host

routes, the mask is "all ones" (0xffffffff).

Optional Capabilities

When the destination is a router (either an area border router or an

AS boundary router) this field indicates the optional OSPF

capabilities supported by the destination router. The two optional

capabilities currently defined by this specification are the ability

to route based on IP TOS and the ability to process AS external

advertisements. For a further discussion of OSPF's optional

capabilities, see Section 4.5.

The set of paths to use for a destination may vary based on IP Type of

Service and the OSPF area to which the paths belong. This means that

there may be multiple routing table entries for the same destination,

depending on the values of the next two fields.

Type of Service

There can be a separate set of routes for each IP Type of Service.

The encoding of TOS in OSPF link state advertisements is described

in Section 12.3.

Area

This field indicates the area whose link state information has led

to the routing table entry's collection of paths. This is called

the entry's associated area. For sets of AS external paths, this

field is not defined. For destinations of type "area border

router", there may be separate sets of paths (and therefore separate

routing table entries) associated with each of several areas. This

will happen when two area border routers share multiple areas in

common. For all other destination types, only the set of paths

associated with the best area (the one providing the shortest route)

is kept.

The rest of the routing table entry describes the set of paths to the

destination. The following fields pertain to the set of paths as a

whole. In other words, each one of the paths contained in a routing

table entry is of the same path-type and cost (see below).

Path-type

There are four possible types of paths used to route traffic to the

destination, listed here in order of preference: intra-area, inter-

area, type 1 external or type 2 external. Intra-area paths indicate

destinations belonging to one of the router's attached areas.

Inter-area paths are paths to destinations in other OSPF areas.

These are discovered through the examination of received summary

link advertisements. AS external paths are paths to destinations

external to the AS. These are detected through the examination of

received AS external link advertisements.

Cost

The link state cost of the path to the destination. For all paths

except type 2 external paths this describes the entire path's cost.

For Type 2 external paths, this field describes the cost of the

portion of the path internal to the AS. This cost is calculated as

the sum of the costs of the path's constituent links.

Type 2 cost

Only valid for type 2 external paths. For these paths, this field

indicates the cost of the path's external portion. This cost has

been advertised by an AS boundary router, and is the most

significant part of the total path cost. For example, an external

type 2 path with type 2 cost of 5 is always preferred over a path

with type 2 cost of 10, regardless of the cost of the two paths'

internal components.

Link State Origin

Valid only for intra-area paths, this field indicates the link state

advertisement (router links or network links) that directly

references the destination. For example, if the destination is a

transit network, this is the transit network's network links

advertisement. If the destination is a stub network, this is the

router links advertisement for the attached router. The

advertisement is discovered during the shortest-path tree

calculation (see Section 16.1). Multiple advertisements may

reference the destination, however a tie-breaking scheme always

reduces the choice to a single advertisement.

This field is for informational purposes only. The advertisement

could be used as a root for an SPF calculation when building a

reverse path forwarding tree. This is beyond the scope of this

specification.

When multiple paths of equal path-type and cost exist to a destination

(called elsewhere "equal-cost" paths), they are stored in a single

routing table entry. Each one of the "equal-cost" paths is

distinguished by the following fields:

Next hop

The outgoing router interface to use when forwarding traffic to the

destination. On multi-access networks, the next hop also includes

the IP address of the next router (if any) in the path towards the

destination. This next router will always be one of the adjacent

neighbors.

Advertising router

Valid only for inter-area and AS external paths. This field

indicates the Router ID of the router advertising the summary link

or AS external link that led to this path.

11.1 Routing table lookup

When an IP data packet is received, an OSPF router finds the routing

table entry that best matches the packet's destination. This routing

table entry then provides the outgoing interface and next hop router to

use in forwarding the packet. This section describes the process of

finding the best matching routing table entry. The process consists of a

number of steps, wherein the collection of routing table entries is

progressively pruned. In the end, the single routing table entry

remaining is the called best match.

Note that the steps described below may fail to produce a best match

routing table entry (i.e., all existing routing table entries are pruned

for some reason or another). In this case, the packet's IP destination

is considered unreachable. Instead of being forwarded, the packet should

be dropped and an ICMP destination unreachable message should be

returned to the packet's source.

(1) Select the complete set of "matching" routing table entries from the

routing table. Each routing table entry describes a (set of)

path(s) to a range of IP addresses. If the data packet's IP

destination falls into an entry's range of IP addresses, the routing

table entry is called a match. (It is quite likely that multiple

entries will match the data packet. For example, a default route

will match all packets.)

(2) Suppose that the packet's IP destination falls into one of the

router's configured area address ranges (see Section 3.5), and that

the particular area address range is active. This means that there

are one or more reachable (by intra-area paths) networks contained

in the area address range. The packet's IP destination is then

required to belong to one of these constituent networks. For this

reason, only matching routing table entries with path-type of

intra-area are considered (all others are pruned). If no such

matching entries exist, the destination is unreachable (see above).

Otherwise, skip to step 4.

(3) Reduce the set of matching entries to those having the most

preferential path-type (see Section 11). OSPF has a four level

hierarchy of paths. Intra-area paths are the most preferred,

followed in order by inter-area, Type 1 external and Type 2 external

paths.

(4) Select the remaining routing table entry that provides the longest

(most specific) match. Another way of saying this is to choose the

remaining entry that specifies the narrowest range of IP

addresses.[10] For example, the entry for the address/mask pair of

(128.185.1.0, 0xffffff00) is more specific than an entry for the

pair (128.185.0.0, 0xffff0000). The default route is the least

specific match, since it matches all destinations.

(5) At this point, there may still be multiple routing table entries

remaining. Each routing entry will specify the same range of IP

addresses, but a different IP Type of Service. Select the routing

table entry whose TOS value matches the TOS found in the packet

header. If there is no routing table entry for this TOS, select the

routing table entry for TOS 0. In other words, packets requesting

TOS X are routed along the TOS 0 path if a TOS X path does not

exist.

11.2 Sample routing table, without areas

Consider the Autonomous System pictured in Figure 2. No OSPF areas have

been configured. A single metric is shown per outbound interface,

indicating that routes will not vary based on TOS. The calculation

router RT6's routing table proceeds as described in Section 2.1. The

resulting routing table is shown in Table 12. Destination types are

abbreviated: Network as "N", area border router as "BR" and AS boundary

router as "ASBR".

There are no instances of multiple equal-cost shortest paths in this

example. Also, since there are no areas, there are no inter-area paths.

Routers RT5 and RT7 are AS boundary routers. Intra-area routes have

been calculated to routers RT5 and RT7. This allows external routes to

be calculated to the destinations advertised by RT5 and RT7 (i.e.,

networks N12, N13, N14 and N15). It is assumed all AS external

advertisements originated by RT5 and RT7 are advertising type 1 external

metrics. This results in type 1 external paths being calculated to

destinations N12-N15.

11.3 Sample routing table, with areas

Consider the previous example, this time split into OSPF areas. An OSPF

area configuration is pictured in Figure 6. Router RT4's routing table

will be described for this area configuration. Router RT4 has a

connection to Area 1 and a backbone connection. This causes Router RT4

to view the AS as the concatenation of the two graphs shown in Figures 7

and 8. The resulting routing table is displayed in Table 13.

Again, routers RT5 and RT7 are AS boundary routers. Routers RT3, RT4,

RT7, RT10 and RT11 are area border routers. Note that there are two

routing entries (in this case having identical paths) for router RT7, in

its dual capacities as an area border router and an AS boundary router.

Note also that there are two routing entries for the area border router

RT3, since it has two areas in common with RT4 (Area 1 and the

backbone).

Backbone paths have been calculated to all area border routers (BR).

These are used when determining the inter-area routes. Note that all of

Type Dest Area Path Type Cost Next Hop(s) Adv. Router(s)

__________________________________________________________________________

N N1 0 intra-area 10 RT3 *

N N2 0 intra-area 10 RT3 *

N N3 0 intra-area 7 RT3 *

N N4 0 intra-area 8 RT3 *

N Ib 0 intra-area 7 * *

N Ia 0 intra-area 12 RT10 *

N N6 0 intra-area 8 RT10 *

N N7 0 intra-area 12 RT10 *

N N8 0 intra-area 10 RT10 *

N N9 0 intra-area 11 RT10 *

N N10 0 intra-area 13 RT10 *

N N11 0 intra-area 14 RT10 *

N H1 0 intra-area 21 RT10 *

ASBR RT5 0 intra-area 6 RT5 *

ASBR RT7 0 intra-area 8 RT10 *

__________________________________________________________________________

N N12 * type 1 external 10 RT10 RT7

N N13 * type 1 external 14 RT5 RT5

N N14 * type 1 external 14 RT5 RT5

N N15 * type 1 external 17 RT10 RT7

Table 12: The routing table for Router RT6 (no configured areas).

the inter-area routes are associated with the backbone; this is always

the case when the router is itself an area border router. Routing

information is condensed at area boundaries. In this example, we assume

that Area 3 has been defined so that networks N9-N11 and the host route

to H1 are all condensed to a single route when advertised to the

backbone (by router RT11). Note that the cost of this route is the

minimum of the set of costs to its individual components.

There is a virtual link configured between routers RT10 and RT11.

Without this configured virtual link, RT11 would be unable to advertise

a route for networks N9-N11 and host H1 into the backbone, and there

would not be an entry for these networks in router RT4's routing table.

In this example there are two equal-cost paths to network N12. However,

they both use the same next hop (Router RT5).

Router RT4's routing table would improve (i.e., some of the paths in the

routing table would become shorter) if an additional virtual link were

configured between router RT4 and router RT3. The new virtual link

would itself be associated with the first entry for area border router

RT3 in Table 13 (an intra-area path through Area 1). This would yield a

cost of 1 for the virtual link. The routing table entries changes that

would be caused by the addition of this virtual link are shown in Table

14.

12. Link State Advertisements

Each router in the Autonomous System originates one or more link state

advertisements. There are five distinct types of link state

advertisements, which are described in Section 4.3. The collection of

link state advertisements forms the link state or topological database.

Each separate type of advertisement has a separate function. Router

links and network links advertisements describe how an area's routers

and networks are interconnected. Summary link advertisements provide a

way of condensing an area's routing information. AS external

advertisements provide a way of transparently advertising externally-

derived routing information throughout the Autonomous System.

Each link state advertisement begins with a standard 20-byte header.

This link state header is discussed below.

Type Dest Area Path Type Cost Next Hop(s) Adv. Router(s)

_______________________________________________________________________________

N N1 1 intra-area 4 RT1 *

N N2 1 intra-area 4 RT2 *

N N3 1 intra-area 1 * *

N N4 1 intra-area 3 RT3 *

BR RT3 1 intra-area 1 * *

_______________________________________________________________________________

N Ib 0 intra-area 22 RT5 *

N Ia 0 intra-area 27 RT5 *

BR RT3 0 intra-area 21 RT5 *

BR RT7 0 intra-area 14 RT5 *

BR RT10 0 intra-area 22 RT5 *

BR RT11 0 intra-area 25 RT5 *

ASBR RT5 0 intra-area 8 * *

ASBR RT7 0 intra-area 14 RT5 *

_______________________________________________________________________________

N N6 0 inter-area 15 RT5 RT7

N N7 0 inter-area 19 RT5 RT7

N N8 0 inter-area 18 RT5 RT7

N N9-N11,H1 0 inter-area 26 RT5 RT11

_______________________________________________________________________________

N N12 * type 1 external 16 RT5 RT5,RT7

N N13 * type 1 external 16 RT5 RT5

N N14 * type 1 external 16 RT5 RT5

N N15 * type 1 external 23 RT5 RT7

Table 13: Router RT4's routing table in the presence of areas.

Type Dest Area Path Type Cost Next Hop(s) Adv. Router(s)

__________________________________________________________________________

N Ib 0 intra-area 16 RT3 *

N Ia 0 intra-area 21 RT3 *

BR RT3 0 intra-area 1 * *

BR RT10 0 intra-area 16 RT3 *

BR RT11 0 intra-area 19 RT3 *

__________________________________________________________________________

N N9-N11,H1 0 inter-area 20 RT3 RT11

Table 14: Changes resulting from an additional virtual link.

12.1 The Link State Header

The link state header contains the LS type, Link State ID and

Advertising Router fields. The combination of these three fields

uniquely identifies the link state advertisement.

There may be several instances of an advertisement present in the

Autonomous System, all at the same time. It must then be determined

which instance is more recent. This determination is made be examining

the LS sequence, LS checksum and LS age fields. These fields are also

contained in the 20-byte link state header.

Several of the OSPF packet types list link state advertisements. When

the instance is not important, an advertisement is referred to by its LS

type, Link State ID and Advertising Router (see Link State Request

Packets). Otherwise, the LS sequence number, LS age and LS checksum

fields must also be referenced.

A detailed explanation of the fields contained in the link state header

follows.

12.1.1 LS age

This field is the age of the link state advertisement in seconds. It

should be processed as an unsigned 16-bit integer. It is set to 0 when

the link state advertisement is originated. It must be incremented by

InfTransDelay on every hop of the flooding procedure. Link state

advertisements are also aged as they are held in each router's database.

The age of a link state advertisement is never incremented past MaxAge.

Advertisements having age MaxAge are not used in the routing table

calculation. When an advertisement's age first reaches MaxAge, it is

reflooded. A link state advertisement of age MaxAge is finally flushed

from the database when it is no longer contained on any neighbor Link

state retransmission lists. This indicates that it has been

acknowledged by all adjacent neighbors. For more information on the

aging of link state advertisements, consult Section 14.

Ages are examined when a router receives two instances of a link state

advertisement, both having identical sequence numbers and checksums. An

instance of age MaxAge is then always accepted as most recent; this

allows old advertisements to be flushed quickly from the routing domain.

Otherwise, if the ages differ by more than MaxAgeDiff, the instance

having the smaller age is accepted as most recent.[11] See Section 13.1

for more details.

12.1.2 Options

The options field in the link state header indicates which optional

capabilities are associated with the advertisement. OSPF's optional

capabilities are described in Section 4.5. There are currently two

optional capabilities defined; they are represented by the T-bit and E-

bit found in the options field. The rest of the options field should be

set to zero.

The E-bit represents OSPF's external routing capability. This bit

should be set in all advertisements associated with the backbone, and

all advertisements associated with non-stub areas (see Section 3.6). It

should also be set in all AS external advertisements. It should be

reset in all router links, network links and summary link advertisements

associated with a stub area. For all link state advertisements, the

setting of the E-bit is for informational purposes only; it does not

affect the routing table calculation.

The T-bit represents OSPF's TOS routing capability. This bit should be

set in a router links advertisement if and only if the router is capable

of calculating separate routes for each IP TOS (see Section 2.4). The

T-bit should always be set in network links advertisements. It should

be set in summary link and AS external link advertisements if and only

if the advertisement describes paths for all TOS values, instead of just

the TOS 0 path. Note that, with the T-bit set, there may still be only

a single metric in the advertisement (the TOS 0 metric). This would

mean that paths for non-zero TOS exist, but are equivalent to the TOS 0

path. A link state advertisement's T-bit is examined when calculating

the routing table's non-zero TOS paths (see Section 16.9).

12.1.3 LS type

The LS type field dictates the format and function of the link state

advertisement. Advertisements of different types have different names

(e.g., router links or network links). All advertisement types, except

the AS external link advertisements (LS type = 5), are flooded

throughout a single area only. AS external link advertisements are

flooded throughout the entire Autonomous System, excluding stub areas

(see Section 3.6). Each separate advertisement type is briefly

described below in Table 15.

LS Type Advertisement description

__________________________________________________

1 These are the router links

advertisements. They describe the

LS Type Advertisement description

__________________________________________________

collected states of the router's

interfaces. For more information,

consult Section 12.4.1.

__________________________________________________

2 These are the network links

advertisements. They describe the set

of routers attached to the network. For

more information, consult

Section 12.4.2.

__________________________________________________

3 or 4 These are the summary link

advertisements. They describe

inter-area routes, and enable the

condensation of routing information at

area borders. Originated by area border

routers, the Type 3 advertisements

describe routes to networks while the

Type 4 advertisements describe routes to

AS boundary routers.

__________________________________________________

5 These are the AS external link

advertisements. Originated by AS

boundary routers, they describe routes

to destinations external to the

Autonomous System. A default route for

the Autonomous System can also be

described by an AS external link

advertisement.

Table 15: OSPF link state advertisements.

12.1.4 Link State ID

This field identifies the piece of the routing domain that is being

described by the advertisement. Depending on the advertisement's LS

type, the Link State ID takes on the values listed in Table 16.

LS Type Link State ID

______________________________________________________________________

1 The originating router's Router ID.

2 The IP interface address of the network's Designated Router.

3 The destination network's IP address.

4 The Router ID of the described AS boundary router.

5 The destination network's IP address.

Table 16: The advertisement's Link State ID.

When the link state advertisement is describing a network, the Link

State ID is either the network's IP address (as in type 3 summary link

advertisements and in AS external link advertisements) or the network's

IP address is easily derivable from the Link State ID (note that masking

a network links advertisement's Link State ID with the network's subnet

mask yields the network's IP address). When the link state

advertisement is describing a router, the Link State ID is always the

described router's OSPF Router ID.

When an AS external advertisement (LS Type = 5) is describing a default

route, its Link State ID is set to DefaultDestination (0.0.0.0).

12.1.5 Advertising Router

This field specifies the OSPF Router ID of the advertisement's

originator. For router links advertisements, this field is identical to

the Link State ID field. Network link advertisements are originated by

the network's Designated Router. Summary link advertisements are

originated by area border routers. Finally, AS external link

advertisements are originated by AS boundary routers.

12.1.6 LS sequence number

The sequence number field is a signed 32-bit integer. It is used to

detect old and duplicate link state advertisements. The space of

sequence numbers is linearly ordered. The larger the sequence number

(when compared as signed 32-bit integers) the more recent the

advertisement. To describe to sequence number space more precisely, let

N refer in the discussion below to the constant 2**31.

The sequence number -N (0x80000000) is reserved (and unused). This

leaves -N + 1 (0x80000001) as the smallest (and therefore oldest)

sequence number. A router uses this sequence number the first time it

originates any link state advertisement. Afterwards, the

advertisement's sequence number is incremented each time the router

originates a new instance of the advertisement. When an attempt is made

to increment the sequence number past the maximum value of of N - 1

(0x7fffffff), the current instance of the advertisement must first be

flushed from the routing domain. This is done by prematurely aging the

advertisement (see Section 14.1) and reflooding it. As soon as this

flood has been acknowledged by all adjacent neighbors, a new instance

can be originated with sequence number of -N + 1 (0x80000001).

The router may be forced to promote the sequence number of one of its

advertisements when a more recent instance of the advertisement is

unexpectedly received during the flooding process. This should be a

rare event. This may indicate that an out-of-date advertisement,

originated by the router itself before its last restart/reload, still

exists in the Autonomous System. For more information see Section 13.4.

,uh "12.1.7 LS checksum"

This field is the checksum of the complete contents of the

advertisement, excepting the age field. The age field is excepted so

that an advertisement's age can be incremented without updating the

checksum. The checksum used is the same that is used for ISO

connectionless datagrams; it is commonly referred to as the Fletcher

checksum. It is documented in Annex C of [RFC994]. The link state

header also contains the length of the advertisement in bytes;

subtracting the size of the age field (two bytes) yields the amount of

data to checksum.

The checksum is used to detect data corruption of an advertisement.

This corruption can occur while an advertisement is being flooded, or

while it is being held in a router's memory. The LS checksum field

cannot take on the value of zero; the occurrence of such a value should

be considered a checksum failure. In other words, calculation of the

checksum is not optional.

The checksum of a link state advertisement is verified in two cases: a)

when it is received in a Link State Update Packet and b) at times during

the aging of the link state database. The detection of a checksum

failure leads to separate actions in each case. See Sections 13 and 14

for more details.

Whenever the LS sequence number field indicates that two instances of an

advertisement are the same, the LS checksum field is examined. If there

is a difference, the instance with the larger checksum is considered to

be most recent.[12] See Section 13.1 for more details.

12.2 The link state database

A router has a separate link state database for every area to which it

belongs. The link state database has been referred to elsewhere in the

text as the topological database. All routers belonging to the same

area have identical topological databases for the area.

The databases for each individual area are always dealt with separately.

The shortest path calculation is performed separately for each area (see

Section 16). Components of the area topological database are flooded

throughout the area only. Finally, when an adjacency (belonging to Area

A) is being brought up, only the database for Area A is synchronized

between the two routers.

The area database is composed of router links advertisements, network

links advertisements, and summary link advertisements (all listed in the

area data structure). In addition, external routes (AS external

advertisements) are included in all non-stub area databases (see Section

3.6).

An implementation of OSPF must be able to access individual pieces of an

area database. This lookup function is based on an advertisement's LS

type, Link State ID and Advertising Router.[13] There will be a single

instance (the most up-to-date) of each link state advertisement in the

database. The database lookup function is invoked during the link state

flooding procedure (Section 13) and the routing table calculation

(Section 16). In addition, using this lookup function the router can

determine whether it has itself ever originated a particular link state

advertisement, and if so, with what LS sequence number.

A link state advertisement is added to a router's database when either

a) it is received during the flooding process (Section 13) or b) it is

originated by the router itself (Section 12.4). A link state

advertisement is deleted from a router's database when either a) it has

been overwritten by a newer instance during the flooding process

(Section 13) or b) the router originates a newer instance of one of its

self-originated advertisements (Section 12.4) or c) the advertisement

ages out and is flushed from the routing domain (Section 14). Whenever

a link state advertisement is deleted from the database it must also be

removed from all neighbors' Link state retransmission lists (see Section

10).

12.3 Representation of TOS

All OSPF link state advertisements (with the exception of network links

advertisements) specify metrics. In router links advertisements, the

metrics indicate the costs of the described interfaces. In summary link

and AS external link advertisements, the metric indicates the cost of

the described path. In all of these advertisements, a separate metric

can be specified for each IP TOS. TOS is encoded in an OSPF link state

advertisement as the following mapping of the Delay (D), Throughput (T)

and Reliability (R) flags found in the IP packet header's TOS field (see

[RFC791]).

OSPF encoding D T R

_________________________

0 0 0 0

4 0 0 1

8 0 1 0

12 0 1 1

16 1 0 0

20 1 0 1

24 1 1 0

28 1 1 1

Table 17: Representing TOS in OSPF.

Each OSPF link state advertisement must specify the TOS 0 metric. Other

TOS metrics, if they appear, must appear in order of increasing TOS

encoding. For example, the TOS 8 (high throughput) metric must always

appear before the TOS 16 (low delay) metric when both are specified. If

a metric for some non-zero TOS is not specified, its cost defaults to

the cost for TOS 0, unless the T-bit is reset in the advertisement's

options field (see Section 12.1.2 for more details).

Note that if more TOS types are defined in a future IP architecture,

OSPF's TOS encoding can be extended in a straightforward manner.

12.4 Originating link state advertisements

A router may originate many types of link state advertisements. A

router originates a router links advertisement for each area to which it

belongs. If the router is also the Designated Router for any of its

attached networks, it will originate a network links advertisement for

that network.

Area border routers originate a single summary links advertisement for

each known inter-area destination. AS boundary routers originate a

single AS external links advertisement for each known AS external

destination. Destinations are advertised one at a time so that the

change in any single route can be flooded without reflooding the entire

collection of routes. During the flooding procedure, many link state

advertisements can be carried by a single Link State Update packet.

As an example, consider router RT4 in Figure 6. It is an area border

router, having a connection to Area 1 and the backbone. Router RT4

originates 5 distinct link state advertisements into the backbone (one

router links, and one summary link for each of the networks N1-N4).

Router RT4 will also originate 8 distinct link state advertisements into

Area 1 (one router links and seven summary link advertisements as

pictured in Figure 7). If RT4 has been selected as Designated Router

for network N3, it will also originate a network links advertisement for

N3 into Area 1.

In this same figure, router RT5 will be originating 3 distinct AS

external link advertisements (one for each of the networks N12-N14).

These will be flooded throughout the entire AS, assuming that none of

the areas have been configured as stubs. However, if area 3 has been

configured as a stub area, the external advertisements for networks

N12-N14 will not be flooded into area 3 (see Section 3.6). Instead,

router RT11 would originate a default summary link advertisement that

would be flooded throughout area 3 (see Section 12.4.3). This instructs

all of area 3's internal routers to send their AS external traffic to

RT11.

Whenever a new instance of a link state advertisement is originated, its

LS sequence number is incremented, its LS age is set to 0, its LS

checksum is calculated, and the advertisement is added to the link state

database and flooded out the appropriate interfaces. See Section 13.2

for details concerning the installation of the advertisement into the

link state database. See Section 13.3 for details concerning the

flooding of newly originated advertisements.

The eight events that cause a new instance of a link state advertisement

to be originated are:

(1) The LS refresh timer firing. There is a LS refresh timer for each

link state advertisement that the router has originated. The LS

refresh timer is an interval timer, with length LSRefreshTimer. The

LS refresh timer guarantees periodic originations regardless of any

other events that cause new instances. This periodic updating of

link state advertisements adds robustness to the link state

algorithm. Link state advertisements that solely describe

unreachable destinations should not be refreshed, but should instead

be flushed from the routing domain (see Section 14.1).

When whatever is being described by a link state advertisement changes,

a new advertisement is originated. Two instances of the same link state

advertisement may not be originated within the time period

MinLSInterval. This may require that the generation of the next

instance to be delayed by up to MinLSInterval. The following changes

may cause a router to originate a new instance of an advertisement.

These changes should cause new originations only if the contents of the

new advertisement would be different.

(2) An interface's state changes (see Section 9.1). This may mean that

it is necessary to produce a new instance of the router links

advertisement.

(3) An attached network's Designated Router changes. A new router links

advertisement should be originated. Also, if the router itself is

now the Designated Router, a new network links advertisement should

be produced.

(4) One of the neighboring routers changes to/from the FULL state. This

may mean that it is necessary to produce a new instance of the

router links advertisement. Also, if the router is itself the

Designated Router for the attached network, a new network links

advertisement should be produced.

The next three events concern area border routers only.

(5) An intra-area route has been added/deleted/modified in the routing

table. This may cause a new instance of a summary links

advertisement (for this route) to be originated in each attached

area (this includes the backbone).

(6) An inter-area route has been added/deleted/modified in the routing

table. This may cause a new instance of a summary links

advertisement (for this route) to be originated in each attached

area (but NEVER for the backbone).

(7) The router becomes newly attached to an area. The router must then

originate summary link advertisements into the newly attached area

for all pertinent intra-area and inter-area routes in its routing

table. See Section 12.4.3 for more details.

The last event concerns AS boundary routers only.

(8) An external route gained through direct experience with an external

routing protocol (like EGP) changes. This will cause the AS

boundary router to originate a new instance of an external links

advertisement.

The construction of each type of the link state advertisement is

explained below. In general, these sections describe the contents of

the advertisement body (i.e., the part coming after the 20-byte

advertisement header). For information concerning the building of the

link state advertisement header, see Section 12.1.

12.4.1 Router links

A router originates a router links advertisement for each area that it

belongs to. Such an advertisement describes the collected states of the

router's links to the area. The advertisement is flooded throughout the

particular area, and no further.

The format of a router links advertisement is shown in Appendix A

(Section A.4.2). The first 20 bytes of the advertisement consist of the

generic link state header that was discussed in Section 12.1. Router

links advertisements have LS type = 1. The router indicates whether it

is willing to calculate separate routes for each IP TOS by setting (or

resetting) the T-bit of the link state advertisement's Options field.

A router also indicates whether it is an area border router, or an AS

boundary router, by setting the appropriate bits in its router links

advertisements. This enables paths to those types of routers to be

saved in the routing table, for later processing of summary link

advertisements and AS external link advertisements.

The router links advertisement then describes the router's working

connections (links) to the area. Each link is typed according to the

_________________________________________

(Figure not included in text version.)

Figure 15: Area 1 with IP addresses shown

Figure 16: Forwarding address example

_________________________________________

kind of attached network. Each link is also labelled with its Link ID.

This ID gives a name to the entity that is on the other end of the link.

Table 18 summarizes the values used for the type and Link ID fields.

Link type Description Link ID

____________________________________________________________________________

1 Point-to-point link Neighbor Router ID

2 Link to transit network Interface address of Designated Router

3 Link to stub network IP network number

4 Virtual link Neighbor Router ID

Table 18: Link descriptions in the router links advertisement.

In addition, the Link Data field is specified for each link. This field

gives 32 bits of extra information for the link. For links to routers

and transit networks, this field specifies the IP interface address of

the associated router interface (this is needed by the routing table

calculation, see Section 16.3). For links to stub networks, this field

specifies the network's IP address mask.

Finally, the cost of using the link for output (possibly specifying a

different cost for each type of service) is specified. The output cost

of a link is configurable. It must always be non-zero.

To further describe the process of building the list of link records,

suppose a router wishes to build router links advertisement for an Area

A. The router examines its collection of interface data structures.

For each interface, the following steps are taken:

o If the attached network does not belong to Area A, no links are

added to the advertisement, and the next interface should be

examined.

o Else, if the state of the interface is Down, no links are added.

o Else, if the state of the interface is Point-to-Point, then add

links according to the following:

- If the neighboring router is fully adjacent, add a Type 1 link

(point-to-point) if this is an interface to a point-to-point

network, or add a type 4 link (virtual link) if this is a

virtual link. The Link ID should be set to the Router ID of the

neighboring router, and the Link Data should specify the

interface IP address.

- If this is a numbered point-to-point network (i.e, not a virtual

link and not an unnumbered point-to-point network) and the

neighboring router's IP address is known, add a Type 3 link

(stub network) whose Link ID is the neighbor's IP address, whose

Link Data is the mask 0xffffffff indicating a host route, and

whose cost is the interface's configured output cost.

o Else if the state of the interface is Loopback, add a Type 3 link

(stub network) as long as this is not an interface to an unnumbered

serial line. The Link ID should be set to the IP interface address,

the Link Data set to the mask 0xffffffff (indicating a host route),

and the cost set to 0.

o Else if the state of the interface is Waiting, add a Type 3 link

(stub network) whose Link ID is the IP network number of the

attached network and whose Link Data is the attached network's

address mask.

o Else, there has been a Designated Router selected for the attached

network. If the router is fully adjacent to the Designated Router,

or if the router itself is Designated Router and is fully adjacent

to at least one other router, add a single Type 2 link (transit

network) whose whose link ID is the IP interface address of the

attached network's Designated Router (which may be the router

itself) and whose Link Data is the interface IP address. Otherwise,

add a link as if the interface state were Waiting (see above).

Unless otherwise specified, the cost of each link generated by the above

procedure is equal to the output cost of the associated interface. Note

that in the case of serial lines, multiple links may be generated by a

single interface.

After consideration of all the router interfaces, host links are added

to the advertisement by examining the list of attached hosts. A host

route is represented as a Type 3 link (stub network) whose link ID is

the host's IP address and whose Link Data is the mask of all ones

(0xffffffff).

As an example, consider the router links advertisements generated by

router RT3, as pictured in Figure 6. The area containing router RT3

(Area 1) has been redrawn, with actual network addresses, in Figure 15.

Assume that the last byte of all of RT3's interface addresses is 3,

giving it the interface addresses 192.1.1.3 and 192.1.4.3, and that the

other routers have similar addressing schemes. In addition, assume that

all links are functional, and that Router IDs are assigned as the

smallest IP interface address.

RT3 originates two router links advertisements, one for Area 1 and one

for the backbone. Assume that router RT4 has been selected as the

Designated router for network 192.1.1.0. RT3's router links

advertisement for Area 1 is then shown below. It indicates that RT3 has

two connections to Area 1, the first a link to the transit network

192.1.1.0 and the second a link to the stub network 192.1.4.0. Note

that the transit network is identified by the IP interface of its

Designated Router (i.e., the Link ID = 192.1.1.4 which is the Designated

Router RT4's IP interface to 192.1.1.0). Note also that RT3 has

indicated that it is capable of calculating separate routes based on IP

TOS, through setting the T-bit in the Options field. It has also

indicated that it is an area border router.

; RT3's router links advertisement for Area 1

LS age = 0 ;always true on origination

Options = (T-bitE-bit) ;TOS-capable

LS type = 1 ;indicates router links

Link State ID = 192.1.1.3 ;RT3's Router ID

Advertising Router = 192.1.1.3 ;RT3's Router ID

bit E = 0 ;not an AS boundary router

bit B = 1 ;RT3 is an area border router

#links = 2

Link ID = 192.1.1.4 ;IP address of Designated Router

Link Data = 192.1.1.3 ;RT3's IP interface to net

Type = 2 ;connects to transit network

# other metrics = 0

TOS 0 metric = 1

Link ID = 192.1.4.0 ;IP Network number

Link Data = 0xffffff00 ;Network mask

Type = 3 ;connects to stub network

# other metrics = 0

TOS 0 metric = 2

Next RT3's router links advertisement for the backbone is shown. It

indicates that RT3 has a single attachment to the backbone. This

attachment is via an unnumbered point-to-point link to router RT6. RT3

has again indicated that it is TOS-capable, and that it is an area

border router.

; RT3's router links advertisement for the backbone

LS age = 0 ;always true on origination

Options = (T-bitE-bit) ;TOS-capable

LS type = 1 ;indicates router links

Link State ID = 192.1.1.3 ;RT3's router ID

Advertising Router = 192.1.1.3 ;RT3's router ID

bit E = 0 ;not an AS boundary router

bit B = 1 ;RT3 is an area border router

#links = 1

Link ID = 18.10.0.6 ;Neighbor's Router ID

Link Data = 0.0.0.0 ;Interface to unnumbered SL

Type = 1 ;connects to router

# other metrics = 0

TOS 0 metric = 8

Even though router RT3 has indicated that it is TOS-capable in the above

examples, only a single metric (the TOS 0 metric) has been specified for

each interface. Different metrics can be specified for each TOS. The

encoding of TOS in OSPF link state advertisements is described in

Section 12.3.

As an example, suppose the point-to-point link between routers RT3 and

RT6 in Figure 15 is a satellite link. The AS administrator may want to

encourage the use of the line for high bandwidth traffic. This would be

done by setting the metric artificially low for that TOS. Router RT3

would then originate the following router links advertisement for the

backbone (IP TOS 8 = high bandwidth):

; RT3's router links advertisement for the backbone

LS age = 0 ;always true on origination

Options = (T-bitE-bit) ;TOS-capable

LS type = 1 ;indicates router links

Link State ID = 192.1.1.3 ;RT3's Router ID

Advertising Router = 192.1.1.3

bit E = 0 ;not an AS boundary router

bit B = 1 ;RT3 is an area border router

#links = 1

Link ID = 18.10.0.6 ; Neighbor's Router ID

Link Data = 0.0.0.0 ;Interface to unnumbered SL

Type = 1 ;connects to router

# other metrics = 1

TOS 0 metric = 8

TOS = 8 ;High bandwidth

metric = 1 ;traffic preferred

12.4.2 Network links

A network links advertisement is generated for every transit multi-

access network. (A transit network is a network having two or more

attached routers). The network links advertisement describes all the

routers that are attached to the network.

The Designated Router for the network originates the advertisement. The

Designated Router originates the advertisement only if it is fully

adjacent to at least one other router on the network. The network links

advertisement is flooded throughout the area that contains the transit

network, and no further. The networks links advertisement lists those

routers that are fully adjacent to the Designated Router; each fully

adjacent router is identified by its OSPF Router ID. The Designated

Router includes itself in this list.

The Link State ID for a network links advertisement is the IP interface

address of the Designated Router. This value, masked by the network's

address mask (which is also contained in the network links

advertisement) yields the network's IP address.

A router that has formerly been the Designated Router for a network, but

is no longer, should flush the network links advertisement that it had

previously originated. This advertisement is no longer used in the

routing table calculation. It is flushed by prematurely incrementing

the advertisement's age to MaxAge and reflooding (see Section 14.1).

As an example of a network links advertisement, again consider the area

configuration in Figure 6. Network links advertisements are originated

for network N3 in Area 1, networks N6 and N8 in Area 2, and network N9

in Area 3. Assuming that router RT4 has been selected as the Designated

Router for network N3, the following network links advertisement is

generated by RT4 on behalf of network N3 (see Figure 15 for the address

assignments):

; network links advertisement for network N3

LS age = 0 ;always true on origination

Options = (T-bitE-bit) ;TOS-capable

LS type = 2 ;indicates network links

Link State ID = 192.1.1.4 ;IP address of Designated Router

Advertising Router = 192.1.1.4 ;RT4's Router ID

Network Mask = 0xffffff00

Attached Router = 192.1.1.4 ;Router ID

Attached Router = 192.1.1.1 ;Router ID

Attached Router = 192.1.1.2 ;Router ID

Attached Router = 192.1.1.3 ;Router ID

12.4.3 Summary links

Each summary link advertisement describes a route to a single

destination. Summary link advertisements are flooded throughout a

single area only. The destination described is one that is external to

the area, yet still belonging to the Autonomous System.

The DefaultDestination can also be specified in summary link

advertisements. This is used when implementing OSPF's stub area

functionality (see Section 3.6). In a stub area, instead of importing

external routes each area border router originates a "default summary

link" (Link State ID = DefaultDestination) into the area.

Summary link advertisements are originated by area border routers. The

precise summary routes to advertise into an area are determined by

examining the routing table structure (see Section 11). Only intra-area

routes are advertised into the backbone. Both intra-area and inter-area

routes are advertised into the other areas.

To determine which routes to advertise into an attached Area A, each

routing table entry is processed as follows:

o Only Destination types of network and AS boundary router are

advertised in summary link advertisements. If the routing table

entry's Destination type is area border router, examine the next

routing table entry.

o AS external routes are never advertised in summary link

advertisements. If the routing table entry has Path-type type 1

external or type 2 external, examine the next routing table entry.

o Else, if the area associated with this set of paths is the Area A

itself, do not generate a summary link advertisement for the

route.[14]

o Else, if the destination of this route is an AS boundary router,

generate a Type 4 link state advertisement for the destination, with

Link State ID equal to the AS boundary router's ID and metric equal

to the routing table entry's cost. These advertisements should not

be generated if area A has been configured as a stub area.

o Else, the Destination type is network. If this is an inter-area

route, generate a Type 3 advertisement for the destination, with

Link State ID equal to the network's address and metric equal to the

routing table cost.

o The one remaining case is an intra-area route to a network. This

means that the network is contained in one of the router's directly

attached areas. In general, this information must be condensed

before appearing in summary link advertisements. Remember that an

area has been defined as a list of address ranges, each range

consisting of an [address,mask] pair. A single Type 3 advertisement

must be made for each range, with Link State ID equal to the range's

address and cost equal to the smallest cost of any of the component

networks.

If virtual links are being used to provide/increase connectivity of

the backbone, routing information concerning the backbone networks

should not be condensed before being summarized into the virtual

links' transit areas. In other words, the backbone ranges should be

ignored when originating summary links into these areas. The

existence of virtual links can be determined during the shortest

path calculation for the backbone (see Section 16.1).

In addition, if area A has been configured as a stub area and the router

is an area border router, it should advertise a default summary link

into Area A. The Link State ID for the advertisement should be set to

DefaultDestination, and the metric set to the (per-area) configurable

parameter StubDefaultCost.

If a router advertises a summary advertisement for a destination which

then becomes unreachable, the router must then flush the advertisement

from the routing domain by setting its age to MaxAge and reflooding (see

Section 14.1). Also, if the destination is still reachable, yet can no

longer be advertised according to the above procedure (e.g., it is now

an inter-area route, when it used to be an intra-area route associated

with some non-backbone area; it would thus no longer be advertisable to

the backbone), the advertisement should also be flushed from the routing

domain.

For an example of summary link advertisements, consider again the area

configuration in Figure 6. Routers RT3, RT4, RT7, RT10 and RT11 are all

area border routers, and therefore are originating summary links

advertisements. Consider in particular router RT4. Its routing table

was calculated as the example in Section 11.3. RT4 originates summary

link advertisements into both the backbone and Area 1. Into the

backbone, router RT4 originates separate advertisements for each of the

networks N1-N4. Into Area 1, router RT4 originates separate

advertisements for networks N6-N8 and the AS boundary routers RT5,RT7.

It also condenses host routes Ia and Ib into a single summary

advertisement. Finally, the routes to networks N9,N10,N11 and host H9

are advertised by a single summary link. This condensation was

originally performed by the router RT11.

These advertisements are illustrated graphically in Figures 7 and 8.

Two of the summary link advertisements originated by router RT4 follow.

The actual IP addresses for the networks and routers in question have

been assigned in Figure 15.

; summary link advertisement for network N1,

; originated by router RT4 into the backbone

LS age = 0 ;always true on origination

Options = (T-bitE-bit) ;TOS-capable

LS type = 3 ;indicates summary link to IP net

Link State ID = 192.1.2.0 ;N1's IP network number

Advertising Router = 192.1.1.4 ;RT4's ID

TOS = 0

metric = 4

; summary link advertisement for AS boundary router RT7

; originated by router RT4 into Area 1

LS age = 0 ;always true on origination

Options = (T-bitE-bit) ;TOS-capable

LS type = 4 ;indicates summary link to ASBR

Link State ID = router RT7's ID

Advertising Router = 192.1.1.4 ;RT4's ID

TOS = 0

metric = 14

Summary link advertisements pertain to a single destination (IP network

or AS boundary router). However, for a single destination there may be

separate sets of paths, and therefore separate routing table entries,

for each Type of Service. All these entries must be considered when

building the summary link advertisement for the destination; a single

advertisement must specify the separate costs (if they exist) for each

TOS. The encoding of TOS in OSPF link state advertisements is described

in Section 12.3.

Clearing the T-bit in the Options field of a summary link advertisement

indicates that there is a TOS 0 path to the destination, but no paths

for non-zero TOS. This can happen when non-TOS capable routers exist in

the routing domain (see Section 2.4).

12.4.4 AS external links

AS external link advertisements describe routes to destinations external

to the Autonomous System. Most AS external link advertisements describe

routes to specific external destinations. However, a default route for

the Autonomous System can be described in an AS external advertisement

by setting the advertisement's Link State ID to DefaultDestination

(0.0.0.0). AS external link advertisements are originated by AS

boundary routers. An AS boundary router originates a single AS external

link advertisement for each external route that it has learned, either

through another routing protocol (such as EGP), or through configuration

information.

In general, AS external link advertisements are the only type of link

state advertisements that are flooded throughout the entire Autonomous

System; all other types of link state advertisements are specific to a

single area. However, AS external advertisements are not flooded

into/throughout stub areas (see Section 3.6). This enables a reduction

in link state database size for routers internal to stub areas.

The metric that is advertised for an external route can be one of two

types. Type 1 metrics are comparable to the link state metric. Type 2

metrics are assumed to be larger than the cost of any intra-AS path. As

with summary link advertisements, if separate paths exist based on TOS,

separate TOS costs can be included in the AS external link

advertisement. The encoding of TOS in OSPF link state advertisements is

described in Section 12.3. If the T-bit of the advertisement's Options

field is clear, no non-zero TOS paths to the destination exist.

If a router advertises an AS external link advertisement for a

destination which then becomes unreachable, the router must then flush

the advertisement from the routing domain by setting its age to MaxAge

and reflooding (see Section 14.1).

For an example of AS external link advertisements, consider once again

the AS pictured in Figure 6. There are two AS boundary routers: RT5 and

RT7. Router RT5 originates three external link advertisements, for

networks N12-N14. Router RT7 originates two external link

advertisements, for networks N12 and N15. Assume that RT7 has learned

its route to N12 via EGP, and that it wishes to advertise a Type 2

metric to the AS. RT7 would then originate the following advertisement

for N12:

; AS external link advertisement for network N12,

; originated by router RT7

LS age = 0 ;always true on origination

Options = (T-bitE-bit) ;TOS-capable

LS type = 5 ;indicates AS external link

Link State ID = N12's IP network number

Advertising Router = Router RT7's ID

bit E = 1 ;Type 2 metric

TOS = 0

metric = 2

Forwarding address = 0.0.0.0

In the above example, the forwarding address field has been set to

0.0.0.0, indicating that packets for the external destination should be

forwarded to the advertising OSPF router (RT7). This is not always

desirable. Consider the example pictured in Figure 16. There are three

OSPF routers (RTA, RTB and RTC) connected to a common network. Only one

of these routers, RTA, is exchanging EGP information with the non-OSPF

router RTX. RTA must then originate AS external link state

advertisements for those destinations it has learned from RTX. By using

the AS external advertisement's forwarding address field, RTA can

specify that packets for these destinations be forwarded directly to

RTX. Without this feature, routers RTB and RTC would take an extra hop

to get to these destinations.

Note that when the forwarding address field is non-zero, it should point

to a router belonging to another Autonomous System.

A forwarding address can also be specified for the default route. For

example, in figure 16 RTA may want to specify that all externally-

destined packets should by default be forwarded to its EGP peer RTX.

The resulting AS external link advertisement is pictured below. Note

that the Link State ID is set to DefaultDestination.

; Default route, originated by router RTA

; Packets forwarded through RTX

LS age = 0 ;always true on origination

Options = (T-bitE-bit) ;TOS-capable

LS type = 5 ;indicates AS external link

Link State ID = DefaultDestination ; default route

Advertising Router = Router RTA's ID

bit E = 1 ;Type 2 metric

TOS = 0

metric = 1

Forwarding address = RTX's IP address

In figure 16, suppose instead that both RTA and RTB exchange EGP

information with RTX. In this case, RTA and RTB would originate the

same set of external advertisements. These advertisements, if they

specify the same metric, would be functionally equivalent since they

would specify the same destination and forwarding address (RTX). This

leads to a clear duplication of effort. If only one of RTA or RTB

originated the set of external advertisements, the routing would remain

the same, and the size of the link state database would decrease.

However, it must be unambiguously defined as to which router originates

the advertisements (otherwise neither may, or the identity of the

originator may oscillate). The following rule is thereby established:

if two routers, both reachable from one another, originate functionally

equivalent AS external advertisements (i.e., same destination, cost and

non-zero forwarding address), then the advertisement originated by the

router having the highest OSPF Router ID is used. The router having the

lower OSPF Router ID can then flush its advertisement. Flushing a link

state advertisement is discussed in Section 14.1.

13. The Flooding Procedure

Link State Update packets provide the mechanism for flooding link state

advertisements. A Link State Update packet may contain several distinct

advertisements, and floods each advertisement one hop further from its

point of origination. To make the flooding procedure reliable, each

advertisement must be acknowledged separately. Acknowledgments are

transmitted in Link State Acknowledgment packets. Many separate

acknowledgments can be grouped together into a single packet.

The flooding procedure starts when a Link State Update packet has been

received. Many consistency checks have been made on the received packet

before being handed to the flooding procedure (see Section 8.2). In

particular, the Link State Update packet has been associated with a

particular neighbor, and a particular area. If the neighbor is in a

lesser state than Exchange, the packet should be dropped without further

processing.

All types of link state advertisements, other than AS external links,

are associated with a specific area. However, link state advertisements

do not contain an area field. A link state advertisement's area must be

deduced from the Link State Update packet header.

For each link state advertisement contained in the packet, the following

steps are taken:

(1) Validate the advertisement's link state checksum. If the checksum

turns out to be invalid, discard the advertisement and get the next

one from the Link State Update packet.

(2) Examine the link state advertisement's LS type. If the LS type is

unknown, discard the advertisement and get the next one from the

Link State Update Packet. This specification defines LS Types 1-5

(see Section 4.3).

(3) Else if this is a AS external advertisement (LS type = 5), and the

area has been configured as a stub area, discard the advertisement

and get the next one from the Link State Update Packet. AS external

advertisements are not flooded into/throughout stub areas (see

Section 3.6).

(4) Else if the advertisement's age is equal to MaxAge, and there is

currently no instance of the advertisement in the router's link

state database, then take the following actions:

(a) Acknowledge the receipt of the advertisement by sending a Link

State Acknowledgment packet back to the sending neighbor (see

Section 13.5).

(b) Purge all outstanding requests for equal or previous instances

of the advertisement from the sending neighbor's Link State

Request list (see Section 10).

(c) If the sending neighbor is in state Exchange or in state

Loading, then install the MaxAge advertisement in the link state

database. Otherwise, simply discard the advertisement. In

either case, examine the next advertisement (if any) listed in

the Link State Update packet.

(5) Otherwise, find the instance of this advertisement that is currently

contained in the router's link state database. If there is no

database copy, or the received advertisement is more recent than the

database copy (see Section 13.1 below for the determination of which

advertisement is more recent) the following steps must be performed:

(a) If there is already a database copy, and if the database copy

was installed less than MinLSInterval seconds ago, discard the

new advertisement (without acknowledging it) and examine the

next advertisement (if any) listed in the Link State Update

packet.

(b) Otherwise immediately flood the new advertisement out some

subset of the router's interfaces (see Section 13.3). In some

cases (e.g., the state of the receiving interface is DR and the

advertisement was received from a router other than the Backup

DR) the advertisement will be flooded back out the receiving

interface. This occurrence should be noted for later use by the

acknowledgment process (Section 13.5).

(c) Remove the current database copy from all neighbors' Link state

retransmission lists.

(d) Install the new advertisement in the link state database

(replacing the current database copy). This may cause the

routing table calculation to be scheduled. In addition,

timestamp the new advertisement with the current time (i.e., the

time it was received). The flooding procedure cannot overwrite

the newly installed advertisement until MinLSInterval seconds

have elapsed. The advertisement installation process is

discussed further in Section 13.2.

(e) Possibly acknowledge the receipt of the advertisement by sending

a Link State Acknowledgment packet back out the receiving

interface. This is explained below in Section 13.5.

(f) If this new link state advertisement indicates that it was

originated by this router itself, the router must advance the

advertisement's link state sequence number, and issue a new

instance of the advertisement (see Section 13.4).

(6) Else, if there is an instance of the advertisement on the sending

neighbor's Link state request list, an error has occurred in the

Database Description process. In this case, restart the Database

Description process by generating the neighbor event BadLSReq for

the sending neighbor and stop processing the Link State Update

packet.

(7) Else, if the received advertisement is the same instance as the

database copy (i.e., neither one is more recent) the following two

steps should be performed:

(a) If the advertisement is listed in the Link state retransmission

list for the receiving adjacency, the router itself is expecting

an acknowledgment for this advertisement. The router should

treat the received advertisement as an acknowledgment, by

removing the advertisement from the Link state retransmission

list. This is termed an "implied acknowledgment". Its

occurrence should be noted for later use by the acknowledgment

process (Section 13.5).

(b) Possibly acknowledge the receipt of the advertisement by sending

a Link State Acknowledgment packet back out the receiving

interface. This is explained below in Section 13.5.

(8) Else, the database copy is more recent. Note an unusual event to

network management, discard the advertisement and process the next

link state advertisement contained in the packet.

13.1 Determining which link state is newer

When a router encounters two instances of a link state advertisement, it

must determine which is more recent. This occurred above when comparing

a received advertisement to the database copy. This comparison must

also be done during the database exchange procedure which occurs during

adjacency bring-up.

A link state advertisement is identified by its LS type, Link State ID

and Advertising Router. For two instances of the same advertisement,

the LS sequence number, LS age, and LS checksum fields are used to

determine which instance is more recent:

o The advertisement having the newer LS sequence number is more

recent. See Section 12.1.6 for an explanation of the LS sequence

number space. If both instances have the same LS sequence number,

then:

o If the two instances have different LS checksums, then the instance

having the larger LS checksum (when considered as a 16-bit unsigned

integer) is considered more recent.

o Else, if only one of the instances is of age MaxAge, the instance of

age MaxAge is considered to be more recent.

o Else, if the ages of the two instances differ by more than

MaxAgeDiff, the instance having the smaller (younger) age is

considered to be more recent.

o Else, the two instances are considered to be identical.

13.2 Installing link state advertisements in the database

Installing a new link state advertisement in the database, either as the

result of flooding or a newly self originated advertisement, may cause

the routing table structure to be recalculated. The contents of the new

advertisement should be compared to the old instance, if present. If

there is no difference, there is no need to recalculate the routing

table. (Note that even if the contents are the same, the LS checksum

will probably be different, since the checksum covers the LS sequence

number.)

If the contents are different, the following pieces of the routing table

must be recalculated, depending on the LS type field:

Router links, network links

The entire routing table must be recalculated, starting with the

shortest path calculations for each area (not just the area whose

topological database has changed). The reason that the shortest

path calculation cannot be restricted to the single changed area has

to do with the fact that AS boundary routers may belong to multiple

areas. A change in the area currently providing the best route may

force the router to use an intra-area route provided by a different

area.[15]

Summary link

The best route to the destination described by the summary link

advertisement must be re-examined (see Section 16.5). If this

destination is an AS boundary router, it may also be necessary to

re-examine all the AS external link advertisements.

AS external link

The best route to the destination described by the AS external link

advertisement must be re-examined (see Section 16.6).

Also, any old instance of the advertisement must be removed from the

database when the new advertisement is installed. This old instance

must also be removed from all neighbors' Link state retransmission lists

(see Section 10).

13.3 Next step in the flooding procedure

When a new (and more recent) advertisement has been received, it must be

flooded out some set of the router's interfaces. This section describes

the second part of flooding procedure (the first part being the

processing that occurred in Section 13), namely, selecting the outgoing

interfaces and adding the advertisement to the appropriate neighbors'

Link state retransmission lists. Also included in this part of the

flooding procedure is the maintenance of the neighbors' Link state

request lists.

This section is equally applicable to the flooding of an advertisement

that the router itself has just originated (see Section 12.4). For

these advertisements, this section provides the entirety of the flooding

procedure (i.e., the processing of Section 13 is not performed, since,

for example, the advertisement has not been received from a neighbor and

therefore does not need to be acknowledged).

Depending upon the advertisement's LS type, the advertisement can be

flooded out only certain interfaces. These interfaces, defined by the

following, are called the eligible interfaces:

AS external links (LS Type = 5)

AS external links are flooded throughout the entire AS, with the

exception of stub areas (see Section 3.6). The eligible interfaces

are all the router's interfaces, excluding virtual links and those

interfaces attaching to stub areas.

All other types

All other types are specific to a single area (Area A). The

eligible interfaces are all those interfaces attaching to the Area

A. If Area A is the backbone, this includes all the virtual links.

Link state databases must remain synchronized over all adjacencies

associated with the above eligible interfaces. This is accomplished by

executing the following steps on each eligible interface. It should be

noted that this procedure may decide not to flood a link state

advertisement out a particular interface, if there is a high probability

that the attached neighbors have already received the advertisement.

However, in these cases the flooding procedure must be absolutely sure

that the neighbors eventually do receive the advertisement, so the

advertisement is still added to each adjacency's Link state

retransmission list. For each eligible interface:

(1) Each of the neighbors attached to this interface are examined, to

determine whether they must receive the new advertisement. The

following steps are executed for each neighbor:

(a) If the neighbor is in a lesser state than Exchange, it does not

participate in flooding, and the next neighbor should be

examined.

(b) Else, if the adjacency is not yet full (neighbor state is

Exchange or Loading), examine the Link state request list

associated with this adjacency. If there is an instance of the

new advertisement on the list, it indicates that the neighboring

router has an instance of the advertisement already. Compare

the new advertisement to the neighbor's copy:

o If the new advertisement is less recent, then try the next

neighbor.

o If the two copies are the same instance, then delete the

advertisement from the Link state request list, and try the

next neighbor.[16]

o Else, the new advertisement is more recent. Delete the

advertisement from the Link state request list.

(c) If the new advertisement was received from this neighbor, try

the next neighbor.

(d) At this point we are not positive that the new neighbor has an

up-to-date instance of this new advertisement. Add the new

advertisement to the Link state retransmission list for the

adjacency. This ensures that the flooding procedure is

reliable; the advertisement will be retransmitted at intervals

until an acknowledgment is seen from the neighbor.

(2) The router must now decide whether to flood the new link state

advertisement out this interface. If in the previous step, the link

state advertisement was NOT added to any of the Link state

retransmission lists, there is no need to flood the advertisement

and the next interface should be examined.

(3) If the new advertisement was received on this interface, and it was

received from either the Designated Router or the Backup Designated

Router, chances are all the neighbors have received the

advertisement already. Therefore, examine the next interface.

(4) If the new advertisement was received on this interface, and the

interface state is Backup (i.e., the router itself is the Backup

Designated Router), examine the next interface. The Designated

Router will do the flooding on this interface. If the Designated

Router fails, this router will end up retransmitting the updates.

(5) If this step is reached, the advertisement must be flooded out the

interface. Send a Link State Update packet (with the new

advertisement as contents) out the interface. The advertisement's

LS age must be incremented by InfTransDelay (which must be > 0) when

copied into the outgoing packet (until the LS age field reaches its

maximum value of MaxAge).

On broadcast networks, the Link State Update packets are multicast.

The destination IP address specified for the Link State Update

Packet depends on the state of the interface. If the interface

state is DR or Backup, the address AllSPFRouters should be used.

Otherwise, the address AllDRouters should be used.

On non-broadcast, multi-access networks, separate Link State Update

packets must be sent, as unicasts, to each adjacent neighbor (i.e.,

those in state Exchange or greater). The destination IP addresses

for these packets are the neighbors' IP addresses.

13.4 Receiving self-originated link state

It is a common occurrence to receive a self-originated link state

advertisement via the flooding procedure. If the advertisement received

is a newer instance than the last instance that the router actually

originated, the router must take special action.

The reception of such an advertisement indicates that there are link

state advertisements in the routing domain that were originated before

the last time the router was restarted. In this case, the router must

advance the sequence number for the advertisement one past the received

sequence number, and originate a new instance of the advertisement.

Note also that if the type of the advertisement is Summary link or AS

external link, the router may no longer have an (advertisable) route to

the destination. In this case, the advertisement should be flushed from

the routing domain by incrementing the advertisement's LS age to MaxAge

and reflooding (see Section 14.1).

13.5 Sending Link State Acknowledgment packets

Each newly received link state advertisement must be acknowledged. This

is usually done by sending Link State Acknowledgment packets. However,

acknowledgments can also be accomplished implicitly by sending Link

State Update packets (see step 7a of Section 13).

Many acknowledgments may be grouped together into a single Link State

Acknowledgment packet. Such a packet is sent back out the interface

that has received the advertisements. The packet can be sent in one of

two ways: delayed and sent on an interval timer, or sent directly (as a

unicast) to a particular neighbor. The particular acknowledgment

strategy used depends on the circumstances surrounding the receipt of

the advertisement.

Sending delayed acknowledgments accomplishes several things: it

facilitates the packaging of multiple acknowledgments in a single

packet; it enables a single packet to indicate acknowledgments to

several neighbors at once (through multicasting); and it randomizes the

acknowledgment packets sent by the various routers attached to a multi-

access network. The fixed interval between a router's delayed

transmissions must be short (less than RxmtInterval) or needless

retransmissions will ensue.

Direct acknowledgments are sent to a particular neighbor in response to

the receipt of duplicate link state advertisements. These

acknowledgments are sent as unicasts, and are sent immediately when the

duplicate is received.

The precise procedure for sending Link State Acknowledgment packets is

described in Table 19. The circumstances surrounding the receipt of the

advertisement are listed in the left column. The acknowledgment action

then taken is listed in one of the two right columns. This action

depends on the state of the concerned interface; interfaces in state

Backup behave differently from interfaces in all other states.

Action taken in state

Circumstances Backup All other states

______________________________________________________________

Action taken in state

Circumstances Backup All other states

______________________________________________________________

Advertisement has No acknowledgment No acknowledgment

been flooded back sent. sent.

out receiving in-

terface (see Sec-

tion 13, step 5b).

______________________________________________________________

Advertisement is Delayed ack- Delayed ack-

more recent than nowledgment sent nowledgment sent.

database copy, but if advertisement

was not flooded received from DR,

back out receiving otherwise do noth-

interface ing

______________________________________________________________

Advertisement is a Delayed ack- No acknowledgment

duplicate, and was nowledgment sent sent.

treated as an im- if advertisement

plied acknowledg- received from DR,

ment (see Section otherwise do noth-

13, step 7a). ing

______________________________________________________________

Advertisement is a Direct acknowledg- Direct acknowledg-

duplicate, and was ment sent. ment sent.

not treated as an

implied ack-

nowledgment.

______________________________________________________________

Advertisement's age Direct acknowledg- Direct acknowledg-

is equal to MaxAge, ment sent. ment sent.

and there is no

current instance of

the advertisement in

the link state

database (see

Section 13, step 4).

Table 19: Sending link state acknowledgements.

Delayed acknowledgments must be delivered to all adjacent routers

associated with the interface. On broadcast networks, this is

accomplished by sending the delayed Link State Acknowledgment packets as

multicasts. The Destination IP address used depends on the state of the

interface. If the state is DR or Backup, the destination AllSPFRouters

is used. In other states, the destination AllDRouters is used. On

non-broadcast networks, delayed acks must be unicast separately over

each adjacency (neighbor whose state is >= Exchange).

The reasoning behind sending the above packets as multicasts is best

explained by an example. Consider the network configuration depicted in

Figure 15. Suppose RT4 has been elected as DR, and RT3 as Backup for

the network N3. When router RT4 floods a new advertisement to network

N3, it is received by routers RT1, RT2, and RT3. These routers will not

flood the advertisement back onto net N3, but they still must ensure

that their topological databases remain synchronized with their adjacent

neighbors. So RT1, RT2, and RT4 are waiting to see an acknowledgment

from RT3. Likewise, RT4 and RT3 are both waiting to see acknowledgments

from RT1 and RT2. This is best achieved by sending the acknowledgments

as multicasts.

The reason that the acknowledgment logic for Backup DRs is slightly

different is because they perform differently during the flooding of

link state advertisements (see Section 13.3, step 4).

13.6 Retransmitting link state advertisements

Advertisements flooded out an adjacency are placed on the adjacency's

Link state retransmission list. In order to ensure that flooding is

reliable, these advertisements are retransmitted until they are

acknowledged. The length of time between retransmissions is a

configurable per-interface value, RxmtInterval. If this is set too low

for an interface, needless retransmissions will ensue. If the value is

set too high, the speed of the flooding, in the face of lost packets,

may be affected.

Several retransmitted advertisements may fit into a single Link State

Update packet. When advertisements are to be retransmitted, only the

number fitting in a single Link State Update packet should be

transmitted. Another packet of retransmissions can be sent when some of

the advertisements are acknowledged, or on the next firing of the

retransmission timer.

Link State Update Packets carrying retransmissions are always sent as

unicasts (directly to the physical address of the neighbor). They are

never sent as multicasts. Each advertisement's LS age must be

incremented by InfTransDelay (which must be > 0) when copied into the

outgoing packet (until the LS age field reaches its maximum value of

MaxAge).

If the adjacent router goes down, retransmissions may occur until the

adjacency is destroyed by OSPF's Hello Protocol. When the adjacency is

destroyed, the Link state retransmission list is cleared.

13.7 Receiving link state acknowledgments

Many consistency checks have been made on a received Link State

Acknowledgment packet before it is handed to the flooding procedure. In

particular, it has been associated with a particular neighbor. If this

neighbor is in a lesser state than Exchange, the packet is discarded.

Otherwise, for each acknowledgment in the packet, the following steps

are performed:

o Does the advertisement acknowledged have an instance on the Link

state retransmission list for the neighbor? If not, examine the

next acknowledgment. Otherwise:

o If the acknowledgment is for the same instance that is contained on

the list, remove the item from the list and examine the next

acknowledgment. Otherwise:

o Log the questionable acknowledgment, and examine the next one.

14. Aging The Link State Database

Each link state advertisement has an age field. The age is expressed in

seconds. An advertisement's age field is incremented while it is

contained in a router's database. Also, when copied into a Link State

Update Packet for flooding out a particular interface, the

advertisement's age is incremented by InfTransDelay.

An advertisement's age is never incremented past the value MaxAge.

Advertisements having age MaxAge are not used in the routing table

calculation. As a router ages its link state database, an

advertisement's age may reach MaxAge.[17] At this time, the router must

attempt to flush the advertisement from the routing domain. This is

done simply by reflooding the MaxAge advertisement just as if it was a

newly originated advertisement (see Section 13.3).

When a Database summary list for a newly adjacent neighbor is formed,

any MaxAge advertisements present in the link state database are added

to the neighbor's Link state retransmission list instead of the

neighbor's Database summary list. See Section 10.3 for more details.

A MaxAge advertisement is removed entirely from the router's link state

database when a) it is no longer contained on any neighbor Link state

retransmission lists and b) none of the router's neighbors are in states

Exchange or Loading.

When, in the process of aging the link state database, an

advertisement's age hits a multiple of CheckAge, its checksum should be

verified. If the checksum is incorrect, a program or memory error has

been detected, and at the very least the router itself should be

restarted.

14.1 Premature aging of advertisements

A link state advertisement can be flushed from the routing domain by

setting its age to MaxAge and reflooding the advertisement. This

procedure follows the same course as flushing an advertisement whose age

has naturally reached the value MaxAge (see Section 14). In particular,

the MaxAge advertisement is removed from the router's link state

database as soon as a) it is no longer contained on any neighbor Link

state retransmission lists and b) none of the router's neighbors are in

states Exchange or Loading. We call the setting of an advertisement's

age to MaxAge premature aging.

Premature aging is used when it is time for a self-originated

advertisement's sequence number field to wrap. At this point, the

current advertisement instance (having LS sequence number of 0x7fffffff)

must be prematurely aged and flushed from the routing domain before a

new instance with sequence number 0x80000001 can be originated. See

Section 12.1.6 for more information.

Premature aging can also be used when, for example, one of the router's

previously advertised external routes is no longer reachable. In this

circumstance, the router can flush its external advertisement from the

routing domain via premature aging. This procedure is preferable to the

alternative, which is to originate a new advertisement for the

destination specifying a metric of LSInfinity.

A router may only prematurely age its own (self-originated) link state

advertisements. These are the link state advertisements having the

router's own OSPF Router ID in the Advertising Router field.

15. Virtual Links

The single backbone area (Area ID = 0) cannot be disconnected, or some

areas of the Autonomous System will become unreachable. To

establish/maintain connectivity of the backbone, virtual links can be

configured through non-backbone areas. Virtual links serve to connect

separate components of the backbone. The two endpoints of a virtual

link are area border routers. The virtual link must be configured in

both routers. The configuration information in each router consists of

the other virtual endpoint (the other area border router), and the non-

backbone area the two routers have in common (called the transit area).

Virtual links cannot be configured through stub areas (see Section 3.6).

The virtual link is treated as if it were an unnumbered point-to-point

network (belonging to the backbone) joining the two area border routers.

An attempt is made to establish an adjacency over the virtual link.

When this adjacency is established, the virtual link will be included in

backbone router links advertisements, and OSPF packets pertaining to the

backbone area will flow over the adjacency. Such an adjacency has been

referred to as a "virtual adjacency".

In each endpoint router, the cost and viability of the virtual link is

discovered by examining the routing table entry for the other endpoint

router. (The entry's associated area must be the configured transit

area). Actually, there may be a separate routing table entry for each

Type of Service. These are called the virtual link's corresponding

routing table entries. The Interface Up event occurs for a virtual link

when its corresponding TOS 0 routing table entry becomes reachable.

Conversely, the Interface Down event occurs when its TOS 0 routing table

entry becomes unreachable.[18] In other words, the virtual link's

viability is determined by the existence of an intra-area path, through

the transit area, between the two endpoints. The other details

concerning virtual links are as follows:

o AS external links are NEVER flooded over virtual adjacencies. This

would be duplication of effort, since the same AS external links are

already flooded throughout the virtual link's transit area. For

this same reason, AS external link advertisements are not summarized

over virtual adjacencies during the database exchange process.

o The cost of a virtual link is NOT configured. It is defined to be

the cost of the intra-area path between the two defining area border

routers. This cost appears in the virtual link's corresponding

routing table entry. When the cost of a virtual link changes, a new

router links advertisement should be originated for the backbone

area.

o Just as the virtual link's cost and viability are determined by the

routing table build process (through construction of the routing

table entry for the other endpoint), so are the IP interface address

for the virtual interface and the virtual neighbor's IP address.

These are used when sending protocol packets over the virtual link.

o In each endpoint's router links advertisement for the backbone, the

virtual link is represented as a link having link type 4, Link ID

set to the virtual neighbor's OSPF Router ID and Link Data set to

the virtual interface's IP address. See Section 12.4.1 for more

information. Also, it may be the case that there is a TOS 0 path,

but no non-zero TOS paths to the other endpoint router. In this

case, non-zero TOS costs must be set to LSInfinity in the router

links advertisement.

o When virtual links are configured for the backbone, information

concerning backbone networks should not be condensed before being

summarized for the transit areas. In other words, each backbone

network should be advertised in a separate summary link

advertisement, regardless of the backbone's configured area address

ranges. See Section 12.4.3 for more information.

o The time between link state retransmissions, RxmtInterval, is

configured for a virtual link. This should be well over the

expected round-trip delay between the two routers. This may be hard

to estimate for a virtual link. It is better to err on the side of

making it too large.

16. Calculation Of The Routing Table

This section details the OSPF routing table calculation. Using its

attached areas' link state databases as input, a router runs the

following algorithm, building its routing table step by step. At each

step, the router must access individual pieces of the link state

databases (e.g., a router links advertisement originated by a certain

router). This access is performed by the lookup function discussed in

Section 12.2. The lookup process may return a link state advertisement

whose LS age is equal to MaxAge. Such an advertisement should not be

used in the routing table calculation, and is treated just as if the

lookup process had failed.

The OSPF routing table's organization is explained in Section 11. Two

examples of the routing table build process are presented in Sections

11.2 and 11.3. This process can be broken into the following steps:

(1) The present routing table is invalidated. The routing table is

built again from scratch. The old routing table is saved so that

changes in routing table entries can be identified.

(2) The intra-area routes are calculated by building the shortest path

tree for each attached area. In particular, all routing table

entries whose Destination type is "area border router" are

calculated in this step. This step is described in two parts. At

first the tree is constructed by only considering those links

between routers and transit networks. Then the stub networks are

incorporated into the tree.

(3) The inter-area routes are calculated, through examination of summary

link advertisements. If the router is attached to multiple areas

(i.e., it is an area border router), only backbone summary link

advertisements are examined.

(4) For those routing entries whose next hop is over a virtual link, a

real (physical) next hop is calculated. The real next hop will be

on one of the router's directly attached networks. This step only

concerns routers having configured virtual links.

(5) Routes to external destinations are calculated, through examination

of AS external link advertisements. The location of the AS boundary

routers (which originate the AS external link advertisements) has

been determined in steps 2-4.

Steps 2-5 are explained in further detail below. The explanations

describe the calculations for TOS 0 only. It may also be necessary to

perform each step (separately) for each of the non-zero TOS values.[19]

For more information concerning the building of non-zero TOS routes see

Section 16.9.

Changes made to routing table entries as a result of these calculations

can cause the OSPF protocol to take further actions. For example, a

change to an intra-area route will cause an area border router to

originate new summary link advertisements (see Section 12.4). See

Section 16.7 for a complete list of the OSPF protocol actions resulting

from routing table table changes.

16.1 Calculating the shortest-path tree for an area

This calculation yields the set of intra-area routes associated with an

area (called hereafter Area A). A router calculates the shortest-path

tree using itself as the root.[20] The formation of the shortest path

tree is done here in two stages. In the first stage, only links between

routers and transit networks are considered. Using the Dijkstra

algorithm, a tree is formed from this subset of the link state database.

In the second stage, leaves are added to the tree by considering the

links to stub networks.

The procedure will be explained using the graph terminology that was

introduced in Section 2. The area's link state database is represented

as a directed graph. The graph's vertices are routers, transit networks

and stub networks. The first stage of the procedure concerns only the

transit vertices (routers and transit networks) and their connecting

links. Throughout the shortest path calculation, the following data is

also associated with each transit vertex:

Vertex (node) ID

A 32-bit number uniquely identifying the vertex. For router

vertices this is the OSPF Router ID. For network vertices, this is

the IP address of the network's Designated Router.

A link state advertisement

Each transit vertex has an associated link state advertisement. For

router vertices, this is a router links advertisement. For transit

networks, this is a network links advertisement (which is actually

originated by the network's Designated Router). In any case, the

advertisement's Link State ID is always equal to the above Vertex

ID.

List of next hops

The list of next hops for the current shortest paths from the root

to this vertex. There can be multiple shortest paths due to the

equal-cost multipath capability. Each next hop indicates the

outgoing router interface to use when forwarding traffic to the

destination. On multi-access networks, the next hop also includes

the IP address of the next router (if any) in the path towards the

destination.

Distance from root

The link state cost of the current shortest path(s) from the root to

the vertex. The link state cost of a path is calculated as the sum

of the costs of the path's constituent links (as advertised in

router links and network links advertisements). One path is said to

be "shorter" than another if it has a smaller link state cost.

The first stage of the procedure can now be summarized as follows. At

each iteration of the algorithm, there is a list of candidate vertices.

The shortest paths from the root to these vertices have not

(necessarily) been found. The candidate vertex closest to the root is

added to the shortest-path tree, removed from the candidate list, and

its adjacent vertices are examined for possible addition to/modification

of the candidate list. The algorithm then iterates again. It

terminates when the candidate list becomes empty.

The following steps describe the first stage in detail. Remember that

we are computing the shortest path tree for Area A. All references to

link state database lookup below are from Area A's database.

(1) Initialize the algorithm's data structures. Clear the list of

candidate vertices. Initialize the shortest-path tree to only the

root (which is the router doing the calculation).

(2) Call the vertex just added to the tree vertex V. Examine the link

state advertisement associated with vertex V. This is a lookup in

the area link state database based on the Vertex ID. Each link

described by the advertisement gives the cost to an adjacent vertex.

For each described link, (say it joins vertex V to vertex W):

(a) If this is a link to a stub network, examine the next link in

V's advertisement. Links to stub networks will be considered in

the second stage of the shortest path calculation.

(b) Otherwise, W is a transit vertex (router or transit network).

Look up the vertex W's link state advertisement (router links or

network links) in Area A's link state database. If the

advertisement does not exist, or its age is equal to MaxAge, or

it does not have a link back to vertex V, examine the next link

in V's advertisement. Both ends of a link must advertise the

link before it will be used for data traffic.[21]

(c) If vertex W is already on the shortest-path tree, examine the

next link in the advertisement.

(d) If the cost of the link (from V to W) is LSInfinity, the link

should not be used for data traffic. In this case, examine the

next link in the advertisement.

(e) Calculate the link state cost D of the resulting path from the

root to vertex W. D is equal to the sum of the link state cost

of the (already calculated) shortest path to vertex V and the

advertised cost of the link between vertices V and W. If D is:

o Greater than the value that already appears for vertex W on

the candidate list, then examine the next link.

o Equal to the value that appears for vertex W on the the

candidate list, calculate the set of next hops that result

from using the advertised link. Input to this calculation

is the destination (W), and its parent (V). This

calculation is shown in Section 16.1.1. This set of hops

should be added to the next hop values that appear for W on

the candidate list.

o Less than the value that appears for vertex W on the the

candidate list, or if W does not yet appear on the candidate

list, then set the entry for W on the candidate list to

indicate a distance of D from the root. Also calculate the

list of next hops that result from using the advertised

link, setting the next hop values for W accordingly. The

next hop calculation is described in Section 16.1.1; it

takes as input the destination (W) and its parent (V).

(3) If at this step the candidate list is empty, the shortest-path tree

(of transit vertices) has been completely built and this stage of

the algorithm terminates. Otherwise, choose the vertex belonging to

the candidate list that is closest to the root, and add it to the

shortest-path tree (removing it from the candidate list in the

process).

(4) Possibly modify the routing table. For those routing table entries

modified, the associated area will be set to Area A, the path type

will be set to intra-area, and the cost will be set to the newly

discovered shortest path's calculated distance.

If the newly added vertex is an area border router, a routing table

entry is added whose destination type is "area border router". The

Options field found in the associated router links advertisement is

copied into the routing table entry's Optional capabilities field.

If the newly added vertex is an AS boundary router, the routing

table entry of type "AS boundary router" for the destination is

located. Since routers can belong to more than one area, it is

possible that several sets of intra-area paths exist to the AS

boundary router, each set using a different area. However, the AS

boundary router's routing table entry must indicate a set of paths

which utilize a single area. The area leading to the routing table

entry is selected as follows: A set of intra-area paths having no

virtual next hops is always preferred over a set of intra-area paths

in which some virtual next hops appear[22] ; all other things being

equal the set of paths having lower cost is preferred. Note that

whenever an AS boundary router's routing table entry is

added/modified, the Options found in the associated router links

advertisement is copied into the routing table entry's Optional

capabilities field.

If the newly added vertex is a transit network, the routing table

entry for the network is located. The entry's destination ID is the

IP network number, which can be obtained by masking the Vertex ID

(Link State ID) with its associated subnet mask (found in the

associated network links advertisement). If the routing table entry

already exists (i.e., there is already an intra-area route to the

destination installed in the routing table), multiple vertices have

mapped to the same IP network. For example, this can occur when a

new Designated Router is being established. In this case, the

current routing table entry should be overwritten if and only if the

newly found path is just as short and the current routing table

entry's Link State Origin has a smaller Link State ID than the newly

added vertex' link state advertisement.

If there is no routing table entry for the network (the usual case),

a routing table entry for the IP network should be added. The

routing table entry's Link State origin should be set to the newly

added vertex' link state advertisement.

(5) Iterate the algorithm by returning to Step 2.

The stub networks are added to the tree in the procedure's second stage.

In this stage, all router vertices are again examined. Those that have

been determined to be unreachable in the above first phase are

discarded. For each reachable router vertex (call it V), the associated

router links advertisement is found in the link state database. Each

stub network link appearing in the advertisement is then examined, and

the following steps are executed:

(1) If the cost of the stub network link is LSInfinity, the link should

not be used for data traffic. In this case, go on to examine the

next stub network link in the advertisement.

(2) Otherwise, Calculate the distance D of stub network from the root.

D is equal to the distance from the root to the router vertex

(calculated in stage 1), plus the stub network link's advertised

cost. Compare this distance to the current best cost to the stub

network. This is done by looking up the network's current routing

table entry. If the calculated distance D is larger, go on to

examine the next stub network link in the advertisement.

(3) If this step is reached, the stub network's routing table entry must

be updated. Calculate the set of next hops that would result from

using the stub network link. This calculation is shown in Section

16.1.1; input to this calculation is the destination (the stub

network) and the parent vertex (the router vertex). If the distance

D is the same as the current routing table cost, simply add this set

of next hops to the routing table entry's list of next hops. In

this case, the routing table already has a Link State origin. If

this Link State origin is a router links advertisement whose Link

State ID is smaller than V's Router ID, reset the Link State origin

to V's router links advertisement.

Otherwise D is smaller than the routing table cost. Overwrite the

current routing table entry by setting the routing table entry's

cost to D, and by setting the entry's list of next hops to the newly

calculated set. Set the routing table entry's Link State origin to

V's router links advertisement. Then go on to examine the next stub

network link.

For all routing table entries added/modified in the second stage, the

associated area will be set to Area A and the path type will be set to

intra-area. When the list of reachable router links is exhausted, the

second stage is completed. At this time, all intra-area routes

associated with Area A have been determined.

The specification does not require that the above two stage method be

used to calculate the shortest path tree. However, if another algorithm

is used, an identical tree must be produced. For this reason, it is

important to note that links between transit vertices must be

bidirectional in ordered to be included in the above tree. It should

also be mentioned that algorithms exist for incrementally updating the

shortest-path tree (see [BBN]).

16.1.1 The next hop calculation

This section explains how to calculate the current set of next hops to

use for a destination. Each next hop consists of the outgoing interface

to use in forwarding packets to the destination together with the next

hop router (if any). The next hop calculation is invoked each time a

shorter path to the destination is discovered. This can happen in

either stage of the shortest-path tree calculation (see Section 16.1).

In stage 1 of the shortest-path tree calculation a shorter path is found

as the destination is added to the candidate list, or when the

destination's entry on the candidate list is modified (Step 2e of Stage

1). In stage 2 a shorter path is discovered each time the destination's

routing table entry is modified (Step 3 of Stage 2).

The set of next hops to use for the destination may be recalculated

several times during the shortest-path tree calculation, as shorter and

shorter paths are discovered. In the end, the destination's routing

table entry will always reflect the next hops resulting from the

absolute shortest path(s).

Input to the next hop calculation is a) the destination and b) its

parent in the current shortest path between the root (the calculating

router) and the destination. The parent is always a transit vertex

(i.e., always a router or a transit network).

If there is at least one intervening router in the current shortest path

between the destination and the root, the destination simply inherits

the set of next hops from the parent. Otherwise, there are two cases.

In the first case, the parent vertex is the root (the calculating router

itself). This means that the destination is either a directly connected

network or directly connected router. The next hop in this case is

simply the OSPF interface connecting to the network/router; no next hop

router is required.

In the second case, the destination is a router, and its parent vertex

is a network. The list of next hops is then determined by examining the

destination's router links advertisement. For each link in the

advertisement that points back to the parent network, the link's Link

Data field provides the IP address of a next hop router. The outgoing

interface to use can then be derived from the next hop IP address (or it

can be inherited from the parent network).

16.2 Calculating the inter-area routes

The inter-area routes are calculated by examining summary link

advertisements. If the router has active attachments to multiple areas,

only backbone summary link advertisements are examined. Routers

attached to a single area examine that area's summary links. In either

case, the summary links examined below are all part of a single area's

link state database (call it Area A).

Summary link advertisements are originated by the area border routers.

Each summary link advertisement in Area A is considered in turn.

Remember that the destination described by a summary link advertisement

is either a network (type 3 summary link advertisements) or an AS

boundary router (type 4 summary link advertisements). For each summary

link advertisement:

(1) If the cost specified by the advertisement is LSInfinity, then

examine the next advertisement.

(2) If the advertisement was originated by the calculating router

itself, examine the next advertisement.

(3) If the collection of destinations described by the summary link

falls into one of the router's configured area address ranges (see

Section 3.5) and the particular area address range is active, the

summary link should be ignored. Active means that there are one or

more reachable (by intra-area paths) networks contained in the area

range. In this case, all addresses in the area range are assumed to

be either reachable via intra-area paths, or else to be unreachable

by any other means.

(4) Else, call the destination described by the advertisement N, and the

area border originating the advertisement BR. Look up the routing

table entry for BR having A as its associated area. If no such

entry exists for router BR (i.e., BR is unreachable in Area A), do

nothing with this advertisement and consider the next in the list.

Else, this advertisement describes an inter-area path to destination

N, whose cost is the distance to BR plus the cost specified in the

advertisement. Call the cost of this inter-area path IAC.

(5) Next, look up the routing table entry for the destination N. (The

entry's Destination type is either Network or AS boundary router.)

If no entry exists for N or if the entry's path type is "AS

external", install the inter-area path to N, with associated area A,

cost IAC, next hop equal to the list of next hops to router BR, and

advertising router equal to BR.

(6) Else, if the paths present in the table are intra-area paths, do

nothing with the advertisement (intra-area paths are always

preferred).

(7) Else, the paths present in the routing table are also inter-area

paths. Install the new path through BR if it is cheaper, overriding

the paths in the routing table. Otherwise, if the new path is the

same cost, add it to the list of paths that appear in the routing

table entry.

16.3 Resolving virtual next hops

This step is only necessary in area-border routers having configured

virtual links. In these routers, some of the routing table entries may

have virtual next hops. That is, one or more of the next hops installed

in Sections 16.1 and 16.2 may be over a virtual link. However, when

forwarding data traffic to a destination, the next hops must always be

on a directly attached network.

In this section, each virtual next hop is replaced by a real next hop.

In the process a new routing table distance is calculated that may be

smaller than the previously calculated distance. In this case, the list

of next hops is pruned so that only those giving rise to the new

shortest distance are included, and the routing table entry's distance

is updated accordingly.

______________________________________

(Figure not included in text version.)

Figure 17: Resolving virtual next hops

______________________________________

This resolution of virtual next hops is done only for Destination types

Network or AS Boundary router. Suppose that one of a routing table

entry's next hops is a virtual link. This is determined by the

following combination: the routing table entry's path type is either

intra-area or inter-area, the area associated with the routing table

entry must be the backbone, yet the next hop belongs to a different area

(the virtual link's transit area).

Let N be the above entry's destination, and A the virtual link's transit

area. The real next hop (and new distance) is calculated as follows.

Let D be a distance counter, and set the real next hop NH to null.

Then, look up all the summary link advertisements for N in area A's

database, performing the following steps for each advertisement:[23]

(1) Call the border router that originated the advertisement BR. If

there is no routing table entry for BR having A as associated area

(i.e., BR is unreachable through Area A), examine the next

advertisement.

(2) Else, let X be the distance to BR via Area A. If the cost

advertised by BR (call it Y) to the destination is LSInfinity,

examine the next summary link advertisement. Else, the cost to

destination N through area border router BR is X+Y.

(3) If next hop NH is null or X+Y is smaller is smaller than D, set D to

X+Y and set the next hop NH to the next hop specified in router BR's

routing table entry.

At this point, the real next hop NH should be set, and the distance D

calculated should be less than or equal to the cost originally specified

in destination N's routing table entry. This same calculation should be

done for all of N's virtual next hops, and then N's new cost set to the

minimum calculated distance, with the its new set of next hops that

combination of non-virtual and recalculated next hops that correspond to

this (possibly same as original) distance.

The resolving of virtual next hops may produce unexpected results.

After the virtual next hops are resolved, traffic that was originally

scheduled to go over the virtual link may instead take a different path

through the virtual link's transit area. In other words, virtual links

allow transit traffic to be forwarded through an area, but do not

dictate the precise path that the traffic will take.

As an example, consider the Autonomous System pictured in Figure 17.

There is a single non-backbone area (Area 1) that physically divides the

backbone into two separate pieces. To maintain connectivity of the

backbone, a virtual link has been configured between routers RT1 and

RT4. On the right side of the figure, network N1 belongs to the

backbone. The dotted lines indicate that there is a much shorter

intra-area backbone path between router RT5 and network N1 (cost 20)

than there is between router RT4 and network N1 (cost 100). Both router

RT4 and router RT5 will inject summary link advertisements for network

N1 into Area 1.

After the shortest-path tree has been calculated for the backbone,

router RT1 (one end of the virtual link) will have selected router RT4

as the virtual next hop for all data traffic destined for network N1.

However, since router RT5 is so much closer to network N1, all routers

internal to Area 1 (e.g., routers RT2 and RT3) will forward their

network N1 traffic towards router RT5, instead of RT4. And indeed,

after resolving the virtual next hop by the above calculation, router

RT1 will also forward network N1 traffic towards RT5. So, in this

example the virtual link enables network N1 traffic to be forwarded

through the transit Area 1, but the actual path the data traffic takes

does not follow the virtual link.

16.4 Calculating AS external routes

AS external routes are calculated by examining AS external link

advertisements. Each of the AS external link advertisements is

considered in turn. Most AS external advertisements describe routes to

specific IP destinations. An AS external advertisement can also

describe a default route for the Autonomous System (destination =

DefaultDestination). For each AS external link advertisement:

(1) If the cost specified by the advertisement is LSInfinity, then

examine the next advertisement.

(2) If the advertisement was originated by the calculating router

itself, examine the next advertisement.

(3) Call the destination described by the advertisement N. Look up the

routing table entry for the AS boundary router (ASBR) that

originated the advertisement. If no entry exists for router ASBR

(i.e., ASBR is unreachable), do nothing with this advertisement and

consider the next in the list.

Else, this advertisement describes an AS external path to

destination N. Examine the forwarding address specified in the

external advertisement. This indicates the IP address to which

packets for the destination should be forwarded. If forwarding

address is set to 0.0.0.0, packets should be sent to the ASBR

itself. Otherwise, look up the forwarding address in the routing

table.[24] An intra-area or inter-area path must exist to the

forwarding address. If no such path exists, do nothing with the

advertisement and consider the next in the list.

Call the routing table distance to the forwarding address X (when

the forwarding address is set to 0.0.0.0, this is the distance to

the ASBR itself), and the cost specified in the advertisement Y. X

is in terms of the link state metric, and Y is a Type 1 or 2

external metric.

(4) Next, look up the routing table entry for the destination N. If no

entry exists for N, install the AS external path to N, with next hop

equal to the list of next hops to the forwarding address, and

advertising router equal to ASBR. If the external metric type is 1,

then the path-type is set to type 1 external and the cost is equal

to X+Y. If the external metric type is 2, the the path-type is set

to type 2 external, the link state component of the route's cost is

X, and the Type 2 cost is Y.

(5) Else, if the paths present in the table are not type 1 or type 2

external paths, do nothing (AS external paths have the lowest

priority).

(6) Otherwise, compare the cost of this new AS external path to the ones

present in the table. Type 1 external paths are always shorter than

Type 2 external paths. Type 1 external paths are compared by

looking at the sum of the distance to the forwarding address and the

advertised Type 1 metric (X+Y). Type 2 external paths are compared

by looking at the advertised Type 2 metrics, and then if necessary,

the distance to the forwarding addresses.

If the new path is shorter, it replaces the present paths in the

routing table entry. If the new path is the same cost, it is added

to the routing table entry's list of paths.

16.5 Incremental updates --- summary links

When a new summary link advertisement is received, it is not necessary

to recalculate the entire routing table. Call the destination described

by the summary link advertisement N, and let A be the area to which the

advertisement belongs.

Look up the routing table entry for N. If the next hop to N is a

virtual link through Area A (this means that the entry's associated area

is the backbone, and the listed next hop does not belong to the

backbone, but instead belongs to Area A), the real next hop must again

be resolved. This means running the algorithm in Section 16.3 for

destination N only.

Else, if there is an intra-area route to destination N nothing need be

done (intra-area routes always take precedence). Otherwise, if Area A

is the router's sole attached area, or Area A is the backbone, the

procedure in Section 16.2 will have to be performed, but only for those

summary link advertisements whose destination is N. Before this

procedure is performed, the present routing table entry for N should be

invalidated (but kept for comparison purposes). If this procedure leads

to a virtual next hop, the algorithm in Section 16.3 will again have to

be performed in order to calculate the real next hop.

If N's routing table entry changes, and N is an AS boundary router, the

AS external links will have to be reexamined (Section 16.4).

16.6 Incremental updates --- AS external links

When a new AS external link advertisement is received, it is not

necessary to recalculate the entire routing table. Call the destination

described by the AS external link advertisement N. If there is already

an intra-area or inter-area route to the destination, no recalculation

is necessary (these routes take precedence).

Otherwise, the procedure in Section 16.4 will have to be performed, but

only for those AS external link advertisements whose destination is N.

Before this procedure is performed, the present routing table entry for

N should be invalidated.

16.7 Events generated as a result of routing table changes

Changes to routing table entries sometimes cause the OSPF area border

routers to take additional actions. These routers need to act on the

following routing table changes:

o The cost or path type of a routing table entry has changed. If the

destination described by this entry is a Network or AS boundary

router, and this is not simply a change of AS external routes, new

summary link advertisements may have to be generated (potentially

one for each attached area, including the backbone). See Section

12.4.3 for more information. If a previously advertised entry has

been deleted, or is no longer advertisable to a particular area, the

advertisement must be flushed from the routing domain by setting its

age to MaxAge and reflooding (see Section 14.1).

o A routing table entry associated with a configured virtual link has

changed. The destination of such a routing table entry is an area

border router. The change indicates a modification to the virtual

link's cost or viability.

If the entry indicates that the area border router is newly

reachable (via TOS 0), the corresponding virtual link is now

operational. An Interface Up event should be generated for the

virtual link, which will cause a virtual adjacency to begin to form

(see Section 10.3). At this time the virtual interface's IP address

and the virtual neighbor's IP address are also calculated.

If the entry indicates that the area border router is no longer

reachable (via TOS 0), the virtual link and its associated adjacency

should be destroyed. This means an Interface Down event should be

generated for the associated virtual link.

If the cost of the entry has changed, and there is a fully

established virtual adjacency, a new router links advertisement for

the backbone must be originated. This in turn may cause further

routing table changes.

16.8 Equal-cost multipath

The OSPF protocol maintains multiple equal-cost routes to all

destinations. This can be seen in the steps used above to calculate the

routing table, and in the definition of the routing table structure.

Each one of the multiple routes will be of the same type (intra-area,

inter-area, type 1 external or type 2 external), cost, and will have the

same associated area. However, each route specifies a separate next hop

and advertising router.

There is no requirement that a router running OSPF keep track of all

possible equal-cost routes to a destination. An implementation may

choose to keep only a fixed number of routes to any given destination.

This does not affect any of the algorithms presented in this

specification.

16.9 Building the non-zero-TOS portion of the routing table

The OSPF protocol can calculate a different set of routes for each IP

TOS (see Section 2.4). Support for TOS-based routing is optional.

TOS-capable and non-TOS-capable routers can be mixed in an OSPF routing

domain. Routers not supporting TOS calculate only the TOS 0 route to

each destination. These routes are then used to forward all data

traffic, regardless of the TOS indications in the data packet's IP

header. A router that does not support TOS indicates this fact to the

other OSPF routers by clearing the T-bit in the Options field of its

router links advertisement.

The above sections detailing the routing table calculations handle the

TOS 0 case only. In general, for routers supporting TOS-based routing,

each piece of the routing table calculation must be rerun separately for

the non-zero TOS values. When calculating routes for TOS X, only TOS X

metrics can be used. Any link state advertisement may specify a

separate cost for each TOS (a cost for TOS 0 must always be specified).

The encoding of TOS in OSPF link state advertisements is described in

Section 12.3.

An advertisement can specify that it is restricted to TOS 0 (i.e., non-

zero TOS is not handled) by clearing the T-bit in the link state

advertisement's Option field. Such advertisements are not used when

calculating routes for non-zero TOS. For this reason, it is possible

that a destination is unreachable for some non-zero TOS. In this case,

the TOS 0 path is used when forwarding packets (see Section 11.1).

The following lists the modifications needed when running the routing

table calculation for a non-zero TOS value (called TOS X). In general,

routers and advertisements that do not support TOS are omitted from the

calculation.

Calculating the shortest-path tree (Section 16.1).

Routers that do not support TOS-based routing should be omitted from

the shortest-path tree calculation. These routers are identified as

those having the T-bit reset in their router links advertisements.

Such routers should never be added to the Dijktra algorithm's

candidate list, nor should their router links advertisements be

examined when adding the stub networks to the tree.

Calculating the inter-area routes (Section 16.2).

Inter-area paths are the concatenation of a path to an area border

router with a summary link. When calculating TOS X routes, both

path components must also specify TOS X. In other words, only TOS X

paths to the area border router are examined, and the area border

router must be advertising a TOS X route to the destination. Note

that this means that summary link advertisements having the T-bit

reset in their Options field are not considered.

Resolving virtual next hops (Section 16.3).

This calculation again considers the concatenation of a path to an

area border router with a summary link. As with inter-area routes,

only TOS X paths to the area border router are examined, and the

area border router must be advertising a TOS X route to the

destination.

Calculating AS external routes (Section 16.4).

This calculation considers the concatenation of a path to a

forwarding address with an AS external link. Only TOS X paths to

the forwarding address are examined, and the AS boundary router must

be advertising a TOS X route to the destination. Note that this

means that AS external link advertisements having the T-bit reset in

their Options field are not considered.

In addition, the advertising AS boundary router must also be

reachable for its advertisements to be considered (see Section

16.4). However, if the advertising router and the forwarding

address are not one in the same, the advertising router need only be

reachable via TOS 0.

[1]The graph's vertices represent either routers, transit networks,

or stub networks. Since routers may belong to multiple areas, it is

not possible to color the graph's vertices.

[2]It is possible for all of a router's interfaces to be unnumbered

point-to-point links. In this case, an IP address must be assigned

to the router. This address will then be advertised in the router's

router links advertisement as a host route.

[3]Note that in these cases both interfaces, the non-virtual and the

virtual, would have the same IP address.

[4]Note that no host route is generated for, and no IP packets can

be addressed to, interfaces to unnumbered point-to-point networks.

This is regardless of such an interface's state.

[5]It is instructive to see what happens when the Designated Router

for the network crashes. Call the Designated Router for the network

RT1, and the the Backup Designated Router RT2. If router RT1

crashes (or maybe its interface to the network dies), the other

routers on the network will detect RT1's absence within

RouterDeadInterval seconds. All routers may not detect this at

precisely the same time; the routers that detect RT1's absence

before RT2 does will, for a time, select RT2 to be both Designated

Router and Backup Designated Router. When RT2 detects that RT1 is

gone it will move itself to Designated Router. At this time, the

remaining router having highest Router Priority will be selected as

Backup Designated Router.

[6]On point-to-point networks, the lower level protocols indicate

whether the neighbor is up and running. Likewise, existence of the

neighbor on virtual links is indicated by the routing table

calculation. However, in both these cases, the Hello Protocol is

still used. This ensures that communication between the neighbors

is bidirectional, and that each of the neighbors has a functioning

routing protocol layer.

[7]When the identity of the Designated Router is changing, it may be

quite common for a neighbor in this state to send the router a

Database Description packet; this means that there is some momentary

disagreement on the Designated Router's identity.

[8]Note that it is possible for a router to resynchronize any of its

fully established adjacencies by setting the adjacency's state back

to ExStart. This will cause the other end of the adjacency to

process a Seq Number Mismatch event, and therefore to also go back

to ExStart state.

[9]The address space of IP networks and the address space of OSPF

Router IDs may overlap. That is, a network may have an IP address

which is identical (when considered as a 32-bit number) to some

router's Router ID.

[10]It is assumed that, for two different address ranges matching

the destination, one range is more specific than the other. Non-

contiguous subnet masks can be configured to violate this

assumption. Such subnet mask configurations cannot be handled by the

OSPF protocol.

[11]MaxAgeDiff is an architectural constant. It indicates the

maximum dispersion of ages, in seconds, that can occur for a single

link state instance as it is flooded throughout the routing domain.

If two advertisements differ by more than this, they are assumed to

be different instances of the same advertisement. This can occur

when a router restarts and loses track of its previous sequence

number. See Section 13.4 for more details.

[12]When two advertisements have different checksums, they are

assumed to be separate instances. This can occur when a router

restarts, and loses track of its previous sequence number. In this

case, since the two advertisements have the same sequence number, it

is not possible to determine which link state is actually newer. If

the wrong advertisement is accepted as newer, the originating router

will originate another instance. See Section 13.4 for further

details.

[13]There is one instance where a lookup must be done based on

partial information. This is during the routing table calculation,

when a network links advertisement must be found based solely on its

Link State ID. The lookup in this case is still well defined, since

no two network advertisements can have the same Link State ID.

[14]This clause covers the case: Inter-area routes are not

summarized to the backbone. This is because inter-area routes are

always associated with the backbone area.

[15]By keeping more information in the routing table, it is possible

for an implementation to recalculate the shortest path tree only for

a single area. In fact, there are incremental algorithms that allow

an implementation to recalculate only a portion of the shortest path

tree [BBN]. These algorithms are beyond the scope of this

specification.

[16]This is how the Link state request list is emptied, which

eventually causes the neighbor state to transition to Full. See

Section 10.9 for more details.

[17]It should be a relatively rare occurrence for an advertisement's

age to reach MaxAge. Usually, the advertisement will be replaced by

a more recent instance before it ages out.

[18]Only the TOS 0 routes are important here. This is because all

routing protocol packets are sent with TOS= 0. See Appendix A.

[19]It may be the case that paths to certain destinations do not

vary based on TOS. For these destinations, the routing calculation

need not be repeated for each TOS value. In addition, there need

only be a single routing table entry for these destinations (instead

of a separate entry for each TOS value).

[20]Strictly speaking, because of equal-cost multipath, the

algorithm does not create a tree. We continue to use the "tree"

terminology because that is what occurs most often in the existing

literature.

[21]This means that before data traffic will flow between a pair of

neighboring routers, their link state databases must be

synchronized. Before synchronization (neighbor state < Full), a

router will not include the connection to its neighbor in its link

state advertisements.

[22]As a result of this clause, when a virtual link exists between

the calculating router and an AS boundary router, the intra-area

path through the virtual link's transit area is always preferred

over the virtual link itself.

[23]Note the similarity between this procedure and the calculation

of inter-area routes by a router internal to Area A.

[24]When the forwarding address is non-zero, it should point to a

router belonging to another Autonomous System. See Section 12.4.4

for more details.

References

[BBN] McQuillan, J.M., Richer, I. and Rosen, E.C. ARPANET

Routing Algorithm Improvements. BBN Technical Report 3803,

April 1978.

[DEC] Digital Equipment Corporation. Information processing

systems -- Data communications -- Intermediate System to

Intermediate System Intra-Domain Routing Protocol. October

1987.

[McQuillan] McQuillan, J. et.al. The New Routing Algorithm for the

Arpanet. IEEE Transactions on Communications, May 1980.

[Perlman] Perlman, Radia. Fault-Tolerant Broadcast of Routing

Information. Computer Networks, Dec. 1983.

[RFC791] Postel, Jon. Internet Protocol. September 1981

[RFC944] ANSI X3S3.3 86-60. Final Text of DIS 8473, Protocol for

Providing the Connectionless-mode Network Service. March

1986.

[RFC1060] Reynolds, J. and Postel, J. Assigned Numbers. March 1990.

[RFC1112] Deering, S.E. Host extensions for IP multicasting. May

1988.

[RFC1131] Moy, J. The OSPF Specification. October 1989.

[RS-85-153] Leiner, Dr. Barry M., et.al. The DARPA Internet Protocol

Suite. DDN Protocol Handbook, April 1985.

A. OSPF data formats

This appendix describes the format of OSPF protocol packets and OSPF

link state advertisements. The OSPF protocol runs directly over the IP

network layer. Before any data formats are described, the details of

the OSPF encapsulation are explained.

Next the OSPF options field is described. This field describes various

capabilities that may or may not be supported by pieces of the OSPF

routing domain. It is contained both in OSPF protocol packets and in

OSPF link state advertisements.

OSPF packet formats are detailed in Section A.3. A description of OSPF

link state advertisements appears in Section A.4.

A.1 Encapsulation of OSPF packets

OSPF runs directly over the Internet Protocol's network layer. OSPF

packets are therefore encapsulated solely by IP and local network

headers.

OSPF does not define a way to fragment its protocol packets, and depends

on IP fragmentation when transmitting packets larger than the network

MTU. The OSPF packet types that are likely to be large (Database

Description Packets, Link State Request, Link State Update, and Link

State Acknowledgment packets) can usually be split into several separate

protocol packets, without loss of functionality. This is recommended;

IP fragmentation should be avoided whenever possible. Using this

reasoning, an attempt should be made to limit the sizes of packets sent

over virtual links to 576 bytes. However, if necessary, the length of

OSPF packets can be up to 65,535 bytes (including the IP header).

The other important features of OSPF's IP encapsulation are:

o Use of IP multicast. Some OSPF messages are multicast, when sent

over multi-access networks. Two distinct IP multicast addresses are

used. Packets destined to these multicast addresses should never be

forwarded. Such packets are meant to travel a single hop only. To

ensure that these packets will not travel multiple hops, their IP

TTL must be set to 1.

AllSPFRouters

This multicast address has been assigned the value 224.0.0.5.

All routers running OSPF should be prepared to receive packets

sent to this address. Hello packets are always sent to this

destination. Also, certain protocol packets are sent to this

address during the flooding procedure.

AllDRouters

This multicast address has been assigned the value 224.0.0.6.

Both the Designated Router and Backup Designated Router must be

prepared to receive packets destined to this address. Certain

packets are sent to this address during the flooding procedure.

o OSPF is IP protocol number 89. This number has been registered with

the Network Information Center. IP protocol number assignments are

documented in [RFC1060].

o Routing protocol packets are sent with IP TOS of 0. The OSPF

protocol supports TOS-based routing. Routes to any particular

destination may vary based on TOS. However, all OSPF routing

protocol packets are sent with the DTR bits in the IP header's TOS

field (see [RFC791]) set to 0.

o Routing protocol packets are sent with IP precedence set to

Internetwork Control. OSPF protocol packets should be given

precedence over regular IP data traffic, in both sending and

receiving. Setting the IP precedence field in the IP header to

Internetwork Control [RFC791] may help implement this objective.

A.2 The options field

The OSPF options field is present in OSPF Hello packets, Database

Description packets and all link state advertisements. The options

field enables OSPF routers to support (or not support) optional

capabilities, and to communicate their capability level to other OSPF

routers. Through this mechanism routers of differing capabilities can

be mixed within an OSPF routing domain.

When used in Hello packets, the options field allows a router to reject

a neighbor because of a capability mismatch. Alternatively, when

capabilities are exchanged in Database Description packets a router can

choose not to forward certain LSA types to a neighbor because of its

reduced functionality. Lastly, listing capabilities in LSAs allows

routers to route traffic around reduced functionality routers, by

excluding them from parts of the routing table calculation.

Two capabilities are currently defined. For each capability, the effect

of the capability's appearance (or lack of appearance) in Hello packets,

Database Description packets and link state advertisements is specified

below. For example, the external routing capability (below called the

E-bit) has meaning only in OSPF Hello Packets. Routers should reset

(i.e. clear) the unassigned part of the capability field when sending

Hello packets or Database Description packets and when originating link

state advertisements.

Additional capabilities may be assigned in the future. Routers

encountering unrecognized capabilities in received Hello Packets,

Database Description packets or link state advertisements should ignore

the capability and process the packet/advertisement normally.

+-+-+-+-+-+-+-+-+

ET

+-+-+-+-+-+-+-+-+

The options field

T-bit

This describes the router's TOS capability. If the T-bit is reset,

then the router supports only a single TOS (TOS 0). Such a router

is also said to be incapable of TOS-routing. The absence of the T-

bit in a router links advertisement causes the router to be skipped

when building a non-zero TOS shortest-path tree (see Section 16.9).

In other words, routers incapable of TOS routing will be avoided as

much as possible when forwarding data traffic requesting a non-zero

TOS. The absence of the T-bit in a summary link advertisement or an

AS external link advertisement indicates that the advertisement is

describing a TOS 0 route only (and not routes for non-zero TOS).

E-bit

AS external link advertisements are not flooded into/through OSPF

stub areas (see Section 3.6). The E-bit ensures that all members of

a stub area agree on that area's configuration. The E-bit is

meaningful only in OSPF Hello packets. When the E-bit is reset in

the Hello packet sent out a particular interface, it means that the

router will neither send nor receive AS external link state

advertisements on that interface (in other words, the interface

connects to a stub area). Two routers will not become neighbors

unless they agree on the state of the E-bit.

A.3 OSPF Packet Formats

There are five distinct OSPF packet types. All OSPF packet types begin

with a standard 24 byte header. This header is described first. Each

packet type is then described in a succeeding section. In these

sections each packet's division into fields is displayed, and then the

field definitions are enumerated.

All OSPF packet types (other than the OSPF Hello packets) deal with

lists of link state advertisements. For example, Link State Update

packets implement the flooding of advertisements throughout the OSPF

routing domain. Because of this, OSPF protocol packets cannot be parsed

unless the format of link state advertisements is also understood. The

format of Link state advertisements is described in Section A.4.

The receive processing of OSPF packets is detailed in Section 8.2. The

sending of OSPF packets is explained in Section 8.1.

A.3.1 The OSPF packet header

Every OSPF packet starts with a common 24 byte header. This header

contains all the necessary information to determine whether the packet

should be accepted for further processing. This determination is

described in Section 8.2 of the specification.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version # Type Packet length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Router ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Area ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Checksum Autype

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version #

The OSPF version number. This specification documents version 2 of

the protocol.

Type

The OSPF packet types are as follows. The format of each of these

packet types is described in a succeeding section.

Type Description

________________________________

1 Hello

2 Database Description

3 Link State Request

4 Link State Update

5 Link State Acknowledgment

Packet length

The length of the protocol packet in bytes. This length includes

the standard OSPF header.

Router ID

The Router ID of the packet's source. In OSPF, the source and

destination of a routing protocol packet are the two ends of an

(potential) adjacency.

Area ID

A 32 bit number identifying the area that this packet belongs to.

All OSPF packets are associated with a single area. Most travel a

single hop only. Packets travelling over a virtual link are

labelled with the backbone area ID of 0.

Checksum

The standard IP checksum of the entire contents of the packet,

excluding the 64-bit authentication field. This checksum is

calculated as the 16-bit one's complement of the one's complement

sum of all the 16-bit words in the packet, excepting the

authentication field. If the packet's length is not an integral

number of 16-bit words, the packet is padded with a byte of zero

before checksumming.

AuType

Identifies the authentication scheme to be used for the packet.

Authentication is discussed in Appendix E of the specification.

Consult Appendix E for a list of the currently defined

authentication types.

Authentication

A 64-bit field for use by the authentication scheme.

A.3.2 The Hello packet

Hello packets are OSPF packet type 1. These packets are sent

periodically on all interfaces (including virtual links) in order to

establish and maintain neighbor relationships. In addition, Hellos are

multicast on those physical networks having a multicast or broadcast

capability, enabling dynamic discovery of neighboring routers.

All routers connected to a common network must agree on certain

parameters (network mask, hello and dead intervals). These parameters

are included in Hello packets, so that differences can inhibit the

forming of neighbor relationships. A detailed explanation of the

receive processing for Hello packets is presented in Section 10.5. The

sending of Hello packets is covered in Section 9.5.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version # 1 Packet length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Router ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Area ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Checksum Autype

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Network Mask

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

HelloInt Options Rtr Pri

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

DeadInt

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Designated Router

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Backup Designated Router

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Neighbor

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

...

Network mask

The network mask associated with this interface. For example, if

the interface is to a class B network whose third byte is used for

subnetting, the network mask is 0xffffff00.

Options

The optional capabilities supported by the router, as documented in

Section A.2.

HelloInt

The number of seconds between this router's Hello packets.

Rtr Pri

This router's Router Priority. Used in (Backup) Designated Router

election. If set to 0, the router will be ineligible to become

(Backup) Designated Router.

Deadint

The number of seconds before declaring a silent router down.

Designated Router

The identity of the Designated Router for this network, in the view

of the advertising router. The Designated Router is identified here

by its IP interface address on the network. Set to 0 if there is no

Designated Router.

Backup Designated Router

The identity of the Backup Designated Router for this network, in

the view of the advertising router. The Backup Designated Router is

identified here by its IP interface address on the network. Set to

0 if there is no backup Designated Router.

Neighbor

The Router IDs of each router from whom valid Hello packets have

been seen recently on the network. Recently means in the last

DeadInt seconds.

A.3.3 The Database Description packet

Database Description packets are OSPF packet type 2. These packets are

exchanged when an adjacency is being initialized. They describe the

contents of the topological database. Multiple packets may be used to

describe the database. For this purpose a poll-response procedure is

used. One of the routers is designated to be master, the other a slave.

The master sends Database Description packets (polls) which are

acknowledged by Database Description packets sent by the slave

(responses). The responses are linked to the polls via the packets'

sequence numbers.

The format of the Database Description packet is very similar to both

the Link State Request and Link State Acknowledgment packets. The main

part of all three is a list of items, each item describing a piece of

the topological database. The sending of Database Description Packets

is documented in Section 10.8. The reception of Database Description

packets is documented in Section 10.6.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version # 2 Packet length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Router ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Area ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Checksum Autype

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

0 0 Options 00000IMMS

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

DD sequence number

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+- -+

A

+- Link State Advertisement -+

Header

+- -+

+- -+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

...

0 These fields are reserved. They must be 0.

Options

The optional capabilities supported by the router, as documented in

Section A.2.

I-bit

The Init bit. When set to 1, this packet is the first in the

sequence of database descriptions.

M-bit

The More bit. When set to 1, it indicates that more database

descriptions are to follow.

MS-bit

The Master/Slave bit. When set to 1, it indicates that the router

is the master during the database exchange process. Otherwise, the

router is the slave.

DD sequence number

Used to sequence the collection of database description packets.

The initial value (indicated by the Init bit being set) should be

unique. The sequence number then increments until the complete

database description has been sent.

The rest of the packet consists of a (possibly partial) list of the

topological database's pieces. Each link state advertisement in the

database is described by its link state header. The link state header

is documented in Section A.4.1. It contains all the information

required to uniquely identify both the advertisement and the

advertisement's current instance.

A.3.4 The Link State Request packet

Link State Request packets are OSPF packet type 3. After exchanging

Database Description packets with a neighboring router, a router may

find that parts of its topological database are out of date. The Link

State Request packet is used to request the pieces of the neighbor's

database that are more up to date. Multiple Link State Request packets

may need to be used. The sending of Link State Request packets is the

last step in bringing up an adjacency.

A router that sends a Link State Request packet has in mind the precise

instance of the database pieces it is requesting (defined by LS sequence

number, LS checksum, and LS age). It may receive even more recent

instances in response.

The sending of Link State Request packets is documented in Section 10.9.

The reception of Link State Request packets is documented in Section

10.7.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version # 3 Packet length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Router ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Area ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Checksum Autype

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS type

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Link State ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Advertising Router

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

...

Each advertisement requested is specified by its LS type, Link State ID,

and Advertising Router. This uniquely identifies the advertisement, but

not its instance. Link State Request packets are understood to be

requests for the most recent instance (whatever that might be).

A.3.5 The Link State Update packet

Link State Update packets are OSPF packet type 4. These packets

implement the flooding of link state advertisements. Each Link State

Update packet carries a collection of link state advertisements one hop

further from its origin. Several link state advertisements may be

included in a single packet.

Link State Update packets are multicast on those physical networks that

support multicast/broadcast. In order to make the flooding procedure

reliable, flooded advertisements are acknowledged in Link State

Acknowledgment packets. If retransmission of certain advertisements is

necessary, the retransmitted advertisements are always carried by

unicast Link State Update packets. For more information on the reliable

flooding of link state advertisements, consult Section 13.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version # 4 Packet length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Router ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Area ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Checksum Autype

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

# advertisements

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+- +-+

Link state advertisements

+- +-+

...

# advertisements

The number of link state advertisements included in this update.

The body of the Link State Update packet consists of a list of link

state advertisements. Each advertisement begins with a common 20 byte

header, the link state advertisement header. This header is described

in Section A.4.1. Otherwise, the format of each of the five types of

link state advertisements is different. Their formats are described in

Section A.4.

A.3.6 The Link State Acknowledgment packet

Link State Acknowledgment Packets are OSPF packet type 5. To make the

flooding of link state advertisements reliable, flooded advertisements

are explicitly acknowledged. This acknowledgment is accomplished

through the sending and receiving of Link State Acknowledgment packets.

Multiple link state advertisements can be acknowledged in a single

packet.

Depending on the state of the sending interface and the source of the

advertisements being acknowledged, a Link State Acknowledgment packet is

sent either to the multicast address AllSPFRouters, to the multicast

address AllDRouters, or as a unicast. The sending of Link State

Acknowledgement packets is documented in Section 13.5. The reception of

Link State Acknowledgement packets is documented in Section 13.7.

The format of this packet is similar to that of the Data Description

packet. The body of both packets is simply a list of link state

advertisement headers.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version # 5 Packet length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Router ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Area ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Checksum Autype

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+- -+

A

+- Link State Advertisement -+

Header

+- -+

+- -+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

...

Each acknowledged link state advertisement is described by its link

state header. The link state header is documented in Section A.4.1. It

contains all the information required to uniquely identify both the

advertisement and the advertisement's current instance.

A.4 Link state advertisement formats

There are five distinct types of link state advertisements. Each link

state advertisement begins with a standard 20-byte link state header.

This header is explained in Section A.4.1. Succeeding sections then

diagram the separate link state advertisement types.

Each link state advertisement describes a piece of the OSPF routing

domain. Every router originates a router links advertisement. In

addition, whenever the router is elected Designated Router, it

originates a network links advertisement. Other types of link state

advertisements may also be originated (see Section 12.4). All link

state advertisements are then flooded throughout the OSPF routing

domain. The flooding algorithm is reliable, ensuring that all routers

have the same collection of link state advertisements. (See Section 13

for more information concerning the flooding algorithm). This

collection of advertisements is called the link state (or topological)

database.

From the link state database, each router constructs a shortest path

tree with itself as root. This yields a routing table (see Section 11).

For the details of the routing table build process, see Section 16.

A.4.1 The Link State Advertisement header

All link state advertisements begin with a common 20 byte header. This

header contains enough information to uniquely identify the

advertisement (LS type, Link State ID, and Advertising Router).

Multiple instances of the link state advertisement may exist in the

routing domain at the same time. It is then necessary to determine

which instance is more recent. This is accomplished by examining the LS

age, LS sequence number and LS checksum fields that are also contained

in the link state advertisement header.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS age Options LS type

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Link State ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Advertising Router

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS sequence number

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS checksum length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS age

The time in seconds since the link state advertisement was

originated.

Options

The optional capabilities supported by the described portion of the

routing domain. OSPF's optional capabilities are documented in

Section A.2.

LS type

The type of the link state advertisement. Each link state type has

a separate advertisement format. The link state types are as

follows (see Section 12.1.3 for further explanation):

LS Type Description

___________________________________

1 Router links

2 Network links

3 Summary link (IP network)

4 Summary link (ASBR)

5 AS external link

Link State ID

This field identifies the portion of the internet environment that

is being described by the advertisement. The contents of this field

depend on the advertisement's LS type. For example, in network

links advertisements the Link State ID is set to the IP interface

address of the network's Designated Router (from which the network's

IP address can be derived). The Link State ID is further discussed

in Section 12.1.4.

Advertising Router

The Router ID of the router that originated the link state

advertisement. For example, in network links advertisements this

field is set to the Router ID of the network's Designated Router.

LS sequence number

Detects old or duplicate link state advertisements. Successive

instances of a link state advertisement are given successive LS

sequence numbers. See Section 12.1.6 for more details.

LS checksum

The Fletcher checksum of the complete contents of the link state

advertisement. See Section 12.1.7 for more details.

length

The length in bytes of the link state advertisement. This includes

the 20 byte link state header.

A.4.2 Router links advertisements

Router links advertisements are the Type 1 link state advertisements.

Each router in an area originates a router links advertisement. The

advertisement describes the state and cost of the router's links (or

interfaces) to the area. All of the router's links to the area must be

described in a single router links advertisement. For details

concerning the construction of router links advertisements, see Section

12.4.1.

In router links advertisements, the Link State ID field is set to the

router's OSPF Router ID. The T-bit is set in the advertisement's Option

field if and only if the router is able to calculate a separate set of

routes for each IP TOS. Router links advertisements are flooded

throughout a single area only.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS age Options 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Link State ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Advertising Router

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS sequence number

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS checksum length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

0 EB 0 # links

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Link ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Link Data

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type # TOS TOS 0 metric

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

TOS 0 metric

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

...

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

TOS 0 metric

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Link ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Link Data

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

...

bit E

When set, the router is an AS boundary router (E is for external)

bit B

When set, the router is an area border router (B is for border)

# links

The number of router links described by this advertisement. This

must be the total collection of router links to the area.

The following fields are used to describe each router link. Each router

link is typed (see the below Type field). The type field indicates the

kind of link being described. It may be a link to a transit network, to

another router or to a stub network. The values of all the other fields

describing a router link depend on the link's type. For example, each

link has an associated 32-bit data field. For links to stub networks

this field specifies the network's IP address mask. For the other link

types the Link Data specifies the router's associated IP interface

address.

Type

A quick description of the router link. One of the following. Note

that host routes are classified as links to stub networks whose

network mask is 0xffffffff.

Type Description

__________________________________________________

1 Point-to-point connection to another router

2 Connection to a transit network

3 Connection to a stub network

4 Virtual link

Link ID

Identifies the object that this router link connects to. Value

depends on the link's type. When connecting to an object that also

originates a link state advertisement (i.e., another router or a

transit network) the Link ID is equal to the other advertisement's

Link State ID. This provides the key for looking up said

advertisement in the link state database. See Section 12.2 for more

details.

Type Link ID

______________________________________

1 Neighboring router's ID

2 IP address of Designated Router

3 IP network/subnet number

4 Neighboring router's ID

Link Data

Contents again depend on the link's Type field. For connections to

stub network, it specifies the network mask. For the other link

types it specifies the router's associated IP interface address.

This latter piece of information is needed during the routing table

build process, when calculating the IP address of the next hop. See

Section 16.1.1 for more details.

#metrics

The number of different TOS metrics given for this link, not

counting the required metric for TOS 0. For example, if no

additional TOS metrics are given, this field should be set to 0.

TOS 0 metric

The cost of using this router link for TOS 0.

For each link, separate metrics may be specified for each Type of

Service (TOS). The metric for TOS 0 must always be included, and was

discussed above. Metrics for non-zero TOS are described below. The

encoding of TOS in OSPF link state advertisements is described in

Section 12.3. Note that the cost for non-zero TOS values that are not

specified defaults to the TOS 0 cost. Metrics must be listed in order

of increasing TOS encoding. For example, the metric for TOS 16 must

always follow the metric for TOS 8 when both are specified.

TOS IP type of service that this metric refers to. The encoding of TOS

in OSPF link state advertisements is described in Section 12.3.

metric

The cost of using this outbound router link, for traffic of the

specified TOS.

A.4.3 Network links advertisements

Network links advertisements are the Type 2 link state advertisements.

A network links advertisement is originated for each transit network in

the area. A transit network is a multi-access network that has more

than one attached router. The network links advertisement is originated

by the network's Designated Router. The advertisement describes all

routers attached to the network, including the Designated Router itself.

The advertisement's Link State ID field lists the IP interface address

of the Designated Router.

The distance from the network to all attached routers is zero, for all

types of service. This is why the TOS and metric fields need not be

specified in the network links advertisement. For details concerning

the construction of network links advertisements, see Section 12.4.2.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS age Options 2

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Link State ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Advertising Router

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS sequence number

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS checksum length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Network Mask

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Attached Router

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

...

Network Mask

The IP address mask for the network. For example, a class A network

would have the mask 0xff000000.

Attached Router

The Router IDs of each of the routers attached to the network.

Actually, only those routers that are fully adjacent to the

Designated Router are listed. The Designated Router includes itself

in this list. The number of routers included can be deduced from

the link state advertisement's length field.

A.4.4 Summary link advertisements

Summary link advertisements are the Type 3 and 4 link state

advertisements. These advertisements are originated by area border

routers. A separate summary link advertisement is made for each

destination (known to the router) which belongs to the AS, yet is

outside the area. For details concerning the construction of summary

link advertisements, see Section 12.4.3.

Type 3 link state advertisements are used when the destination is an IP

network. In this case the advertisement's Link State ID field is an IP

network number. When the destination is an AS boundary router, a Type 4

advertisement is used, and the Link State ID field is the AS boundary

router's OSPF Router ID. (To see why it is necessary to advertise the

location of each ASBR, consult Section 16.4.) Other than the difference

in the Link State ID field, the format of Type 3 and 4 link state

advertisements is identical.

For stub areas, type 3 summary link advertisements can also be used to

describe a (per-area) default route. Default summary routes are used in

stub areas instead of flooding a complete set of external routes. When

describing a default summary route, the advertisement's Link State ID is

always set to DefaultDestination (0.0.0.0) and the Network Mask is set

to 0.0.0.0.

Separate costs may be advertised for each IP Type of Service. The

encoding of TOS in OSPF link state advertisements is described in

Section 12.3. Note that the cost for TOS 0 must be included, and is

always listed first. If the T-bit is reset in the advertisement's

Option field, only a route for TOS 0 is described by the advertisement.

Otherwise, routes for the other TOS values are also described; if a cost

for a certain TOS is not included, its cost defaults to that specified

for TOS 0.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS age Options 3 or 4

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Link State ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Advertising Router

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS sequence number

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS checksum length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Network Mask

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

TOS metric

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

...

Network Mask

For Type 3 link state advertisements, this indicates the

destination's IP network mask. For example, when advertising the

location of a class A network the value 0xff000000 would be used.

This field is not meaningful and must be zero for Type 4 link state

advertisements.

For each specified type of service, the following fields are defined.

The number of TOS routes included can be calculated from the link state

advertisement's length field. Values for TOS 0 must be specified; they

are listed first. Other values must be listed in order of increasing

TOS encoding. For example, the cost for TOS 16 must always follow the

cost for TOS 8 when both are specified.

TOS The Type of Service that the following cost concerns. The encoding

of TOS in OSPF link state advertisements is described in Section

12.3.

metric

The cost of this route. Expressed in the same units as the

interface costs in the router links advertisements.

A.4.5 AS external link advertisements

AS external link advertisements are the Type 5 link state

advertisements. These advertisements are originated by AS boundary

routers. A separate advertisement is made for each destination (known

to the router) which is external to the AS. For details concerning the

construction of AS external link advertisements, see Section 12.4.3.

AS external link advertisements usually describe a particular external

destination. For these advertisements the Link State ID field specifies

an IP network number. AS external link advertisements are also used to

describe a default route. Default routes are used when no specific

route exists to the destination. When describing a default route, the

Link State ID is always set to DefaultDestination (0.0.0.0) and the

Network Mask is set to 0.0.0.0.

Separate costs may be advertised for each IP Type of Service. The

encoding of TOS in OSPF link state advertisements is described in

Section 12.3. Note that the cost for TOS 0 must be included, and is

always listed first. If the T-bit is reset in the advertisement's

Option field, only a route for TOS 0 is described by the advertisement.

Otherwise, routes for the other TOS values are also described; if a cost

for a certain TOS is not included, its cost defaults to that specified

for TOS 0.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS age Options 5

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Link State ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Advertising Router

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS sequence number

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS checksum length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Network Mask

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

E TOS metric

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Forwarding address

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

External Route Tag

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

...

Network Mask

The IP network mask for the advertised destination. For example,

when advertising a class A network the mask 0xff000000 would be

used.

For each specified type of service, the following fields are defined.

The number of TOS routes included can be calculated from the link state

advertisement's length field. Values for TOS 0 must be specified; they

are listed first. Other values must be listed in order of increasing

TOS encoding. For example, the cost for TOS 16 must always follow the

cost for TOS 8 when both are specified.

bit E

The type of external metric. If bit E is set, the metric specified

is a Type 2 external metric. This means the metric is considered

larger than any link state path. If bit E is zero, the specified

metric is a Type 1 external metric. This means that is is

comparable directly (without translation) to the link state metric.

Forwarding address

Data traffic for the advertised destination will be forwarded to

this address. If the Forwarding address is set to 0.0.0.0, data

traffic will be forwarded instead to the advertisement's originator

(i.e., the responsible AS boundary router).

TOS The Type of Service that the following cost concerns. The encoding

of TOS in OSPF link state advertisements is described in Section

12.3.

metric

The cost of this route. Interpretation depends on the external type

indication (bit E above).

External Route Tag

A 32-bit field attached to each external route. This is not used by

the OSPF protocol itself. It may be used to communicate information

between AS boundary routers; the precise nature of such information

is outside the scope of this specification.

B. Architectural Constants

Several OSPF protocol parameters have fixed architectural values. These

parameters have been referred to in the text by names such as

LSRefreshTimer. The same naming convention is used for the configurable

protocol parameters. They are defined in appendix C.

The name of each architectural constant follows, together with its value

and a short description of its function.

LSRefreshTime

The maximum time between distinct originations of any particular

link state advertisement. For each link state advertisement that a

router originates, an interval timer should be set to this value.

Firing of this timer causes a new instance of the link state

advertisement to be originated. The value of LSRefreshTime is set

to 30 minutes.

MinLSInterval

The minimum time between distinct originations of any particular

link state advertisement. The value of MinLSInterval is set to 5

seconds.

MaxAge

The maximum age that a link state advertisement can attain. When an

advertisement's age reaches MaxAge, it is reflooded. It is then

removed from the database as soon as this flood is acknowledged,

i.e., as soon as it has been removed from all neighbor Link state

retransmission lists. Advertisements having age MaxAge are not used

in the routing table calculation. The value of MaxAge must be

greater than LSRefreshTime. The value of MaxAge is set to 1 hour.

CheckAge

When the age of a link state advertisement (that is contained in the

link state database) hits a multiple of CheckAge, the

advertisement's checksum is verified. An incorrect checksum at this

time indicates a serious error. The value of CheckAge is set to 5

minutes.

MaxAgeDiff

The maximum time dispersion that can occur, as a link state

advertisement is flooded throughout the AS. Most of this time is

accounted for by the link state advertisements sitting on router

output queues (and therefore not aging) during the flooding process.

The value of MaxAgeDiff is set to 15 minutes.

LSInfinity

The link state metric value indicating that the destination is

unreachable. It is defined to be the binary value of all ones. It

depends on the size of the metric field, which is 16 bits in router

links advertisements, and 24 bits in both summary and AS external

links advertisements.

DefaultDestination

The Destination ID that indicates the default route. This route is

used when no other matching routing table entry can be found. The

default destination can only be advertised in AS external link

advertisements and in type 3 summary link advertisements for stub

areas. Its value is the IP address 0.0.0.0.

C. Configurable Constants

The OSPF protocol has quite a few configurable parameters. These

parameters are listed below. They are grouped into general functional

categories (area parameters, interface parameters, etc.). Sample values

are given for some of the parameters.

Some parameter settings need to be consistent among groups of routers.

For example, all routers in an area must agree on that area's

parameters, and all routers attached to a network must agree on that

network's IP network number and mask.

Some parameters may be determined by router algorithms outside of this

specification (e.g., the address of a host connected to the router via a

SLIP line). From OSPF's point of view, these items are still

configurable.

C.1 Global parameters

In general, a separate copy of the OSPF protocol is run for each area.

Because of this, most configuration parameters are defined on a per-area

basis. The few global configuration parameters are listed below.

Router ID

This is a 32-bit number that uniquely identifies the router in the

Autonomous System. One algorithm for Router ID assignment is to

choose the largest or smallest IP address assigned to the router.

If a router's OSPF Router ID is changed, the router's OSPF software

should be restarted before the new Router ID takes effect.

TOS capability

This item indicates whether the router will calculate separate

routes based on TOS. For more information, see Sections 4.5 and

16.9.

C.2 Area parameters

All routers belonging to an area must agree on that area's

configuration. Disagreements between two routers will lead to an

inability for adjacencies to form between them, with a resulting

hindrance to the flow of routing protocol traffic. The following items

must be configured for an area:

Area ID

This is a 32-bit number that identifies the area. The Area ID of 0

is reserved for the backbone. If the area represents a subnetted

network, the IP network number of the subnetted network may be used

for the area ID.

List of address ranges

An OSPF area is defined as a list of [IP address, mask] pairs. Each

pair describes a range of IP addresses. Networks and hosts are

assigned to an area depending on whether their addresses fall into

one of the area's defining address ranges. Routers are viewed as

belonging to multiple areas, depending on their attached networks'

area membership. Routing information is condensed at area

boundaries. External to the area, a single route is advertised for

each address range.

As an example, suppose an IP subnetted network is to be its own OSPF

area. The area would be configured as a single address range, whose

IP address is the address of the subnetted network, and whose mask

is the natural class A, B, or C internet mask. A single route would

be advertised external to the area, describing the entire subnetted

network.

Authentication type

Each area can be configured for a separate type of

authentication. See Appendix E for a discussion of the

defined authentication types.

External routing capability

Whether AS external advertisements will be flooded into/throughout

the area. If AS external advertisements are excluded from the area,

the area is called a "stub". Internal to stub areas, routing to

external destinations will be based solely on a default summary

route. The backbone cannot be configured as a stub area. Also,

virtual links cannot be configured through stub areas. For more

information, see Section 3.6.

StubDefaultCost

If the area has been configured as a stub area, and the router

itself is an area border router, then the StubDefaultCost indicates

the cost of the default summary link that the router should

advertise into the area. There can be a separate cost configured

for each IP TOS. See Section 12.4.3 for more information.

C.3 Router interface parameters

Some of the configurable router interface parameters (such as IP

interface address and subnet mask) actually imply properties of the

attached networks, and therefore must be consistent across all the

routers attached to that network. The parameters that must be

configured for a router interface are:

IP interface address

The IP protocol address for this interface. This uniquely

identifies the router over the entire internet. An IP address is

not required on serial lines. Such a serial line is called

"unnumbered".

IP interface mask

This denotes the portion of the IP interface address that

identifies the attached network. This is often referred to

as the subnet mask.

Interface output cost(s)

The cost of sending a packet on the interface, expressed in the link

state metric. This is advertised as the link cost for this

interface in the router's router links advertisement. There may be

a separate cost for each IP Type of Service. The interface output

cost(s) must always be greater than 0.

RxmtInterval

The number of seconds between link state advertisement

retransmissions, for adjacencies belonging to this interface. Also

used when retransmitting Database Description and Link State Request

Packets. This should be well over the expected round-trip delay

between any two routers on the attached network. The setting of

this value should be conservative or needless retransmissions will

result. It will need to be larger on low speed serial lines and

virtual links. Sample value for a local area network: 5 seconds.

InfTransDelay

The estimated number of seconds it takes to transmit a Link State

Update Packet over this interface. Link state advertisements

contained in the update packet must have their age incremented by

this amount before transmission. This value should take into

account the transmission and propagation delays for the interface.

It must be greater than 0. Sample value for a local area network: 1

second.

Router Priority

An 8-bit unsigned integer. When two routers attached to a network

both attempt to become Designated Router, the one with the highest

Router Priority takes precedence. If there is still a tie, the

router with the highest Router ID takes precedence. A router whose

Router Priority is set to 0 is ineligible to become Designated

Router on the attached network. Router Priority is only configured

for interfaces to multi-access networks.

HelloInterval

The length of time, in seconds, between the Hello packets that the

router sends on the interface. This value is advertised in the

router's Hello packets. It must be the same for all routers

attached to a common network. The smaller the hello interval, the

faster topological changes will be detected, but more routing

traffic will ensue. Sample value for a X.25 PDN network: 30

seconds. Sample value for a local area network: 10 seconds.

RouterDeadInterval

The number of seconds that a router's Hellos have not been seen

before its neighbors declare the router down. This is also

advertised in the router's Hello Packets in the DeadInt field. This

should be some multiple of the HelloInterval (say 4). This value

again must be the same for all routers attached to a common network.

Authentication key

This configured data allows the authentication procedure to generate

and/or verify the authentication field in the OSPF header. For

example, if the authentication type indicates simple password, the

authentication key would be a 64-bit password. This key would be

inserted directly into the OSPF header when originating routing

protocol packets. There could be a separate password for each

network.

C.4 Virtual link parameters

Virtual links are used to restore/increase connectivity of the backbone.

Virtual links may be configured between any pair of area border routers

having interfaces to a common (non-backbone) area. The virtual link

appears as an unnumbered point-to-point link in the graph for the

backbone. The virtual link must be configured in both of the area

border routers.

A virtual link appears in router links advertisements (for the backbone)

as if it were a separate router interface to the backbone. As such, it

has all of the parameters associated with a router interface (see

Section C.3). Although a virtual link acts like an unnumbered point-

to-point link, it does have an associated IP interface address. This

address is used as the IP source in protocol packets it sends along the

virtual link, and is set dynamically during the routing table build

process. Interface output cost is also set dynamically on virtual links

to be the cost of the intra-area path between the two routers. The

parameter RxmtInterval must be configured, and should be well over the

expected round-trip delay between the two routers. This may be hard to

estimate for a virtual link. It is better to err on the side of making

it too large. Router Priority is not used on virtual links.

A virtual link is defined by the following two configurable parameters:

the Router ID of the virtual link's other endpoint, and the (non-

backbone) area through which the virtual link runs (referred to as the

virtual link's transit area). Virtual links cannot be configured

through stub areas.

C.5 Non-broadcast, multi-access network parameters

OSPF treats a non-broadcast, multi-access network much like it treats a

broadcast network. Since there many be many routers attached to the

network, a Designated Router is selected for the network. This

Designated Router then originates a networks links advertisement, which

lists all routers attached to the non-broadcast network.

However, due to the lack of broadcast capabilities, it is necessary to

use configuration parameters in the Designated Router selection. These

parameters need only be configured in those routers that are themselves

eligible to become Designated Router (i.e., those router's whose DR

Priority for the network is non-zero):

List of all other attached routers

The list of all other routers attached to the non-broadcast network.

Each router is listed by its IP interface address on the network.

Also, for each router listed, that router's eligibility to become

Designated Router must be defined. When an interface to a non-

broadcast network comes up, the router sends Hello packets only to

those neighbors eligible to become Designated Router, until the

identity of the Designated Router is discovered.

PollInterval

If a neighboring router has become inactive (hellos have not been

seen for RouterDeadInterval seconds), it may still be necessary to

send Hellos to the dead neighbor. These Hellos will be sent at the

reduced rate PollInterval, which should be much larger than

HelloInterval. Sample value for a PDN X.25 network: 2 minutes.

C.6 Host route parameters

Host routes are advertised in network links advertisements as stub

networks with mask 0xffffffff. They indicate either router interfaces

to point-to-point networks, looped router interfaces, or IP hosts that

are directly connected to the router (e.g., via a SLIP line). For each

host directly connected to the router, the following items must be

configured:

Host IP address

The IP address of the host.

Cost of link to host

The cost of sending a packet to the host, in terms of the link state

metric. There may be multiple costs configured, one for each IP

TOS. However, since the host probably has only a single connection

to the internet, the actual configured cost(s) in many cases is

unimportant (i.e., will have no effect on routing).

D. Required Statistics

An OSPF implementation must provide a minimum set of statistics

indicating the operational state of the protocol. These statistics must

be accessible to the user; this will probably be accomplished through

some sort of network management interface.

It is hoped that these statistics will aid in the debugging of the

implementation, and in the analysis of the protocol's performance.

The statistics can be broken into two broad categories. The first

consists of what we will call logging messages. These are messages

produced in real time, with generally a single message produced as the

result of a single protocol event. Such messages are also commonly

referred to as traps.

The second category will be referred to as cumulative statistics. These

are counters whose value have collected over time, such as the count of

link state retransmissions over the last hour. Also falling into this

category are dumps of the various routing data structures.

D.1 Logging messages

A logging message should be produced on every significant protocol

event. The major events are listed below. Most of these events

indicate a topological change in the routing domain. However, some

number of logging messages can be expected even when the routing domain

remains intact for long periods of time. For example, link state

originations will still happen due to the link state refresh timer

firing.

Any of the messages that refer to link state advertisements should print

the area associated with the advertisement. There is no area associated

with AS external link advertisements.

The following list of logging messages indicate topological changes in

the routing domain:

T1 The state of a router interface changes. Interface state changes

are documented in Section 9.3. In general, they will cause new link

state advertisements to be originated. The logging message produced

should include the interface's IP address (or other name), interface

type (virtual link, etc.) and old and new state values (as

documented in Section 9.1).

T2 The state of a neighbor changes. Neighbor state changes are

documented in Section 10.3. The logging message produced should

include the neighbor IP address, and old and new state values.

T3 The (Backup) Designated Router has changed on one of the attached

networks. See Section 9.4. The logging message produced should

include the network IP address, and the old and new (Backup)

Designated Routers.

T4 The router is originating a new instance of a link state

advertisement. The logging message produced should indicate the LS

type, Link State ID and Advertising Router associated with the

advertisement (see Section 12.4).

T5 The router has received a new instance of a link state

advertisement. The router receives these in Link State Update

packets. This will cause recalculation of the routing table. The

logging message produced should indicate the advertisement's LS

type, Link State ID and Advertising Router. The message should also

include the neighbor from whom the advertisement was received.

T6 An entry in the routing table has changed (see Section 11). The

logging message produced should indicate the Destination type,

Destination ID, and the old and new paths to the destination.

The following logging messages may indicate that there is a network

configuration error:

C1 A received OSPF packet is rejected due to errors in its IP/OSPF

header. The reasons for rejection are documented in Section 8.2.

They include OSPF checksum failure, authentication failure, and

inability to match the source with an active OSPF neighbor. The

logging message produced should include the IP source and

destination addresses, the router ID in the OSPF header, and the

reason for the rejection.

C2 An incoming Hello packet is rejected due to mismatches between the

Hello's parameters and those configured for the receiving interface

(see Section 10.5). This indicates a configuration problem on the

attached network. The logging message should include the Hello's

source, the receiving interface, and the non-matching parameters.

C3 An incoming Database Description packet, Link State Request Packet,

Link State Acknowledgment Packet or Link State Update packet is

rejected due to the source neighbor being in the wrong state (see

Sections 10.6, 10.7, 13.7 , and 13 respectively). This can be

normal when the identity of the network's Designated Router changes,

causing momentary disagreements over the validity of adjacencies.

The logging message should include the source neighbor, its state,

and the packet's type.

C4 A Database Description packet has been retransmitted. This may mean

that the value of RxmtInterval that has been configured for the

associated interface is too small. The logging message should

include the neighbor to whom the packet is being sent.

The following messages can be caused by packet transmission errors, or

software errors in an OSPF implementation:

E1 The checksum in a received link state advertisement is incorrect.

The advertisement is discarded (see Section 13). The logging

message should include the advertisement's LS type, Link State ID

and Advertising Router (which may be incorrect). The message should

also include the neighbor from whom the advertisement was received.

E2 During the aging process, it is discovered that one of the link

state advertisements in the database has an incorrect checksum.

This indicates memory corruption or a software error in the router

itself. The router should be dumped and restarted.

The following messages are an indication that a router has restarted,

losing track of its previous LS sequence number. Should these messages

continue, it may indicate the presence of duplicate Router IDs:

R1 Two link state advertisements have been seen, whose LS type, Link

State ID, Advertising Router and LS sequence number are the same,

yet with differing LS checksums. These are considered to be

different instances of the same advertisement. The instance with

the larger checksum is accepted as more recent (see Section 12.1.7,

13.1). The logging message should include the LS type, Link State

ID, Advertising Router, LS sequence number and the two differing

checksums.

R2 Two link state advertisements have been seen, whose LS type, Link

State ID, Advertising Router, LS sequence number and LS checksum are

the same, yet can be distinguished by their LS age fields. This

means that one of the advertisement's LS age is MaxAge, or the two

LS age fields differ by more than MaxAgeDiff. The logging message

should include the LS type, Link State ID, Advertising Router, LS

sequence number and the two differing ages.

R3 The router has received an instance of one of its self-originated

advertisements, that is considered to be more recent. This forces

the router to originate a new advertisement (see Section 13.4). The

logging message should include the advertisement's LS type, Link

State ID, and Advertising Router along with the neighbor from whom

the advertisement was received.

R4 An acknowledgment has been received for an instance of an

advertisement that is not currently contained in the router's

database (see Section 13.7). The logging message should detail the

instance being acknowledged and the database copy (if any), along

with the neighbor from whom the acknowledgment was received.

R5 An advertisement has been received through the flooding procedure

that is LESS recent the the router's current database copy (see

Section 13). The logging message should include the received

advertisement's LS type, Link State ID, Advertising Router, LS

sequence number, LS age and LS checksum. Also, the message should

display the neighbor from whom the advertisement was received.

The following messages are indication of normal, yet infrequent protocol

events. These messages will help in the interpretation of some of the

above messages:

N1 The Link state refresh timer has fired for one of the router's

self-originated advertisements (see Section 12.4). A new instance

of the advertisement must be originated. The message should include

the advertisement's LS type, Link State ID and Advertising Router.

N2 One of the advertisements in the router's link state database has

aged to MaxAge (see Section 14). At this point, the advertisement

is no longer included in the routing table calculation, and is

reflooded. The message should list the advertisement's LS type,

Link State ID and Advertising Router.

N3 An advertisement of age MaxAge has been flushed from the router's

database. This occurs after the advertisement has been acknowledged

by all adjacent neighbors. The message should list the

advertisement's LS type, Link State ID and Advertising Router.

D.2 Cumulative statistics

These statistics display collections of the routing data structures.

They should be able to be obtained interactively, through some kind of

network management facility.

All the following statistics displays, with the exception of the area

list, routing table and the AS external links, are specific to a single

area. As noted in Section 4, most OSPF protocol mechanisms work on each

area separately.

The following statistics displays should be available:

(1) A list of all the areas attached to the router, along with the

authentication type to use for the area, the number of router

interfaces attaching to the area, and the total number of nets and

routers belonging to the area.

For example, consider the router RT3 pictured in Figure 15. It has

interfaces to two separate areas, Area 1 and the backbone (Area 0).

Table 20 then indicates that the backbone is using a simple password

for authentication, and that Area 1 is not using any authentication.

The number of nets includes IP networks, subnets, and hosts (this is

the reason for 2 backbone nets -- they are the host routes

corresponding to the serial line between backbone routers RT6 and

RT10).

Area ID # ifcs AuType # nets # routers

______________________________________________

0 1 1 2 7

1 2 0 4 4

Table 20: Sample OSPF area display.

(2) A list of all the router's interfaces to an area, along with their

addresses, output cost, current state, the (Backup) Designated

Router for the attached network, and the number of neighbors

currently associated with the interface. Some number of these

neighbors will have become adjacent, the number of these is noted in

the display also.

Again consider router RT3 in Figure 15. Table 21 below indicates

that RT4 has been selected as Designated Router for network N3, and

router RT1 has been selected as Backup. Adjacencies have been

established to both of these routers. There are no routers besides

RT3 attached to network N4, so it becomes DR, yet still advertises

the network as a stub in its router links advertisements.

Ifc IP address state cost DR Backup # nbrs # adjs

__________________________________________________________________________

192.1.1.3 DR other 1 192.1.1.4 192.1.1.1 3 2

192.1.4.3 DR 2 192.1.4.3 none 0 0

Table 21: Sample OSPF interface display.

(3) The list of neighbors associated with a particular interface. Each

neighbor's IP address, router ID, state, and the length of the three

link state advertisement queues (see Section 10) to the neighbor is

displayed.

Suppose router RT4 is the Designated Router for network N3, and

router RT1 is the Backup Designated router. Suppose also that the

adjacency between router RT3 and RT1 has not yet fully formed. The

display of router RT3's neighbors (associated with its interface to

network N3) may then look like Table 22. The display indicates that

RT3 and RT1 are still in the database exchange procedure, Router RT3

has more Database Description packets to send to RT1, and RT1 has at

least one link state advertisement that RT3 doesn't. Also, there is

a single link state advertisement that has been flooded, but not

acknowledged, to each neighbor that participates in the flooding

procedure (state >= Exchng). (In the following examples we assume

that a router's Router ID is assigned to be its smallest IP

interface address).

Nbr IP address Router ID state LS rxmt len DB summ len LS req len

____________________________________________________________________________

192.1.1.1 192.1.1.1 Exchng 1 10 1

192.1.1.2 192.1.1.2 2-Way 0 0 0

192.1.1.4 192.1.1.4 Full 1 0 0

Table 22: Sample OSPF neighbor display.

(4) A list of the area's link state database. This is the same in all

of the routers attached to the area. It is composed of that area's

router links, network links, and summary links advertisements.

Also, the AS external link advertisements are a part of all the

areas' databases.

The link state database for Area 1 in Figure 15 might look like

Table 23 (compare this with Figure 7). Assume the the Designated

Router for network N3 is router RT4, as above. Both routers RT3 and

RT4 are originating summary link advertisements into Area 1, since

they are area border routers. Routers RT5 and RT7 are AS external

routers. Their location must be described in summary links

advertisements. Also, their AS external link advertisements are

flooded throughout the entire AS.

Router RT3 can locate its self-originated advertisements by looking

for its own router ID (192.1.1.3) in advertisements' Advertising

Router fields.

The LS sequence number, LS age, and LS checksum fields indicate the

advertisement's instance. Their values are stored in the

advertisement's link state header; we have not bothered to make up

values for the example.

LS type Link State ID Advertising Router LS seq no LS age LS checksum

_______________________________________________________________________________

1 192.1.1.1 192.1.1.1 * * *

1 192.1.1.2 192.1.1.2 * * *

1 192.1.1.3 192.1.1.3 * * *

1 192.1.1.4 192.1.1.4 * * *

_______________________________________________________________________________

2 192.1.1.4 192.1.1.4 * * *

_______________________________________________________________________________

3 Ia,Ib 192.1.1.3 * * *

3 N6 192.1.1.3 * * *

3 N7 192.1.1.3 * * *

3 N8 192.1.1.3 * * *

3 N9-N11,H1 192.1.1.3 * * *

3 Ia,Ib 192.1.1.4 * * *

3 N6 192.1.1.4 * * *

3 N7 192.1.1.4 * * *

3 N8 192.1.1.4 * * *

3 N9-N11,H1 192.1.1.4 * * *

4 RT5 192.1.1.3 * * *

4 RT7 192.1.1.3 * * *

4 RT5 192.1.1.4 * * *

4 RT7 192.1.1.4 * * *

_______________________________________________________________________________

4 N12 RT5's ID * * *

4 N13 RT5's ID * * *

4 N14 RT5's ID * * *

4 N12 RT7's ID * * *

LS type Link State ID Advertising Router LS seq no LS age LS checksum

_______________________________________________________________________________

4 N15 RT7's ID * * *

Table 23: Sample OSPF link state database display.

(5) The contents of any particular link state advertisement. For

example, a listing of the router links advertisement for Area 1,

with LS type = 1 and Link State ID = 192.1.1.3 is shown in Section

12.4.1.

(6) A listing of the entire routing table. Several examples are shown

in Section 11. The routing table is calculated from the combined

databases of each attached area (see Section 16). It may be

desirable to sort the routing table by Type of Service, or by

destination, or a combination of the two.

E. Authentication

All OSPF protocol exchanges are authenticated. The OSPF packet header

(see Section A.3.1) includes an authentication type field, and 64-bits

of data for use by the appropriate authentication scheme (determined by

the type field).

The authentication type is configurable on a per-area basis. Additional

authentication data is configurable on a per-interface basis. For

example, if an area uses a simple password scheme for authentication, a

separate password may be configured for each network contained in the

area.

Authentication types 0 and 1 are defined by this specification. All

other authentication types are reserved for definition by the IANA

(iana@ISI.EDU). The current list of authentication types is described

below in Table 24.

AuType Description

_______________________________________________________________

0 No authentication

1 Simple password

All others Reserved for assignment by the IANA (iana@ISI.EDU)

Table 24: OSPF authentication types.

E.1 Autype 0 -- No authentication

Use of this authentication type means that routing exchanges in the area

are not authenticated. The 64-bit field in the OSPF header can contain

anything; it is not examined on packet reception.

E.2 Autype 1 -- Simple password

Using this authentication type, a 64-bit field is configured on a per-

network basis. All packets sent on a particular network must have this

configured value in their OSPF header 64-bit authentication field. This

essentially serves as a "clear" 64-bit password.

This guards against routers inadvertently coming up in the area. They

must first be configured with their attached networks' passwords before

they can join the routing domain.

F. Version 1 differences

This section documents the changes between OSPF version 1 and OSPF

version 2. The impetus for these changes derives from comments received

on RFC1131 and recent field experience with the OSPF protocol.

Unfortunately, the changes are not backward-compatible. For that

reason, OSPF version 1 will not interoperate with OSPF version 2.

However, the changes are small in scope and should not greatly affect

any existing implementations. In addition, some of the proposed changes

should enable future protocol additions to be made in a backward-

compatible manner (see Section F.4).

F.1 Protocol Enhancements

The following enhancements were made to the OSPF protocol.

F.1.1 Stub area support

In many Autonomous Systems, the majority of the OSPF link state database

consists of AS external advertisements. In these Autonomous Systems,

some OSPF areas may be organized in such a way that external

advertisements can be safely ignored, enabling a reduction of the area's

database size. This applies to OSPF areas where there is only a single

exit/entry that is used by all externally addressed packets, or to cases

where some sub-optimality of external routing is acceptable.

Therefore, an OSPF area configuration option has been added (see

Sections 3.6 and C.2) allowing the import of external advertisements to

be disabled for an area. When this option is enabled, no AS external

advertisements will be flooded into the area (Sections 13, 13.3 and

10.3). Instead, within the area all data traffic to external

destinations will follow a (per-area) default route. These areas are

called "stub" areas.

To implement this, all area border routers attached to stub areas will

originate a default summary link advertisement for the area (Section

12.4.3). This will direct all internal routers to an area border router

when forwarding externally addressed packets. In addition, to ensure

that stub areas are configured consistently, an Options field has been

added to OSPF Hello packets (Sections A.2 and A.3.2). A bit is reset in

the Options field indicating that the attached area is a stub area

(Section 9.5). A router will not accept a neighbor's hellos unless they

both agree on the area's ability to process AS external advertisements

(Section 10.5). In this way, a system administrator will be able to

discover incorrectly configured routers, and data traffic will be routed

around them (in order to avoid potential looping situations) until their

configuration can be repaired.

F.1.2 Optional TOS support

In OSPF there is conceptually a separate routing table for each TOS; the

calculations detailed in steps 1-5 of Section 16 must be done separately

for each TOS. (Note however that link and summary costs need not be

specified separately for each TOS; costs for unspecified TOS values

default to the cost of TOS 0).

In version 1 of the OSPF specification, all OSPF routers were required

to route based on TOS. However, producing a separate routing table for

each TOS may prove costly, both in terms of memory and processor

resources. For this reason, version 2 allows the system administrator

to configure routers to calculate/use only a single routing table (the

TOS 0 table). When this is done, some traffic may take non-optimal

routes. But all packets will still be delivered, and routing will

remain loop free (see Section 2.4).

In order to avoid routing loops, a router (router X) using a single

table must communicate this information to its peers. This is done by

resetting the new TOS-capable bit in the router X's router links

advertisement (Section 12.4.1). Then, when its peers perform the

Dijkstra calculation (Section 16.1) for non-zero TOS values, they will

omit router X from the calculation. In effect, an attempt will be made

to bypass router X when forwarding non-zero TOS traffic. Summary link

and AS external link advertisements can also indicate their non-

availability for non-zero TOS traffic (Sections 12.4.3 and 12.4.4).

The result may be that no route can be found for some non-zero value of

TOS. When this happens, the packet is routed along the TOS 0 route

instead (Section 11.1).

It is still mandatory for all OSPF implementations to be able to

construct separate routing tables for each TOS value, if desired by the

system administrator.

F.1.3 Preventing external extra-hops

In some cases, version 1 of the OSPF specification will introduce

extra-hops when calculating routes to external destinations. This is

because it is implicit in the format of AS external advertisements that

packets should be forwarded through the advertising router. However,

consider the situation where multiple OSPF routers share a LAN with an

external router (call it router Y) , and only one OSPF router (call it

router X) exchanges routing information with Y. The OSPF routers on the

LAN other than X will forward packets destined for Y and beyond through

X, generating an extra hop (see Section 2.2).

To fix this, a new field has been added to AS external advertisements.

This field (called the forwarding address) will indicate the router

address to which packets should be forwarded (Section 12.4.4). In the

above example, router X will put Y's IP address into this field. If the

field is 0, packets are (as before) forwarded to the originator of the

advertisement. A different forwarding address can be specified for each

TOS value.

Whenever possible, this new field should be set to 0. This is because

setting it to an actual router address incurs additional cost during the

routing table build process (Section 16.4).

Besides preventing extra-hops, there are two other applications for this

field. The first is for use by "route servers". Using the forwarding

address, a router in the middle of the Autonomous System can gather

external routing information and originate AS external advertisements

that specify the correct exit route to use for each external destination

(Section 2.2).

The other application possibly enables the reduction of the number of AS

external advertisements that need be imported. Suppose in the example

at the beginning of this section that there are two routers (X and Z)

exchanging EGP information with the non-OSPF router Y. It is then

likely that both X and Z will originate the same set of external routes.

Two AS external advertisements that specify the same (non-zero)

forwarding address, destination and cost are obviously functionally

equivalent, regardless of their originators (advertising routers). The

OSPF specification dictates that the advertisement originated by the

router with the largest Router ID will always be used. This allows the

other router to flush its equivalent advertisement (Section 12.4.4).

F.2 Corrected problems

The following problems in OSPF version 1 have been corrected in version

2.

F.2.1 LS sequence number space changes

The LS sequence number space has been changed from version 1's lollipop

shape to a linear sequence space (Section 12.1.6). Sequence numbers

will now be compared as signed 32-bit integers. Link state

advertisements having larger sequence numbers will be considered more

recent. The sequence number space will still begin at (-N+1) (where N =

2**31). The value of -N remains reserved. The LS sequence number of

successive instances of an advertisement will continue to be incremented

until it reaches the maximum possible value: N-1. At this point, when a

new instance of the advertisement must be originated (due either to

topological change of the expiration of the LS refresh timer) the

current instance must first be "prematurely aged".

There will be a new section discussing premature aging (Section 14.1).

This is a method for flushing a link state advertisement from the

routing domain: the advertisement's age is set to MaxAge and

advertisement is reflooded just as if it were a newly received

advertisement. As soon as the new flooding is acknowledged by all of

the router's adjacent neighbors, the advertisement is flushed from the

database.

Premature aging can also be used when, for example, a previously

advertised external route is no longer reachable. In this circumstance,

premature aging is preferable to the alternative, which is to originate

a new advertisement for the destination specifying a metric of

LSInfinity.

A router may only prematurely age its own (self-originated) link state

advertisements. These are the link state advertisements having the

router's own OSPF router ID in the Advertising Router field.

F.2.2 Flooding of unexpected MaxAge advertisements

Version 1 of the OSPF omitted the handling of a special case in the

flooding procedure: the reception of a MaxAge advertisement that has no

database instance. A paragraph has been added to Section 13 to deal

with this occurrence. Without this paragraph, retransmissions of MaxAge

advertisements could possibly delay their being flushed from the routing

domain.

F.2.3 Virtual links and address ranges

When summarizing information into a virtual link's transit area, version

2 of the OSPF specification prohibits the collapsing of multiple

backbone IP networks/subnets into a single summary link. This

restriction has been added to deal with certain anomalous OSPF area

configurations. See Sections 15 and 12.4.3 for more information.

F.2.4 Routing table lookup explained

When forwarding an IP data packet, a router looks up the packet's IP

destination in the routing table. This determines the packet's next

hop. A new section (Section 11.1) has been added describing the routing

table lookup (instead of just specifying a "best match"). This section

clarifies OSPF's four level routing hierarchy (i.e., intra-area, inter-

area, external type 1 and external type 2 routes). It also specifies

the effect of TOS on routing.

F.2.5 Sending Link State Request packets

OSPF Version 2 eases the restrictions on the sending of Link State

Request packets. Link State Request packets can now be sent to a

neighboring router before a complete set of Database Description packets

have been exchanged. This enables a more efficient use of a router's

memory resources; an OSPF version 2 implementation may limit the size of

the neighbor Link state request lists. See Sections 10.9, 10.7 and 10.3

for more details.

F.2.6 Changes to the Database description process

The specification has been modified to ensure that, when two routers are

synchronizing their databases during the Database Description process,

none of the component link state advertisements can have their sequence

numbers decrease. A link state advertisement's sequence number

decreases when it is flushed from the routing domain via premature-

aging, and then reoriginated with the smallest sequence number

0x80000001 (see Section 14.1). So the specification now dictates that

an advertisement cannot be flushed from a router's database until both

a) it no longer appears on any neighbor Link State Retransmission lists

and b) none of the router's neighbors are in states Exchange or Loading.

See Sections 13 (step 4c) and 14.1 for more details.

In addition, a new step has been added to the flooding procedure

(Section 13) in order to make the Database Description process more

robust. This step detects when a neighbor lists one instance of an

advertisement in its Database Description packets, but responds to Link

State Request packets by sending another (earlier) instance. This

behavior now causes the event BadLSReq to be generated, which restarts

the Database Description process with the neighbor. In OSPF version 1,

the neighbor event BadLSReq erroneously did not restart the Database

Description process.

F.2.7 Receiving OSPF Hello packets

The section detailing the receive processing of OSPF Hello packets

(Section 10.5) has been modified to include the generation of the

neighbor Backup Seen event. In addition, the section detailing the

Designated Router election algorithm (Section 9.4) has been modified to

include the algorithm's initial state.

F.2.8 Network mask defined for default route

The network mask for the default route, when it appears as the

destination in either an AS external link advertisement or in a summary

link advertisement, has been set to 0.0.0.0. See Sections A.4.4 and

A.4.5 for more details.

F.2.9 Rate limit imposed on flooding

When an advertisement is installed in the link state database, it is

timestamped. The flooding procedure is then not allowed to install a

new instance of the advertisement until MinLSInterval seconds have

elapsed. This enforces a rate limit on the flooding procedure; a new

instance can be flooded only once every MinLSInterval seconds. This

guards against routers that disregard the limit on self-originated

advertisements (already present in OSPF version 1) of one origination

every MinLSInterval seconds. For more information, see Section 13.

F.3 Packet format changes

The following changes have been made to the format of OSPF packets and

link state advertisements. Some of these changes were required to

support the added functionality listed above. Other changes were made

to further simplify the parsing of OSPF packets.

F.3.1 Adding a Capability bitfield

To support the new "stub area" and "optional TOS" features, a bitfield

listing protocol capabilities has been added to the Hello packet,

Database Description packet and all link state advertisements. When

used in Hello packets, this allows a router to reject a neighbor because

of a capability mismatch. Alternatively, when capabilities are

exchanged in Database Description packets a router can choose not to

forward certain link state advertisements to a neighbor because of its

reduced functionality. Lastly, listing capabilities in link state

advertisements allows routers to route traffic around reduced

functionality router, by excluding them from parts of the routing table

calculation. See Section A.2 for more details.

F.3.2 Packet simplification

To simplify the format of Database Description packets and Link State

Acknowledgment packets, their description of link state advertisements

has been modified. Each advertisement is now be described by its 20-

byte link state header (see Section A.4). This does not consume any

additional space in the packets. The one additional piece of

information that will be present is the LS length. However, this field

need not be used when processing the Database Description and Link State

Acknowledgment packets.

F.3.3 Adding forwarding addresses to AS external advertisements

As discussed in Section F.1.3, a forwarding address field has been added

to the AS external advertisement.

F.3.4 Labelling of virtual links

Virtual links will be labelled as such in router links advertisements.

This separates virtual links from unnumbered point-to-point links,

allowing all backbone routers to discover whether any virtual links are

in use. See Section 12.4.1 for more details.

F.3.5 TOS costs ordered

When a link state advertisement specifies a separate cost depending on

TOS, these costs must be ordered by increasing TOS value. For example,

the cost for TOS 16 must always follow the cost for TOS 8.

F.3.6 OSPF's TOS encoding redefined

The way that OSPF encodes TOS in its link state advertisements has been

redefined in version 2. OSPF's encoding of the Delay (D), Throughput (T)

and Reliability (R) TOS flags defined by [RFC791] is described in

Section 12.3.

F.4 Backward-compatibility provisions

Additional functionality will probably be added to OSPF in the future.

One example of this is a multicast routing capability, which is

currently under development. In order to be able to add such features

in a backward-compatible manner, the following provisions have been made

in the OSPF specification.

New capabilities will probably involve the introduction of new link

state advertisements. If a router receives a link state advertisement

of unknown type during the flooding procedure, the advertisement is

simply ignored (Section 13. The router should not attempt to further

flood the advertisement, nor acknowledge it. The advertisement should

not be installed into the link state database. If the router receives

an advertisement of unknown type during the Database Description

process, this is an error (see Sections 10.6 and 10.3). The Database

Description process is then restarted.

There is also an Options field in both the Hello packets, Database

Description packets and the link state advertisement headers.

Unrecognized capabilities found in these places should be ignored, and

should not affect the normal processing of protocol packets/link state

advertisements (see Sections 10.5 and 10.6). Routers will originate

their Hello packets, Database Description packets and link state

advertisements with unrecognized capabilities set to 0 (see Sections

9.5, 10.8 and 12.1.2).

Security Considerations

All OSPF protocol exchanges are authenticated. This is accomplished

through authentication fields contained in the OSPF packet header. For

more information, see Sections 8.1, 8.2, and Appendix E.

Author's Address

John Moy

Proteon, Inc.

2 Technology Drive

Westborough, MA 01581

Phone: (508) 898-2800

EMail: jmoy@proteon.com

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