RFC3509 - Alternative Implementations of OSPF Area Border Routers

Network Working Group A. Zinin

Request for Comments: 3509 Alcatel

Category: Informational A. Lindem

Redback Networks

D. Yeung

Procket Networks

April 2003

Alternative Implementations of OSPF Area Border Routers

Status of this Memo

This memo provides information for the Internet community. It does

not specify an Internet standard of any kind. Distribution of this

memo is unlimited.

Abstract

Open Shortest Path First (OSPF) is a link-state intra-domain routing

protocol used for routing in IP networks. Though the definition of

the Area Border Router (ABR) in the OSPF specification does not

require a router with multiple attached areas to have a backbone

connection, it is actually necessary to provide sUCcessful routing to

the inter-area and external destinations. If this requirement is not

met, all traffic destined for the areas not connected to such an ABR

or out of the OSPF domain, is dropped. This document describes

alternative ABR behaviors implemented in Cisco and IBM routers.

1 Overview

1.1 Introduction

An OSPF routing domain can be split into several subdomains, called

areas, which limit the scope of LSA flooding. According to [Ref1] a

router having attachments to multiple areas is called an "area border

router" (ABR). The primary function of an ABR is to provide its

attached areas with Type-3 and Type-4 LSAs, which are used for

describing routes and AS boundary routers (ASBRs) in other areas, as

well as to perform actual inter-area routing.

1.2 Motivation

In OSPF domains the area topology is restricted so that there must be

a backbone area (area 0) and all other areas must have either

physical or virtual connections to the backbone. The reason for this

star-like topology is that OSPF inter-area routing uses the

distance-vector approach and a strict area hierarchy permits

avoidance of the "counting to infinity" problem. OSPF prevents

inter-area routing loops by implementing a split-horizon mechanism,

allowing ABRs to inject into the backbone only Summary-LSAs derived

from the

intra-area routes, and limiting ABRs' SPF calculation to consider

only Summary-LSAs in the backbone area's link-state database.

The last restriction leads to a problem when an ABR has no backbone

connection (in OSPF, an ABR does not need to be attached to the

backbone). Consider a sample OSPF domain depicted in the Figure 1.

. .

. Area 0 .

+--+ +--+

..R1.. ..R2..

. +--+ .. +--+ .

. .. .

. +--+ .

. Area1 R3 Area2 .

. +--+ +--+ .

. .. R4 .

. . . +--+ .

....... .......

Figure 1. ABR dropping transit traffic

In this example R1, R2, and R3 are ABRs. R1 and R2 have backbone

connections, while R3 doesn't.

Following the section 12.4.1 of [Ref1], R3 will identify itself as an

ABR by setting the bit B in its router-LSA. Being an ABR, R3 can

only consider summary-LSAs from the backbone when building the

routing table (according to section 16.2 of [Ref1]), so it will not

have any inter-area routes in its routing table, but only intra-area

routes from both Area 1 and Area 2. Consequently, according to

section 12.4.3 of [Ref1], R3 will originate into Areas 1 and 2 only

summary-LSAs covering destinations in the directly attached areas,

i.e., in Area 2---the summary-LSAs for Area 1, and in Area 1---the

summary-LSAs for Area 2.

At the same time, router R2, as an ABR connected to the backbone,

will inject into Area 2 summary-LSAs describing the destinations in

Area 0 (the backbone), Area 1 and other areas reachable through the

backbone.

This results in a situation where internal router R4 calculates its

routes to destinations in the backbone and areas other than Area 1

via R2. The topology of Area 2 itself can be such that the best path

from R4 to R2 is via R3, so all traffic destined for the backbone and

backbone-attached areas goes through R3. Router R3 in turn, having

only intra-area routes for areas 1 and 2, will drop all traffic not

destined for the areas directly attached to it. The same problem can

occur when a backbone-connected ABR loses all of its adjacencies in

the backbone---even if there are other normally functioning ABRs in

the attached areas, all traffic going to the backbone (destined for

it or for other areas) will be dropped.

In a standard OSPF implementation this situation can be remedied by

use of Virtual Links (see section 15 of [Ref1] for more details). In

this case, router R3 will have a virtual backbone connection, will

form an adjacency over it, will receive all LSAs directly from a

backbone-attached router (R1 or R2, or both in our example) and will

install intra- or inter-area routes.

While being an unavoidable technique for repairing a partitioned

backbone area, the use of virtual links in the described situation

adds extra configuration headaches and system traffic overhead.

Another situation where standard ABR behavior does not provide

acceptable results is when it is necessary to provide redundant

connectivity to an ASBR via several different OSPF areas. This would

allow a provider to aggregate all their customers connecting through

a single Access point into one area while still offering a redundant

connection through another access point in a different area, as shown

in Figure 2.

. .

. Area 0 .

+--+ +--+

..R1.. ..R2..

. +--+ .. +--+ .

. .. .

. Area1 .. Area2 .

. .. .

. +--+ .

.......R3.......

ASBR+--+

/.. --+- -+--

CN1 CNx

Customer Networks (CN1--CNx) Advertised

as AS External or NSSA External Routes

Figure 2. Dual Homed Customer Router

This technique is already used in a number of networks including one

of a major provider.

The next section describes alternative ABR behaviors, implemented in

Cisco and IBM routers. The changes are in the ABR definition and

inter-area route calculation. Any other parts of standard OSPF are

not changed.

These solutions are targeted to the situation when an ABR has no

backbone connection. They imply that a router connected to multiple

areas without a backbone connection is not an ABR and should function

as a router internal to every attached area. This solution emulates

a situation where separate OSPF processes are run for each area and

supply routes to the routing table. It remedies the situation

described in the examples above by not dropping transit traffic.

Note that a router following it does not function as a real border

router---it doesn't originate summary-LSAs. Nevertheless such a

behavior may be desirable in certain situations.

Note that the proposed solutions do not obviate the need of virtual

link configuration in case an area has no physical backbone

connection at all. The methods described here improve the behavior

of a router connecting two or more backbone-attached areas.

2 Changes to ABR Behavior

2.1 Definitions

The following definitions will be used in this document to describe

the new ABR behaviors:

Configured area:

An area is considered configured if the router has at least one

interface in any state assigned to that area.

Actively Attached area:

An area is considered actively attached if the router has at least

one interface in that area in the state other than Down.

Active Backbone Connection:

A router is considered to have an active backbone connection if

the backbone area is actively attached and there is at least one

fully adjacent neighbor in it.

Area Border Router (ABR):

Cisco Systems Interpretation:

A router is considered to be an ABR if it has more than one

area Actively Attached and one of them is the backbone area.

IBM Interpretation:

A router is considered to be an ABR if it has more than one

Actively Attached area and the backbone area Configured.

2.2 Implementation Details

The following changes are made to the base OSPF, described in [Ref1]:

1. The algorithm for Type 1 LSA (router-LSA) origination is changed

to prevent a multi-area connected router from identifying itself

as an ABR by the bit B (as described in section 12.4.1 of [Ref1])

until it considers itself as an ABR according to the definitions

given in section 2.1.

2. The algorithm for the routing table calculation is changed to

allow the router to consider the summary-LSAs from all attached

areas if it is not an ABR, but has more than one attached area,

or it does not have an Active Backbone Connection. Definitions

of the terms used in this paragraph are given in section 2.1.

So, the paragraph 1 of section 16.2 of [Ref1] should be

interpreted as follows:

"The inter-area routes are calculated by examining summary-LSAs.

If the router is an ABR and has an Active Backbone Connection,

only backbone summary-LSAs are examined. Otherwise (either the

router is not an ABR or it has no Active Backbone Connection),

the router should consider summary-LSAs from all Actively

Attached areas..."

3. For Cisco ABR approach, the algorithm for the summary-LSAs

origination is changed to prevent loops of summary-LSAs in

situations where the router considers itself an ABR but doesn't

have an Active Backbone Connection (and, consequently, examines

summaries from all attached areas). The algorithm is changed to

allow an ABR to announce only intra-area routes in such a

situation.

So, the paragraph 2 of subsection 12.4.3 of [Ref1] should be

interpreted as follows:

"Summary-LSAs 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) in

accordance with the algorithm described below. Note that while

only intra-area routes are advertised into the backbone, if the

router has an Active Backbone Connection, both intra-area and

inter-area routes are advertised into the other areas; otherwise,

the router only advertises intra-area routes into non-backbone

areas."

For this policy to be applied we change steps 6 and 7 in the

summary origination algorithm to be as follows:

Step 6:

"Else, if the destination of this route is an AS boundary

router, a summary-LSA should be originated if and only if the

routing table entry describes the preferred path to the AS

boundary router (see Step 3 of Section 16.4). If so, a Type 4

summary-LSA is originated for the destination, with Link State

ID equal to the AS boundary router's Router ID and metric

equal to the routing table entry's cost. If the ABR

performing this algorithm does not have an Active Backbone

Connection, it can originate Type 4 summary-LSA only if the

type of the route to the ASBR is intra-area. Note: Type 4

summary-LSAs should not be generated if Area A has been

configured as a stub area."

Step 7:

"Else, the Destination type is network. If this is an

inter-area route and the ABR performing this algorithm has an

Active Backbone Connection, generate a Type 3 summary-LSA for

the destination, with Link State ID equal to the network's

address (if necessary, the Link State ID can also have one or

more of the network's host bits set; see Appendix E for

details) and metric equal to the routing table cost."

The changes in the ABR behavior described in this section allow a

multi-area connected router to successfully route traffic destined

for the backbone and other areas. Note that if the router does not

have a backbone area Configured it does not actively attract

inter-area traffic, because it does not consider itself an ABR and

does not originate summary-LSAs. It still can forward traffic from

one attached area to another along intra-area routes in case other

routers in corresponding areas have the best inter-area paths over

it, as described in section 1.2.

By processing all summaries when the backbone is not active, we

prevent the ABR, which has just lost its last backbone adjacency,

from dropping any packets going through the ABR in question to

another ABR and destined towards the backbone or other areas not

connected to the ABR directly.

3 Virtual Link Treatment

The Cisco ABR approach described in this document requires an ABR to

have at least one active interface in the backbone area. This

requirement may cause problems with virtual links in those rare

situations where the backbone area is purely virtual, as shown in

Figure 3, and the state of the VL is determined as in [Ref1].

....... ........... ......

. . . .

+--+ VL +--+

R1***********R2

+--+ +--+

Area 1 . . Area 2 . . Area 3

....... ........... ......

Figure 3. Purely Virtual Backbone

If R1 and R2 treat virtual links as in [Ref1], their virtual links

will never go up, because their router-LSAs do not contain the B-bit,

which is, in turn, because the routers do not have active interfaces

(virtual links) in the backbone and do not consider themselves ABRs.

Note that this problem does not appear if one of the routers has a

real interface in the backbone, as it usually is in real networks.

Though the situation described is deemed to be rather rare,

implementations supporting Cisco ABR behavior may consider changing

VL-specific code so that a virtual link is reported up (an

InterfaceUp event is generated) when a router with corresponding

router-ID is seen via Dijkstra, no matter whether its router-LSA

indicates that it is an ABR or not. This means that checking of

configured virtual links should be done not in step 4 of the

algorithm in 16.1 of [Ref1] when a router routing entry is added, but

every time a vertex is added to the SPT in step 3 of the same

algorithm.

4 Compatibility

The changes of the OSPF ABR operations do not influence any ASPects

of the router-to-router cooperation and do not create routing loops,

and hence are fully compatible with standard OSPF. Proof of

compatibility is outside the scope of this document.

5 Deployment Considerations

This section discusses the deployments details of the ABR behaviors

described in this document. Note that this approach is fully

compatible with standard ABR behavior, so ABRs acting as described in

[Ref1] and in this document can coexist in an OSPF domain and will

function without problems.

Deployment of ABRs using the alternative methods improves the

behavior of a router connected to multiple areas without a backbone

attachment, but can lead to uneXPected routing asymmetry, as

described below.

Consider an OSPF domain depicted in Figure 4.

. Backbone .

. .

. --------------------- .

. 1 1 .

..+--+.............+--+..

..R1..... ....R4..

. +--+ . . +--+ .

. 1 . . /4 .

. 8 +--+ 4 / .

. +-R3---+ .

. 1 / +--+\4 .

. +--+ / . . \ 4 +--+ .

. R2/8 . . +--R5 .

. +--+ . . +--+ .

. . . .

. --------- . . -------- .

. net N . . net M .

. . . .

. Area 1 . . Area 2 .

........... ..........

Figure 4. Inter-area routing asymmetry

Assume that R3 uses the approach described in this document. In this

case R2 will have inter-area routes to network M via ABR R1 only. R5

in turn will have its inter-area route to network N via R4, but as

far as R4 is only reachable via R3, all traffic destined to network N

will pass through R3. R3 will have an intra-area route to network N

via R2 and will, of course, route it directly to it (because

intra-area routes are always preferred over inter-area ones).

Traffic going back from network N to network M will pass through R2

and will be routed to R1, as R2 will not have any inter-area routes

via R3. So, traffic from N to M will always go through the backbone

while traffic from M to N will cross the areas directly via R3 and,

in this example, will not use a more optimal path through the

backbone.

Note that this problem is not caused by the fact that R3 uses the

alternative approach. The reason for attracting the attention to it

is that R3 is not really functioning as an ABR in case this new

behavior is used, i.e., it does not inject summary-LSAs into the

attached areas, but inter-area traffic can still go through it.

6 Security Considerations

The alternative ABR behaviors specified in this document do not raise

any security issues that are not already covered in [Ref1].

7 Acknowledgements

Authors would like to thank Alvaro Retana, Russ White, and Liem

Nguyen for their review of the document.

8 Disclaimer

This document describes OSPF ABR implementations of respective

vendors "as is", only for informational purposes, and without any

warranties, guarantees or support. These implementations are subject

to possible future changes. For the purposes of easier deployment,

information about software versions where described behavior was

integrated is provided below.

Initial Cisco ABR implementation (slightly different from the one

described in this memo, requiring non-backbone areas to be

configured, and not necessarily actively attached in the ABR

definition) was introduced in Cisco IOS (tm) version 11.1(6). Cisco

ABR behavior described in this document was integrated in Cisco IOS

(tm) in version 12.1(3)T.

The ABR behavior described as IBM ABR approach was implemented by IBM

in IBM Nways Multiprotocol Routing Services (MRS) 3.3.

Note that the authors do not intend to keep this document in sync

with actual implementations.

10 References

[Ref1] Moy, J., "OSPF version 2", STD 54, RFC2328, April 1998.

11 Authors' Addresses

Alex Zinin

Alcatel

EMail: zinin@psg.com

Derek M. Yeung

Procket Networks

1100 Cadillac Ct

Milpitas, CA 95035

Phone: 408-635-7911

EMail: myeung@procket.com

Acee Lindem

Redback Networks

102 Carric Bend Court

Cary, NC 27519 USA

Phone: 919-387-6971

EMail: acee@redback.com

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