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RFC2338 - Virtual Router Redundancy Protocol

王朝other·作者佚名  2008-05-31
窄屏简体版  字體: |||超大  

Network Working Group S. Knight

Request for Comments: 2338 D. Weaver

Category: Standards Track Ascend Communications, Inc.

D. Whipple

Microsoft, Inc.

R. Hinden

D. Mitzel

P. Hunt

Nokia

P. Higginson

M. Shand

Digital Equipment Corp.

A. Lindem

IBM Corporation

April 1998

Virtual Router Redundancy Protocol

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

This memo defines the Virtual Router Redundancy Protocol (VRRP).

VRRP specifies an election protocol that dynamically assigns

responsibility for a virtual router to one of the VRRP routers on a

LAN. The VRRP router controlling the IP address(es) associated with

a virtual router is called the Master, and forwards packets sent to

these IP addresses. The election process provides dynamic fail over

in the forwarding responsibility should the Master become

unavailable. This allows any of the virtual router IP addresses on

the LAN to be used as the default first hop router by end-hosts. The

advantage gained from using VRRP is a higher availability default

path without requiring configuration of dynamic routing or router

discovery protocols on every end-host.

Table of Contents

1. IntrodUCtion...............................................2

2. Required Features..........................................5

3. VRRP Overview..............................................6

4. Sample Configurations......................................8

5. Protocol...................................................9

5.1 VRRP Packet Format....................................10

5.2 IP Field Descriptions.................................10

5.3 VRRP Field Descriptions...............................11

6. Protocol State Machine....................................13

6.1 Parameters............................................13

6.2 Timers................................................15

6.3 State Transition Diagram..............................15

6.4 State Descriptions....................................15

7. Sending and Receiving VRRP Packets........................18

7.1 Receiving VRRP Packets................................18

7.2 Transmitting Packets..................................19

7.3 Virtual MAC Address...................................19

8. Operational Issues........................................20

8.1 ICMP Redirects........................................20

8.2 Host ARP Requests.....................................20

8.3 Proxy ARP.............................................20

9. Operation over FDDI and Token Ring........................21

9.1 Operation over FDDI...................................21

9.2 Operation over Token Ring.............................21

10. Security Considerations...................................23

10.1 No Authentication....................................23

10.2 Simple Text PassWord.................................23

10.3 IP Authentication Header.............................24

11. Acknowledgments...........................................24

12. References................................................24

13. Authors' Addresses........................................25

14. Full Copyright Statement..................................27

1. Introduction

There are a number of methods that an end-host can use to determine

its first hop router towards a particular IP destination. These

include running (or snooping) a dynamic routing protocol such as

Routing Information Protocol [RIP] or OSPF version 2 [OSPF], running

an ICMP router discovery client [DISC] or using a statically

configured default route.

Running a dynamic routing protocol on every end-host may be

infeasible for a number of reasons, including administrative

overhead, processing overhead, security issues, or lack of a protocol

implementation for some platforms. Neighbor or router discovery

protocols may require active participation by all hosts on a network,

leading to large timer values to reduce protocol overhead in the face

of large numbers of hosts. This can result in a significant delay in

the detection of a lost (i.e., dead) neighbor, which may introduce

unacceptably long "black hole" periods.

The use of a statically configured default route is quite popular; it

minimizes configuration and processing overhead on the end-host and

is supported by virtually every IP implementation. This mode of

operation is likely to persist as dynamic host configuration

protocols [DHCP] are deployed, which typically provide configuration

for an end-host IP address and default gateway. However, this

creates a single point of failure. Loss of the default router

results in a catastrophic event, isolating all end-hosts that are

unable to detect any alternate path that may be available.

The Virtual Router Redundancy Protocol (VRRP) is designed to

eliminate the single point of failure inherent in the static default

routed environment. VRRP specifies an election protocol that

dynamically assigns responsibility for a virtual router to one of the

VRRP routers on a LAN. The VRRP router controlling the IP

address(es) associated with a virtual router is called the Master,

and forwards packets sent to these IP addresses. The election

process provides dynamic fail-over in the forwarding responsibility

should the Master become unavailable. Any of the virtual router's IP

addresses on a LAN can then be used as the default first hop router

by end-hosts. The advantage gained from using VRRP is a higher

availability default path without requiring configuration of dynamic

routing or router discovery protocols on every end-host.

VRRP provides a function similar to a Cisco Systems, Inc. proprietary

protocol named Hot Standby Router Protocol (HSRP) [HSRP] and to a

Digital Equipment Corporation, Inc. proprietary protocol named IP

Standby Protocol [IPSTB].

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

document are to be interpreted as described in [RFC2119].

The IESG/IETF take no position regarding the validity or scope of any

intellectual property right or other rights that might be claimed to

pertain to the implementation or use of the technology, or the extent

to which any license under such rights might or might not be

available. See the IETF IPR web page at http://www.ietf.org/ipr.Html

for additional information.

1.1 Scope

The remainder of this document describes the features, design goals,

and theory of operation of VRRP. The message formats, protocol

processing rules and state machine that guarantee convergence to a

single Virtual Router Master are presented. Finally, operational

issues related to MAC address mapping, handling of ARP requests,

generation of ICMP redirect messages, and security issues are

addressed.

This protocol is intended for use with IPv4 routers only. A separate

specification will be produced if it is decided that similar

functionality is desirable in an IPv6 environment.

1.2 Definitions

VRRP Router A router running the Virtual Router Redundancy

Protocol. It may participate in one or more

virtual routers.

Virtual Router An abstract object managed by VRRP that acts

as a default router for hosts on a shared LAN.

It consists of a Virtual Router Identifier and

a set of associated IP address(es) across a

common LAN. A VRRP Router may backup one or

more virtual routers.

IP Address Owner The VRRP router that has the virtual router's

IP address(es) as real interface address(es).

This is the router that, when up, will respond

to packets addressed to one of these IP

addresses for ICMP pings, TCP connections,

etc.

Primary IP Address An IP address selected from the set of real

interface addresses. One possible selection

algorithm is to always select the first

address. VRRP advertisements are always sent

using the primary IP address as the source of

the IP packet.

Virtual Router Master The VRRP router that is assuming the

responsibility of forwarding packets sent to

the IP address(es) associated with the virtual

router, and answering ARP requests for these

IP addresses. Note that if the IP address

owner is available, then it will always become

the Master.

Virtual Router Backup The set of VRRP routers available to assume

forwarding responsibility for a virtual router

should the current Master fail.

2.0 Required Features

This section outlines the set of features that were considered

mandatory and that guided the design of VRRP.

2.1 IP Address Backup

Backup of IP addresses is the primary function of the Virtual Router

Redundancy Protocol. While providing election of a Virtual Router

Master and the additional functionality described below, the protocol

should strive to:

- Minimize the duration of black holes.

- Minimize the steady state bandwidth overhead and processing

complexity.

- Function over a wide variety of multiAccess LAN technologies

capable of supporting IP traffic.

- Provide for election of multiple virtual routers on a network for

load balancing

- Support of multiple logical IP subnets on a single LAN segment.

2.2 Preferred Path Indication

A simple model of Master election among a set of redundant routers is

to treat each router with equal preference and claim victory after

converging to any router as Master. However, there are likely to be

many environments where there is a distinct preference (or range of

preferences) among the set of redundant routers. For example, this

preference may be based upon access link cost or speed, router

performance or reliability, or other policy considerations. The

protocol should allow the eXPression of this relative path preference

in an intuitive manner, and guarantee Master convergence to the most

preferential router currently available.

2.3 Minimization of Unnecessary Service Disruptions

Once Master election has been performed then any unnecessary

transitions between Master and Backup routers can result in a

disruption in service. The protocol should ensure after Master

election that no state transition is triggered by any Backup router

of equal or lower preference as long as the Master continues to

function properly.

Some environments may find it beneficial to avoid the state

transition triggered when a router becomes available that is more

preferential than the current Master. It may be useful to support an

override of the immediate convergence to the preferred path.

2.4 Extensible Security

The virtual router functionality is applicable to a wide range of

internetworking environments that may employ different security

policies. The protocol should require minimal configuration and

overhead in the insecure operation, provide for strong authentication

when increased security is required, and allow integration of new

security mechanisms without breaking backwards compatible operation.

2.5 Efficient Operation over Extended LANs

Sending IP packets on a multiaccess LAN requires mapping from an IP

address to a MAC address. The use of the virtual router MAC address

in an extended LAN employing learning bridges can have a significant

effect on the bandwidth overhead of packets sent to the virtual

router. If the virtual router MAC address is never used as the

source address in a link level frame then the station location is

never learned, resulting in flooding of all packets sent to the

virtual router. To improve the efficiency in this environment the

protocol should: 1) use the virtual router MAC as the source in a

packet sent by the Master to trigger station learning; 2) trigger a

message immediately after transitioning to Master to update the

station learning; and 3) trigger periodic messages from the Master to

maintain the station learning cache.

3.0 VRRP Overview

VRRP specifies an election protocol to provide the virtual router

function described earlier. All protocol messaging is performed

using IP multicast datagrams, thus the protocol can operate over a

variety of multiaccess LAN technologies supporting IP multicast.

Each VRRP virtual router has a single well-known MAC address

allocated to it. This document currently only details the mapping to

networks using the IEEE 802 48-bit MAC address. The virtual router

MAC address is used as the source in all periodic VRRP messages sent

by the Master router to enable bridge learning in an extended LAN.

A virtual router is defined by its virtual router identifier (VRID)

and a set of IP addresses. A VRRP router may associate a virtual

router with its real addresses on an interface, and may also be

configured with additional virtual router mappings and priority for

virtual routers it is willing to backup. The mapping between VRID

and addresses must be coordinated among all VRRP routers on a LAN.

However, there is no restriction against reusing a VRID with a

different address mapping on different LANs. The scope of each

virtual router is restricted to a single LAN.

To minimize network traffic, only the Master for each virtual router

sends periodic VRRP Advertisement messages. A Backup router will not

attempt to pre-empt the Master unless it has higher priority. This

eliminates service disruption unless a more preferred path becomes

available. It's also possible to administratively prohibit all pre-

emption attempts. The only exception is that a VRRP router will

always become Master of any virtual router associated with addresses

it owns. If the Master becomes unavailable then the highest priority

Backup will transition to Master after a short delay, providing a

controlled transition of the virtual router responsibility with

minimal service interruption.

VRRP defines three types of authentication providing simple

deployment in insecure environments, added protection against

misconfiguration, and strong sender authentication in security

conscious environments. Analysis of the protection provided and

vulnerability of each mechanism is deferred to Section 10.0 Security

Considerations. In addition new authentication types and data can be

defined in the future without affecting the format of the fixed

portion of the protocol packet, thus preserving backward compatible

operation.

The VRRP protocol design provides rapid transition from Backup to

Master to minimize service interruption, and incorporates

optimizations that reduce protocol complexity while guaranteeing

controlled Master transition for typical operational scenarios. The

optimizations result in an election protocol with minimal runtime

state requirements, minimal active protocol states, and a single

message type and sender. The typical operational scenarios are

defined to be two redundant routers and/or distinct path preferences

among each router. A side effect when these assumptions are violated

(i.e., more than two redundant paths all with equal preference) is

that duplicate packets may be forwarded for a brief period during

Master election. However, the typical scenario assumptions are

likely to cover the vast majority of deployments, loss of the Master

router is infrequent, and the expected duration in Master election

convergence is quite small ( << 1 second ). Thus the VRRP

optimizations represent significant simplifications in the protocol

design while incurring an insignificant probability of brief network

degradation.

4. Sample Configurations

4.1 Sample Configuration 1

The following figure shows a simple network with two VRRP routers

implementing one virtual router. Note that this example is provided

to help understand the protocol, but is not expected to occur in

actual practice.

+-----+ +-----+

MR1 BR1

VRID=1 +-----+ +-----+

IP A ---------->* *<--------- IP B

------------------+------------+-----+--------+--------+--------+--

^ ^ ^ ^

(IP A) (IP A) (IP A) (IP A)

+--+--+ +--+--+ +--+--+ +--+--+

H1 H2 H3 H4

+-----+ +-----+ +--+--+ +--+--+

Legend:

---+---+---+-- = Ethernet, Token Ring, or FDDI

H = Host computer

MR = Master Router

BR = Backup Router

* = IP Address

(IP) = default router for hosts

The above configuration shows a very simple VRRP scenario. In this

configuration, the end-hosts install a default route to the IP

address of virtual router #1 (IP A) and both routers run VRRP. The

router on the left becomes the Master for virtual router #1 (VRID=1)

and the router on the right is the Backup for virtual router #1. If

the router on the left should fail, the other router will take over

virtual router #1 and its IP addresses, and provide uninterrupted

service for the hosts.

Note that in this example, IP B is not backed up by the router on the

left. IP B is only used by the router on the right as its interface

address. In order to backup IP B, a second virtual router would have

to be configured. This is shown in the next section.

4.2 Sample Configuration 2

The following figure shows a configuration with two virtual routers

with the hosts spitting their traffic between them. This example is

expected to be very common in actual practice.

+-----+ +-----+

MR1 MR2

& &

BR2 BR1

VRID=1 +-----+ +-----+ VRID=2

IP A ---------->* *<---------- IP B

------------------+------------+-----+--------+--------+--------+--

^ ^ ^ ^

(IP A) (IP A) (IP B) (IP B)

+--+--+ +--+--+ +--+--+ +--+--+

H1 H2 H3 H4

+-----+ +-----+ +--+--+ +--+--+

Legend:

---+---+---+-- = Ethernet, Token Ring, or FDDI

H = Host computer

MR = Master Router

BR = Backup Router

* = IP Address

(IP) = default router for hosts

In the above configuration, half of the hosts install a default route

to virtual router #1's IP address (IP A), and the other half of the

hosts install a default route to virtual router #2's IP address (IP

B). This has the effect of load balancing the outgoing traffic,

while also providing full redundancy.

5.0 Protocol

The purpose of the VRRP packet is to communicate to all VRRP routers

the priority and the state of the Master router associated with the

Virtual Router ID.

VRRP packets are sent encapsulated in IP packets. They are sent to

the IPv4 multicast address assigned to VRRP.

5.1 VRRP Packet Format

This section defines the format of the VRRP packet and the relevant

fields in the IP 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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version Type Virtual Rtr ID Priority Count IP Addrs

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Auth Type Adver Int Checksum

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP Address (1)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.

.

.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP Address (n)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication Data (1)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication Data (2)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.2 IP Field Descriptions

5.2.1 Source Address

The primary IP address of the interface the packet is being sent

from.

5.2.2 Destination Address

The IP multicast address as assigned by the IANA for VRRP is:

224.0.0.18

This is a link local scope multicast address. Routers MUST NOT

forward a datagram with this destination address regardless of its

TTL.

5.2.3 TTL

The TTL MUST be set to 255. A VRRP router receiving a packet with

the TTL not equal to 255 MUST discard the packet.

5.2.4 Protocol

The IP protocol number assigned by the IANA for VRRP is 112

(decimal).

5.3 VRRP Field Descriptions

5.3.1 Version

The version field specifies the VRRP protocol version of this packet.

This document defines version 2.

5.3.2 Type

The type field specifies the type of this VRRP packet. The only

packet type defined in this version of the protocol is:

1 ADVERTISEMENT

A packet with unknown type MUST be discarded.

5.3.3 Virtual Rtr ID (VRID)

The Virtual Router Identifier (VRID) field identifies the virtual

router this packet is reporting status for.

5.3.4 Priority

The priority field specifies the sending VRRP router's priority for

the virtual router. Higher values equal higher priority. This field

is an 8 bit unsigned integer field.

The priority value for the VRRP router that owns the IP address(es)

associated with the virtual router MUST be 255 (decimal).

VRRP routers backing up a virtual router MUST use priority values

between 1-254 (decimal). The default priority value for VRRP routers

backing up a virtual router is 100 (decimal).

The priority value zero (0) has special meaning indicating that the

current Master has stopped participating in VRRP. This is used to

trigger Backup routers to quickly transition to Master without having

to wait for the current Master to timeout.

5.3.5 Count IP Addrs

The number of IP addresses contained in this VRRP advertisement.

5.3.6 Authentication Type

The authentication type field identifies the authentication method

being utilized. Authentication type is unique on a per interface

basis. The authentication type field is an 8 bit unsigned integer.

A packet with unknown authentication type or that does not match the

locally configured authentication method MUST be discarded.

The authentication methods currently defined are:

0 - No Authentication

1 - Simple Text Password

2 - IP Authentication Header

5.3.6.1 No Authentication

The use of this authentication type means that VRRP protocol

exchanges are not authenticated. The contents of the Authentication

Data field should be set to zero on transmission and ignored on

reception.

5.3.6.2 Simple Text Password

The use of this authentication type means that VRRP protocol

exchanges are authenticated by a clear text password. The contents

of the Authentication Data field should be set to the locally

configured password on transmission. There is no default password.

The receiver MUST check that the Authentication Data in the packet

matches its configured authentication string. Packets that do not

match MUST be discarded.

Note that there are security implications to using Simple Text

password authentication, and one should see the Security

Consideration section of this document.

5.3.6.3 IP Authentication Header

The use of this authentication type means the VRRP protocol exchanges

are authenticated using the mechanisms defined by the IP

Authentication Header [AUTH] using "The Use of HMAC-MD5-96 within ESP

and AH" [HMAC]. Keys may be either configured manually or via a key

distribution protocol.

If a packet is received that does not pass the authentication check

due to a missing authentication header or incorrect message digest,

then the packet MUST be discarded. The contents of the

Authentication Data field should be set to zero on transmission and

ignored on reception.

5.3.7 Advertisement Interval (Adver Int)

The Advertisement interval indicates the time interval (in seconds)

between ADVERTISEMENTS. The default is 1 second. This field is used

for troubleshooting misconfigured routers.

5.3.8 Checksum

The checksum field is used to detect data corruption in the VRRP

message.

The checksum is the 16-bit one's complement of the one's complement

sum of the entire VRRP message starting with the version field. For

computing the checksum, the checksum field is set to zero.

5.3.9 IP Address(es)

One or more IP addresses that are associated with the virtual router.

The number of addresses included is specified in the "Count IP Addrs"

field. These fields are used for troubleshooting misconfigured

routers.

5.3.10 Authentication Data

The authentication string is currently only utilized for simple text

authentication, similar to the simple text authentication found in

the Open Shortest Path First routing protocol [OSPF]. It is up to 8

characters of plain text. If the configured authentication string is

shorter than 8 bytes, the remaining space MUST be zero-filled. Any

VRRP packet received with an authentication string that does not

match the locally configured authentication string MUST be discarded.

The authentication string is unique on a per interface basis.

There is no default value for this field.

6. Protocol State Machine

6.1 Parameters

6.1.1 Parameters per Interface

Authentication_Type Type of authentication being used. Values

are defined in section 5.3.6.

Authentication_Data Authentication data specific to the

Authentication_Type being used.

6.1.2 Parameters per Virtual Router

VRID Virtual Router Identifier. Configured item

in the range 1-255 (decimal). There is no

default.

Priority Priority value to be used by this VRRP

router in Master election for this virtual

router. The value of 255 (decimal) is

reserved for the router that owns the IP

addresses associated with the virtual

router. The value of 0 (zero) is reserved

for Master router to indicate it is

releasing responsibility for the virtual

router. The range 1-254 (decimal) is

available for VRRP routers backing up the

virtual router. The default value is 100

(decimal).

IP_Addresses One or more IP addresses associated with

this virtual router. Configured item. No

default.

Advertisement_Interval Time interval between ADVERTISEMENTS

(seconds). Default is 1 second.

Skew_Time Time to skew Master_Down_Interval in

seconds. Calculated as:

( (256 - Priority) / 256 )

Master_Down_Interval Time interval for Backup to declare Master

down (seconds). Calculated as:

(3 * Advertisement_Interval) + Skew_time

Preempt_Mode Controls whether a higher priority Backup

router preempts a lower priority Master.

Values are True to allow preemption and

False to not prohibit preemption. Default

is True.

Note: Exception is that the router that owns

the IP address(es) associated with the

virtual router always pre-empts independent

of the setting of this flag.

6.2 Timers

Master_Down_Timer Timer that fires when ADVERTISEMENT has not

been heard for Master_Down_Interval.

Adver_Timer Timer that fires to trigger sending of

ADVERTISEMENT based on

Advertisement_Interval.

6.3 State Transition Diagram

+---------------+

+---------> <-------------+

Initialize

+------ ----------+

+---------------+

V V

+---------------+ +---------------+

---------------------->

Master Backup

<----------------------

+---------------+ +---------------+

6.4 State Descriptions

In the state descriptions below, the state names are identified by

{state-name}, and the packets are identified by all upper case

characters.

A VRRP router implements an instance of the state machine for each

virtual router election it is participating in.

6.4.1 Initialize

The purpose of this state is to wait for a Startup event. If a

Startup event is received, then:

- If the Priority = 255 (i.e., the router owns the IP address(es)

associated with the virtual router)

o Send an ADVERTISEMENT

o Broadcast a gratuitous ARP request containing the virtual

router MAC address for each IP address associated with the

virtual router.

o Set the Adver_Timer to Advertisement_Interval

o Transition to the {Master} state

else

o Set the Master_Down_Timer to Master_Down_Interval

o Transition to the {Backup} state

endif

6.4.2 Backup

The purpose of the {Backup} state is to monitor the availability and

state of the Master Router.

While in this state, a VRRP router MUST do the following:

- MUST NOT respond to ARP requests for the IP address(s) associated

with the virtual router.

- MUST discard packets with a destination link layer MAC address

equal to the virtual router MAC address.

- MUST NOT accept packets addressed to the IP address(es) associated

with the virtual router.

- If a Shutdown event is received, then:

o Cancel the Master_Down_Timer

o Transition to the {Initialize} state

endif

- If the Master_Down_Timer fires, then:

o Send an ADVERTISEMENT

o Broadcast a gratuitous ARP request containing the virtual

router MAC address for each IP address associated with the

virtual router

o Set the Adver_Timer to Advertisement_Interval

o Transition to the {Master} state

endif

- If an ADVERTISEMENT is received, then:

If the Priority in the ADVERTISEMENT is Zero, then:

o Set the Master_Down_Timer to Skew_Time

else:

If Preempt_Mode is False, or If the Priority in the

ADVERTISEMENT is greater than or equal to the local

Priority, then:

o Reset the Master_Down_Timer to Master_Down_Interval

else:

o Discard the ADVERTISEMENT

endif

endif

endif

6.4.3 Master

While in the {Master} state the router functions as the forwarding

router for the IP address(es) associated with the virtual router.

While in this state, a VRRP router MUST do the following:

- MUST respond to ARP requests for the IP address(es) associated

with the virtual router.

- MUST forward packets with a destination link layer MAC address

equal to the virtual router MAC address.

- MUST NOT accept packets addressed to the IP address(es) associated

with the virtual router if it is not the IP address owner.

- MUST accept packets addressed to the IP address(es) associated

with the virtual router if it is the IP address owner.

- If a Shutdown event is received, then:

o Cancel the Adver_Timer

o Send an ADVERTISEMENT with Priority = 0

o Transition to the {Initialize} state

endif

- If the Adver_Timer fires, then:

o Send an ADVERTISEMENT

o Reset the Adver_Timer to Advertisement_Interval

endif

- If an ADVERTISEMENT is received, then:

If the Priority in the ADVERTISEMENT is Zero, then:

o Send an ADVERTISEMENT

o Reset the Adver_Timer to Advertisement_Interval

else:

If the Priority in the ADVERTISEMENT is greater than the

local Priority,

or

If the Priority in the ADVERTISEMENT is equal to the local

Priority and the primary IP Address of the sender is greater

than the local primary IP Address, then:

o Cancel Adver_Timer

o Set Master_Down_Timer to Master_Down_Interval

o Transition to the {Backup} state

else:

o Discard ADVERTISEMENT

endif

endif

endif

7. Sending and Receiving VRRP Packets

7.1 Receiving VRRP Packets

Performed the following functions when a VRRP packet is received:

- MUST verify that the IP TTL is 255.

- MUST verify the VRRP version

- MUST verify that the received packet length is greater than or

equal to the VRRP header

- MUST verify the VRRP checksum

- MUST perform authentication specified by Auth Type

If any one of the above checks fails, the receiver MUST discard the

packet, SHOULD log the event and MAY indicate via network management

that an error occurred.

- MUST verify that the VRID is valid on the receiving interface

If the above check fails, the receiver MUST discard the packet.

- MAY verify that the IP address(es) associated with the VRID are

valid

If the above check fails, the receiver SHOULD log the event and MAY

indicate via network management that a misconfiguration was detected.

If the packet was not generated by the address owner (Priority does

not equal 255 (decimal)), the receiver MUST drop the packet,

otherwise continue processing.

- MUST verify that the Adver Interval in the packet is the same as

the locally configured for this virtual router

If the above check fails, the receiver MUST discard the packet,

SHOULD log the event and MAY indicate via network management that a

misconfiguration was detected.

7.2 Transmitting VRRP Packets

The following operations MUST be performed when transmitting a VRRP

packet.

- Fill in the VRRP packet fields with the appropriate virtual

router configuration state

- Compute the VRRP checksum

- Set the source MAC address to Virtual Router MAC Address

- Set the source IP address to interface primary IP address

- Set the IP protocol to VRRP

- Send the VRRP packet to the VRRP IP multicast group

Note: VRRP packets are transmitted with the virtual router MAC

address as the source MAC address to ensure that learning bridges

correctly determine the LAN segment the virtual router is attached

to.

7.3 Virtual Router MAC Address

The virtual router MAC address associated with a virtual router is an

IEEE 802 MAC Address in the following format:

00-00-5E-00-01-{VRID} (in hex in internet standard bit-order)

The first three octets are derived from the IANA's OUI. The next two

octets (00-01) indicate the address block assigned to the VRRP

protocol. {VRID} is the VRRP Virtual Router Identifier. This

mapping provides for up to 255 VRRP routers on a network.

8. Operational Issues

8.1 ICMP Redirects

ICMP Redirects may be used normally when VRRP is running between a

group of routers. This allows VRRP to be used in environments where

the topology is not symmetric.

The IP source address of an ICMP redirect should be the address the

end host used when making its next hop routing decision. If a VRRP

router is acting as Master for virtual router(s) containing addresses

it does not own, then it must determine which virtual router the

packet was sent to when selecting the redirect source address. One

method to deduce the virtual router used is to examine the

destination MAC address in the packet that triggered the redirect.

It may be useful to disable Redirects for specific cases where VRRP

is being used to load share traffic between a number of routers in a

symmetric topology.

8.2 Host ARP Requests

When a host sends an ARP request for one of the virtual router IP

addresses, the Master virtual router MUST respond to the ARP request

with the virtual MAC address for the virtual router. The Master

virtual router MUST NOT respond with its physical MAC address. This

allows the client to always use the same MAC address regardless of

the current Master router.

When a VRRP router restarts or boots, it SHOULD not send any ARP

messages with its physical MAC address for the IP address it owns, it

should only send ARP messages that include Virtual MAC addresses.

This may entail:

- When configuring an interface, VRRP routers should broadcast a

gratuitous ARP request containing the virtual router MAC address

for each IP address on that interface.

- At system boot, when initializing interfaces for VRRP operation;

delay gratuitous ARP requests and ARP responses until both the IP

address and the virtual router MAC address are configured.

8.3 Proxy ARP

If Proxy ARP is to be used on a VRRP router, then the VRRP router

must advertise the Virtual Router MAC address in the Proxy ARP

message. Doing otherwise could cause hosts to learn the real MAC

address of the VRRP router.

9. Operation over FDDI and Token Ring

9.1 Operation over FDDI

FDDI interfaces remove from the FDDI ring frames that have a source

MAC address matching the device's hardware address. Under some

conditions, such as router isolations, ring failures, protocol

transitions, etc., VRRP may cause there to be more than one Master

router. If a Master router installs the virtual router MAC address

as the hardware address on a FDDI device, then other Masters'

ADVERTISEMENTS will be removed from the ring during the Master

convergence, and convergence will fail.

To avoid this an implementation SHOULD configure the virtual router

MAC address by adding a unicast MAC filter in the FDDI device, rather

than changing its hardware MAC address. This will prevent a Master

router from removing any ADVERTISEMENTS it did not originate.

9.2 Operation over Token Ring

Token ring has several characteristics which make running VRRP

difficult. These include:

- In order to switch to a new master located on a different bridge

token ring segment from the previous master when using source

route bridges, a mechanism is required to update cached source

route information.

- No general multicast mechanism supported across old and new token

ring adapter implementations. While many newer token ring adapters

support group addresses, token ring functional address support is

the only generally available multicast mechanism. Due to the

limited number of token ring functional addresses these may

collide with other usage of the same token ring functional

addresses.

Due to these difficulties, the preferred mode of operation over token

ring will be to use a token ring functional address for the VRID

virtual MAC address. Token ring functional addresses have the two

high order bits in the first MAC address octet set to B'1'. They

range from 03-00-00-00-00-80 to 03-00-02-00-00-00 (canonical format).

However, unlike multicast addresses, there is only one unique

functional address per bit position. The functional addresses

addresses 03-00-00-10-00-00 through 03-00-02-00-00-00 are reserved

by the Token Ring Architecture [TKARCH] for user-defined

applications. However, since there are only 12 user-defined token

ring functional addresses, there may be other non-IP protocols using

the same functional address. Since the Novell IPX [IPX] protocol uses

the 03-00-00-10-00-00 functional address, operation of VRRP over

token ring will avoid use of this functional address. In general,

token ring VRRP users will be responsible for resolution of other

user-defined token ring functional address conflicts.

VRIDs are mapped directly to token ring functional addresses. In

order to decrease the likelihood of functional address conflicts,

allocation will begin with the largest functional address. Most non-

IP protocols use the first or first couple user-defined functional

addresses and it is expected that VRRP users will choose VRIDs

sequentially starting with 1.

VRID Token Ring Functional Address

---- -----------------------------

1 03-00-02-00-00-00

2 03-00-04-00-00-00

3 03-00-08-00-00-00

4 03-00-10-00-00-00

5 03-00-20-00-00-00

6 03-00-40-00-00-00

7 03-00-80-00-00-00

8 03-00-00-01-00-00

9 03-00-00-02-00-00

10 03-00-00-04-00-00

11 03-00-00-08-00-00

Or more succinctly, octets 3 and 4 of the functional address are

equal to (0x4000 >> (VRID - 1)) in non-canonical format.

Since a functional address cannot be used used as a MAC level source

address, the real MAC address is used as the MAC source address in

VRRP advertisements. This is not a problem for bridges since packets

addressed to functional addresses will be sent on the spanning-tree

explorer path [802.1D].

The functional address mode of operation MUST be implemented by

routers supporting VRRP on token ring.

Additionally, routers MAY support unicast mode of operation to take

advantage of newer token ring adapter implementations which support

non-promiscuous reception for multiple unicast MAC addresses and to

avoid both the multicast traffic and usage conflicts associated with

the use of token ring functional addresses. Unicast mode uses the

same mapping of VRIDs to virtual MAC addresses as Ethernet. However,

one important difference exists. ARP request/reply packets contain

the virtual MAC address as the source MAC address. The reason for

this is that some token ring driver implementations keep a cache of

MAC address/source routing information independent of the ARP cache.

Hence, these implementations need have to receive a packet with the

virtual MAC address as the source address in order to transmit to

that MAC address in a source-route bridged network.

Unicast mode on token ring has one limitation which should be

considered. If there are VRID routers on different source-route

bridge segments and there are host implementations which keep their

source-route information in the ARP cache and do not listen to

gratuitous ARPs, these hosts will not update their ARP source-route

information correctly when a switch-over occurs. The only possible

solution is to put all routers with the same VRID on the same source-

bridge segment and use techniques to prevent that bridge segment from

being a single point of failure. These techniques are beyond the

scope this document.

For both the multicast and unicast mode of operation, VRRP

advertisements sent to 224.0.0.18 should be encapsulated as described

in [RFC1469].

10. Security Considerations

VRRP is designed for a range of internetworking environments that may

employ different security policies. The protocol includes several

authentication methods ranging from no authentication, simple clear

text passwords, and strong authentication using IP Authentication

with MD5 HMAC. The details on each approach including possible

attacks and recommended environments follows.

Independent of any authentication type VRRP includes a mechanism

(setting TTL=255, checking on receipt) that protects against VRRP

packets being injected from another remote network. This limits most

vulnerabilities to local attacks.

10.1 No Authentication

The use of this authentication type means that VRRP protocol

exchanges are not authenticated. This type of authentication SHOULD

only be used in environments were there is minimal security risk and

little chance for configuration errors (e.g., two VRRP routers on a

LAN).

10.2 Simple Text Password

The use of this authentication type means that VRRP protocol

exchanges are authenticated by a simple clear text password.

This type of authentication is useful to protect against accidental

misconfiguration of routers on a LAN. It protects against routers

inadvertently backing up another router. A new router must first be

configured with the correct password before it can run VRRP with

another router. This type of authentication does not protect against

hostile attacks where the password can be learned by a node snooping

VRRP packets on the LAN. The Simple Text Authentication combined

with the TTL check makes it difficult for a VRRP packet to be sent

from another LAN to disrupt VRRP operation.

This type of authentication is RECOMMENDED when there is minimal risk

of nodes on a LAN actively disrupting VRRP operation. If this type

of authentication is used the user should be aware that this clear

text password is sent frequently, and therefore should not be the

same as any security significant password.

10.3 IP Authentication Header

The use of this authentication type means the VRRP protocol exchanges

are authenticated using the mechanisms defined by the IP

Authentication Header [AUTH] using "The Use of HMAC-MD5-96 within ESP

and AH", [HMAC]. This provides strong protection against

configuration errors, replay attacks, and packet

corruption/modification.

This type of authentication is RECOMMENDED when there is limited

control over the administration of nodes on a LAN. While this type

of authentication does protect the operation of VRRP, there are other

types of attacks that may be employed on shared media links (e.g.,

generation of bogus ARP replies) which are independent from VRRP and

are not protected.

11. Acknowledgments

The authors would like to thank Glen Zorn, and Michael Lane, Clark

Bremer, Hal Peterson, Tony Li, Barbara Denny, Joel Halpern, Steve

Bellovin, and Thomas Narten for their comments and suggestions.

12. References

[802.1D] International Standard ISO/IEC 10038: 1993, ANSI/IEEE Std

802.1D, 1993 edition.

[AUTH] Kent, S., and R. Atkinson, "IP Authentication Header",

Work in Progress.

[DISC] Deering, S., "ICMP Router Discovery Messages", RFC1256,

September 1991.

[DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC2131,

March 1997.

[HMAC] Madson, C., and R. Glenn, "The Use of HMAC-MD5-96 within

ESP and AH", Work in Progress.

[HSRP] Li, T., Cole, B., Morton, P., and D. Li, "Cisco Hot Standby

Router Protocol (HSRP)", RFC2281, March 1998.

[IPSTB] Higginson, P., M. Shand, "Development of Router Clusters to

Provide Fast Failover in IP Networks", Digital Technical

Journal, Volume 9 Number 3, Winter 1997.

[IPX] Novell Incorporated., "IPX Router Specification", Version

1.10, October 1992.

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

[RIP] Hedrick, C., "Routing Information Protocol", RFC1058,

June 1988.

[RFC1469] Pusateri, T., "IP over Token Ring LANs", RFC1469, June

1993.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC2119, March 1997.

[TKARCH] IBM Token-Ring Network, Architecture Reference, Publication

SC30-3374-02, Third Edition, (September, 1989).

13. Authors' Addresses

Steven Knight Phone: +1 612 943-8990

Ascend Communications EMail: Steven.Knight@ascend.com

High Performance Network Division

10250 Valley View Road, Suite 113

Eden Prairie, MN USA 55344

USA

Douglas Weaver Phone: +1 612 943-8990

Ascend Communications EMail: Doug.Weaver@ascend.com

High Performance Network Division

10250 Valley View Road, Suite 113

Eden Prairie, MN USA 55344

USA

David Whipple Phone: +1 206 703-3876

Microsoft Corporation EMail: dwhipple@microsoft.com

One Microsoft Way

Redmond, WA USA 98052-6399

USA

Robert Hinden Phone: +1 408 990-2004

Nokia EMail: hinden@iprg.nokia.com

232 Java Drive

Sunnyvale, CA 94089

USA

Danny Mitzel Phone: +1 408 990-2037

Nokia EMail: mitzel@iprg.nokia.com

232 Java Drive

Sunnyvale, CA 94089

USA

Peter Hunt Phone: +1 408 990-2093

Nokia EMail: hunt@iprg.nokia.com

232 Java Drive

Sunnyvale, CA 94089

USA

P. Higginson Phone: +44 118 920 6293

Digital Equipment Corp. EMail: higginson@mail.dec.com

Digital Park

Imperial Way

Reading

Berkshire

RG2 0TE

UK

M. Shand Phone: +44 118 920 4424

Digital Equipment Corp. EMail: shand@mail.dec.com

Digital Park

Imperial Way

Reading

Berkshire

RG2 0TE

UK

Acee Lindem Phone: 1-919-254-1805

IBM Corporation E-Mail: acee@raleigh.ibm.com

P.O. Box 12195

Research Triangle Park, NC 27709

USA

14. Full Copyright Statement

Copyright (C) The Internet Society (1998). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

 
 
 
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