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RFC2281 - Cisco Hot Standby Router Protocol (HSRP)

王朝other·作者佚名  2008-05-31
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Network Working Group T. Li

Request for Comments: 2281 Juniper Networks

Category: Informational B. Cole

Juniper Networks

P. Morton

Cisco Systems

D. Li

Cisco Systems

March 1998

Cisco Hot Standby Router Protocol (HSRP)

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.

Copyright Notice

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

IESG Note

This document reflects an existing deployed protocol. The IETF does

have a working group which is in the process of prodUCing a standards

track protocol to address the same issues.

Abstract

The memo specifies the Hot Standby Router Protocol (HSRP). The goal

of the protocol is to allow hosts to appear to use a single router

and to maintain connectivity even if the actual first hop router they

are using fails. Multiple routers participate in this protocol and

in concert create the illusion of a single virtual router. The

protocol insures that one and only one of the routers is forwarding

packets on behalf of the virtual router. End hosts forward their

packets to the virtual router.

The router forwarding packets is known as the active router. A

standby router is selected to replace the active router should it

fail. The protocol provides a mechanism for determining active and

standby routers, using the IP addresses on the participating routers.

If an active router fails a standby router can take over without a

major interruption in the host's connectivity. This memo also

discusses the ARP, MAC address, and security issues with this

protocol.

TABLE OF CONTENTS

1 Introduction .............................................. 2

2 Conditions of Use ......................................... 3

3 Scope ..................................................... 4

3.1 Terminology ............................................... 4

4 Definitions ............................................... 4

5 Protocol .................................................. 4

5.1 Packet formats ............................................ 4

5.2 Operational parameters .................................... 7

5.3 States .................................................... 8

5.4 Timers .................................................... 9

5.5 Events .................................................... 9

5.6 Actions ................................................... 10

5.7 State Transitions.......................................... 11

6 MAC address considerations ................................ 13

6.1 General ................................................... 13

6.2 Address Filter ............................................ 14

6.3 ICMP Redirect ............................................. 14

6.4 Proxy ARP ................................................. 15

7 Security Considerations ................................... 15

8 References ................................................ 15

9 Authors' Addresses ........................................ 16

10 Full Copyright Statement .................................. 17

1. Introduction

The Hot Standby Router Protocol, HSRP, provides a mechanism which is

designed to support non-disruptive failover of IP traffic in certain

circumstances. In particular, the protocol protects against the

failure of the first hop router when the source host cannot learn the

IP address of the first hop router dynamically. The protocol is

designed for use over multi-Access, multicast or broadcast capable

LANs (e.g., Ethernet). HSRP is not intended as a replacement for

existing dynamic router discovery mechanisms and those protocols

should be used instead whenever possible [1]. A large class of

legacy host implementations that do not support dynamic discovery are

capable of configuring a default router. HSRP provides failover

services to those hosts.

All of the routers participating in HSRP are assumed to be running

appropriate IP routing protocols and have a consistent set of routes.

The discussion of which protocols are appropriate and whether routing

is consistent in any given situation is beyond the scope of this

specification.

Using HSRP, a set of routers work in concert to present the illusion

of a single virtual router to the hosts on the LAN. This set is

known as an HSRP group or a standby group. A single router elected

from the group is responsible for forwarding the packets that hosts

send to the virtual router. This router is known as the active

router. Another router is elected as the standby router. In the

event that the active router fails, the standby assumes the packet

forwarding duties of the active router. Although an arbitrary number

of routers may run HSRP, only the active router forwards the packets

sent to the virtual router.

To minimize network traffic, only the active and the standby routers

send periodic HSRP messages once the protocol has completed the

election process. If the active router fails, the standby router

takes over as the active router. If the standby router fails or

becomes the active router, another router is elected as the standby

router.

On a particular LAN, multiple hot standby groups may coexist and

overlap. Each standby group emulates a single virtual router. For

each standby group, a single well-known MAC address is allocated to

the group, as well as an IP address. The IP address SHOULD belong to

the primary subnet in use on the LAN, but MUST differ from the

addresses allocated as interface addresses on all routers and hosts

on the LAN, including virtual IP addresses assigned to other HSRP

groups.

If multiple groups are used on a single LAN, load splitting can be

achieved by distributing hosts among different standby groups.

The remainder of this specification discusses the operation of a

single standby group. In the case of multiple groups, each group

operates independently of other groups on the LAN and according to

this specification. Note that individual routers may participate in

multiple groups. In this case, the router maintains separate state

and timers for each group.

2 Conditions of Use

US Patent number 5,473,599 [2], assigned to Cisco Systems, Inc. may

be applicable to HSRP. If an implementation requires the use of any

claims of patent no. 5,473,599, Cisco will license such claims on

reasonable, nondiscriminatory terms for use in practicing the

standard. More specifically, such license will be available for a

one-time, paid up fee.

3 Scope

This document describes the packets, messages, states, and events

used to implement the protocol. It does not discuss network

management or internal implementation issues.

3.1 Terminology

The language conventions of RFC2119 [3] are used in this document.

4 Definitions

Active Router - the router that is currently forwarding packets

for the virtual router

Standby Router - the primary backup router

Standby Group - the set of routers participating in HSRP that

jointly emulate a virtual router

Hello Time - the interval between successive HSRP Hello

messages from a given router

Hold Time - the interval between the receipt of a Hello

message and the presumption that the sending

router has failed

5 Protocol

Within a standby group, the routers periodically advertise state

information using various messages.

5.1 Packet formats

The standby protocol runs on top of UDP, and uses port number 1985.

Packets are sent to multicast address 224.0.0.2 with TTL 1.

Routers use their actual IP address as the source address for

protocol packets, not the virtual IP address. This is necessary so

that the HSRP routers can identify each other.

The format of the data portion of the UDP datagram is:

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 Op Code State Hellotime

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

Holdtime Priority Group Reserved

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

Authentication Data

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

Authentication Data

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

Virtual IP Address

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

Version: 1 octet

The version of the HSRP messages. This document describes version

0.

Op Code: 1 octet

The Op Code describes the type of message contained in this

packet. Possible values are:

0 - Hello

1 - Coup

2 - Resign

Hello messages are sent to indicate that a router is running and

is capable of becoming the active or standby router.

Coup messages are sent when a router wishes to become the active

router.

Resign messages are sent when a router no longer wishes to be the

active router.

State: 1 octet

Internally, each router in the standby group implements a state

machine. The State field describes the current state of the

router sending the message. Details on the individual states are

described below. Possible values are:

0 - Initial

1 - Learn

2 - Listen

4 - Speak

8 - Standby

16 - Active

Hellotime: 1 octet

This field is only meaningful in Hello messages. It contains the

approximate period between the Hello messages that the router

sends. The time is given in seconds.

If the Hellotime is not configured on a router, then it MAY be

learned from the Hello message from the active router. The

Hellotime SHOULD only be learned if no Hellotime is configured and

the Hello message is authenticated. A router that sends a Hello

message MUST insert the Hellotime that it is using in the

Hellotime field in the Hello message. If the Hellotime is not

learned from a Hello message from the active router and it is not

manually configured, a default value of 3 seconds is RECOMMENDED.

Holdtime: 1 octet

This field is only meaningful in Hello messages. It contains the

amount of time that the current Hello message should be considered

valid. The time is given in seconds.

If a router sends a Hello message, then receivers should consider

that Hello message to be valid for one Holdtime. The Holdtime

SHOULD be at least three times the value of the Hellotime and MUST

be greater than the Hellotime. If the Holdtime is not configured

on a router, then it MAY be learned from the Hello message from

the active router. The Holdtime SHOULD only be learned if the

Hello message is authenticated. A router that sends a Hello

message MUST insert the Holdtime that it is using in the Holdtime

field in the Hello message.

A router which is in active state MUST NOT learn new values for

the Hellotime and the Holdtime from other routers, although it may

continue to use values which it learned from the previous active

router. It MAY also use the Hellotime and Holdtime values learned

through manual configuration. The active router MUST NOT use one

configured time and one learned time. If the Holdtime is not

learned and it is not manually configured, a default value of 10

seconds is RECOMMENDED.

Priority: 1 octet

This field is used to elect the active and standby routers. When

comparing priorities of two different routers, the router with the

numerically higher priority wins. In the case of routers with

equal priority the router with the higher IP address wins.

Group: 1 octet

This field identifies the standby group. For Token Ring, values

between 0 and 2 inclusive are valid. For other media values

between 0 and 255 inclusive are valid.

Authentication Data: 8 octets

This field contains a clear-text 8 character reused passWord.

If no authentication data is configured, the RECOMMENDED default

value is 0x63 0x69 0x73 0x63 0x6F 0x00 0x00 0x00.

Virtual IP Address: 4 octets

The virtual IP address used by this group.

If the virtual IP address is not configured on a router, then it

MAY be learned from the Hello message from the active router. An

address SHOULD only be learned if no address was configured and

the Hello message is authenticated.

5.2 Operational parameters

The following information MUST be known to each router in the standby

group. The mechanisms used to determine this information are outside

of the scope of this document.

Standby group number

Virtual MAC address

Priority

Authentication Data

Hellotime

Holdtime

The following information MUST be known to at least one router in

each standby group and MAY be known by any of the other routers in

the group.

Virtual IP Address

The following information MAY be configured on any router:

Preemption capability

If a router has higher priority than the active router and

preemption is configured, it MAY take over as the active router

using a Coup message.

5.3 States

Each router in the group participates in the protocol by implementing

a simple state machine. This specification describes the externally

visible behavior of this state machine. Implementations MAY vary

their internal implementations within the functional description of

the state machine.

All routers begin in the Initial state. This section discusses the

intent of each state. For specific details on the actions taken in

each state, please see the state transition table in section 5.7.

1. Initial

This is the starting state and indicates that HSRP is not running.

This state is entered via a configuration change or when an

interface first comes up.

2. Learn

The router has not determined the virtual IP address, and not yet

seen an authenticated Hello message from the active router. In

this state the router is still waiting to hear from the active

router.

3. Listen

The router knows the virtual IP address, but is neither the active

router nor the standby router. It listens for Hello messages from

those routers.

4. Speak

The router sends periodic Hello messages and is actively

participating in the election of the active and/or standby router.

A router cannot enter Speak state unless it has the virtual IP

address.

5. Standby

The router is a candidate to become the next active router and

sends periodic Hello messages. Excluding transient conditions,

there MUST be at most one router in the group in Standby state.

6. Active

The router is currently forwarding packets that are sent to the

group's virtual MAC address. The router sends periodic Hello

messages. Excluding transient conditions, there MUST be at most

one router in Active state in the group.

5.4 Timers

Each router maintains three timers, an Active timer, a Standby timer,

and a Hello timer.

The Active timer is used to monitor the active router. The active

timer is started anytime an authenticated Hello message is seen from

the active router. It is set to eXPire in the Holdtime seen in the

Hello message.

The Standby timer is used to monitor the standby router The Standby

timer is started anytime an authenticated Hello message is seen from

the standby router. It is set to expire in the Holdtime seen in the

Hello message.

The Hello timer expires once per Hellotime period. If the router is

in Speak, Standby, or Active states, it should generate a Hello

message upon Hello timer expiry. The Hello timer MUST be jittered.

5.5 Events

These are the events in the HSRP finite state machine.

a - HSRP is configured on an enabled interface.

b - HSRP is disabled on an interface or the interface is disabled.

c - Active timer expiry. The Active timer was set to the Holdtime

when the last Hello message was seen from the active router.

d - Standby timer expiry. The Standby timer was set to the

Holdtime when the last Hello message was seen from the standby

router.

e - Hello timer expiry. The periodic timer for sending Hello

messages has expired.

f - Receipt of a Hello message of higher priority from a router in

Speak state.

g - Receipt of a Hello message of higher priority from the active

router.

h - Receipt of a Hello message of lower priority from the active

router.

i - Receipt of a Resign message from the active router.

j - Receipt of a Coup message from a higher priority router.

k - Receipt of a Hello message of higher priority from the standby

router.

l - Receipt of a Hello message of lower priority from the standby

router.

5.6 Actions

This section specifies the actions to be taken as part of the state

machine.

A Start Active Timer

If this action occurred as the result of the receipt of a an

authenticated Hello message from the active router, the Active

timer is set to the Holdtime field in the Hello message.

Otherwise the Active timer is set to the current Holdtime value

in use by this router. The Active timer is then started.

B Start Standby Timer

If this action occurred as the result of the receipt of an

authenticated Hello message from the standby router, the

Standby timer is set to the Holdtime field in the Hello

message. Otherwise the Standby timer is set to the current

hold time value in use by this router. The Standby timer is

then started.

C Stop Active Timer

The Active timer is stopped.

D Stop Standby Timer

The Standby timer is stopped.

E Learn Parameters

This action is taken when an authenticated message is received

from the active router. If the virtual IP address for this

group was not manually configured, the virtual IP address MAY

be learned from the message. The router MAY learn Hellotime

and Holdtime values from the message.

F Send Hello Message

The router sends a Hello message with its current State,

Hellotime and Holdtime.

G Send Coup Message

The router sends a Coup message to inform the active router

that there is a higher priority router available.

H Send Resign Message

The router sends a Resign message to allow another router to

become the active router.

I Send Gratuitous ARP Message

The router broadcasts an ARP response packet advertising the

group's virtual IP address and virtual MAC address. The packet

is sent using the virtual MAC address as the source MAC address

in the link layer header, as well as within the ARP packet.

5.7 State Transitions

This table describes the state transitions of the state machine. For

each event and current state of the router, the router MUST perform

the set of actions specified and transition to the designated state.

If no action is specified, no action should be taken. If no state

change is specified, no state change should be performed.

The notation used in this table has the specified set of actions

listed as letters corresponding to the actions listed in section 5.6.

The next state is listed as a number as specified in section 5.3. A

slash ('/') separates the actions and states. Certain state

transitions have alternatives which depend on external state.

Alternatives are separated by a ''. See the attached notes for

details on these transitions.

States

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

1 2 3 4 5 6

Initial Learn Listen Speak Standby Active

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

Event

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

a AB/23+

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

b CD/1 CD/1 CD/1 CD/1 CDH/1

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

c AB/4 CDFI/6

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

d B/4 D/5

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

e F F F

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

f B/3 B/3

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

g EAB/3 EA EA EA AB/4

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

h EAB/3 ABGFI/6* ABGFI/6* ABGFI/6* G

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

i AB/4 A CFI/6

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

j ABH/4

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

k B B/3 B/3 B

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

l B/4 D/5 B

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

Notes

+ If the virtual IP address is configured, set state 3 (Listen) If

the virtual IP address is not configured, set state 2 (Learn). In

either case do actions A and B.

* If the router is configured to preempt do actions B, G, F, and I

and set state to 6 (Active). If the router is not configured to

preempt do actions A with no state change.

6 MAC Address Considerations

6.1 General

Each HSRP group has an associated well known virtual MAC address. On

token ring networks, these addresses are actually functional

addresses. The three addresses 0xC0 0x00 0x00 0x01 0x00 0x00, 0xC0

0x00 0x00 0x02 0x00 0x00, and 0xC0 0x00 0x00 0x04 0x00 0x00

correspond to groups 0, 1, and 2 respectively.

On other media, the virtual MAC addresses are 0x00 0x00 0x0C 0x07

0xAC XX where XX represents the HSRP group number. Routers which

implement HSRP SHOULD use well-known HSRP MAC addresses as the

group's virtual MAC address whenever possible.

The active router MUST accept and forward traffic that is destined

for the group's virtual MAC address. It MUST stop accepting or

forwarding such traffic when the router leaves the Active state.

If and only if the router is in the Active state, the router MUST use

the group's virtual MAC address as the source MAC address for its

Hello messages. This is necessary in order to allow learning bridges

to be able to determine which LAN segment the virtual MAC address

currently belongs to.

For each group, there is one virtual IP address and one virtual MAC

address. This is a desirable situation, since the ARP table entries

in the end stations do not need to change over time as the HSRP

active router moves from one router to another.

Additionally, for HSRP to work in bridging environments, the bridges

must be able to quickly update themselves as the virtual MAC address

"moves". Although learning bridges typically are able to do this,

some have been known to have problems with this. It is RECOMMENDED

that only true learning bridges be used with HSRP.

The movement of the virtual MAC address can cause further undesirable

side effects in environments where additional state is tied to the

MAC address. For example on Token Ring, if Source Route Bridging is

in use, a RIF will be stored with the virtual MAC address in a host's

RIF cache. The RIF indicates the path and final ring used to reach

the MAC address. As routers transition into Active state, they will

not be able to affect the RIF caches on the hosts on the bridged

ring. This may lead to packets being bridged to the ring for the

previous active router.

In such circumstances, a router MAY use its normal MAC addresses as

the virtual MAC address. This method of operation is strongly

discouraged. In this mode, the virtual IP address will map to a

different MAC address over time. This can create problems for end

stations, since ARP tables assume a relatively static mapping between

MAC address and IP address. These ARP tables are normally updated

when the end stations receive the gratuitous ARP responses generated

by a router that enters the active state.

6.2 Address Filter

As noted, routers currently emulating a virtual router adopt their

group's MAC and IP addresses. MAC addresses are typically provided

in an address filter or 'list' of MAC addresses in a router's

interface controller. It is desirable for routers to be able to add

one or more virtual MAC addresses to their controllers' MAC address

filter while maintaining their primary MAC addresses.

Unfortunately, some interface controllers support address filtering

for only one unicast MAC address. Or, in the case of Token Ring, the

functional address which HSRP should use is already in use for some

other protocol. In these cases, such routers can still implement

HSRP, but the protocol must change the interface's primary MAC

address when assuming or relinquishing control as the active router.

This is potentially problematic because some traffic may otherwise

wish to use the router's primary MAC address. However, the problem

MAY be mitigated by having the router send out gratuitous ARP packets

regarding its non-HSRP IP addresses. Through this, other network

entities using IP should update their ARP tables to reflect that the

router is now using a group virtual MAC address rather than its

primary MAC address.

Some protocols may not be able to run simultaneously with the standby

protocol due to the interface primary MAC address change. For

example, DECnet phase IV and HSRP will not be able to run at the same

time on some equipment.

6.3 ICMP Redirect

While running HSRP, it is important to prevent the host from

discovering the primary MAC addresses of the routers in its standby

group. Thus, any protocol that informs a host of a router's primary

address should be disabled. Thus, routers participating in HSRP on

an interface MUST NOT send ICMP redirects on that interface.

6.4 Proxy ARP

Typically, hosts learn the HSRP virtual IP address through the

configuration of their default router. These hosts then send packets

for destinations outside of the LAN to the virtual IP address. In

some environments, hosts may instead make use of proxy ARP in order

to route off of the LAN. In this case, the hosts use the MAC address

that is supplied in proxy ARP responses. HSRP functionality is

maintained if the proxy ARP responses specify the HSRP virtual MAC

address.

If an HSRP router is configured to support proxy ARP with HSRP, then

the router MUST specify the HSRP virtual MAC address in any proxy ARP

responses it generates. These proxy ARP responses MUST not be

suppressed based upon HSRP state. Suppression based upon state could

result in lack of any proxy ARP response being generated, since these

proxy ARP responses may be suppressed due to other reasons, such as

split-horizon rules.

7. Security Considerations

This protocol does not provide security. The authentication field

found within the message is useful for preventing misconfiguration.

The protocol is easily subverted by an active intruder on the LAN.

This can result in a packet black hole and a denial-of-service

attack. It is difficult to subvert the protocol from outside the LAN

as most routers will not forward packets addressed to the all-routers

multicast address (224.0.0.2).

8. References

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

September 1991.

[2] United States Patent. Patent Number : 5,473,599. Standby Router

Protocol. Date of Patent: Dec. 5, 1995.

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

Levels", BCP 14, RFC2119, March 1997.

9. Authors' Addresses

Tony Li

Juniper Networks, Inc.

3260 Jay St.

Santa Clara, CA 95054

Phone: (408) 327-1900

EMail: tli@juniper.net

Bruce Cole

Juniper Networks, Inc.

3260 Jay St.

Santa Clara, CA 95054

Phone: (408) 327-1900

EMail: cole@juniper.net

Phil Morton

Cisco Systems

170 Tasman Dr.

San Jose, CA 95143

Phone: (408) 526-7632

EMail: pmorton@cisco.com

Dawn Li

Cisco Systems

170 Tasman Dr.

San Jose, CA 95143

Phone: (408) 527-2014

EMail: dawnli@cisco.com

10. 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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