rfc2281.txt

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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.



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RFC 2281                       Cisco HSRP                     March 1998


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.






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RFC 2281                       Cisco HSRP                     March 1998


   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.






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RFC 2281                       Cisco HSRP                     March 1998


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 RFC 2119 [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:









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RFC 2281                       Cisco HSRP                     March 1998


                          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:







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RFC 2281                       Cisco HSRP                     March 1998


         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