📄 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.
Li, et. al. Informational [Page 1]
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.
Li, et. al. Informational [Page 2]
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.
Li, et. al. Informational [Page 3]
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:
Li, et. al. Informational [Page 4]
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:
Li, et. al. Informational [Page 5]
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
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