The Cisco 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). 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
HSRP runs on top of UDP, and uses port number 1985. Packets
are sent to multicast address 18.104.22.168 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:
Virtual IP address
HSRP version number, 0 for this version.
Type of message contained in the packet. Possible
0 - Hello, sent to indicate that a router is running and is capable of becoming
the active or standby router.
1 - Coup, sent when a router wishes to become the active router.
2 - Resign, sent when a router no longer wishes to be
the active router.
Internally, each router in the standby group
implements a state machine. The State field describes the current state of the
router sending the message. Possible values are:
Approximate period between the Hello messages
that the router sends (for Hello messagesonly). 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.
The amount of time, in seconds, that the current
Hello message should be considered valid. (For Hello messags only.)
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.
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.
(8 bytes) 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 bytes) 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.