8.1 HSRP Operation
8.1.5 HSRP messages
All routers in a standby group send or receive HSRP messages. These messages are used to determine and maintain the router roles within the group. HSRP messages are encapsulated in the data portion of User Datagram Protocol (UDP) packets, and use port number 1985. HSRP messages use the interface IP address not the phantom IP address as the source IP address. The HSRP messages are sent to the destination multicast address (224.0.0.2), which is the multicast address used to communicate to all routers. The Time-To-Live (TTL) on the packet is set to one (1).

The HSRP message format is shown in the Figure and the respective fields are described below.

  • The Version field indicates the version of HSRP.
  • The Op Code describes the type of message contained in this packet. Possible values follow:
  • Hello=0 messages - sent to indicate that a router is running and is capable of becoming either the active or standby router.
  • Coup=1 messages - sent when a router wants to become the active router.
  • Resign=2 messages - sent when a router no longer wants to be the active router.
  • Internally, each router in the standby group implements a state machine. The State field describes the current state (active, standby, and so on) of the router sending the message.
  • The Hellotime field is only meaningful in hello messages. This field stores the approximate period between the hello messages that the router sends. The time is given in seconds.
  • The Holdtime field is meaningful only in hello messages. This field stores the amount of time that the current hello message should be valid. The time is given in seconds.
  • The Priority 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.
  • The Group field identifies the standby group. Values 0 and 255, inclusive, are valid.
  • The Authentication Data field contains a clear text eight-character reused password.
  • The Virtual Address field contains the IP address of the virtual router used by this group.

Only the active and standby routers send periodic HSRP messages after the protocol has completed the election process.