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| 6.3 |  | HSRP Operations |
| 6.3.3 |  | 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 UDP packets and use port number 1985. HSRP messages use the interface IP address not the virtual IP address as the source IP address. The HSRP messages are sent to the destination multicast address (224.0.0.2). This is the multicast address used to communicate to all routers. The Time-To-Live (TTL) on the packet is set to one.

The HSRP message fields are as follows:

- Version field indicates the version of HSRP.
- Op Code describes the type of message contained in this packet.
  - 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.
- Hellotime is only meaningful in hello messages and stores the approximate period, in seconds, between the hello messages that the router sends.
- Holdtime 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.
- Priority 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 identifies the standby group. Values 0 and 255, inclusive, are valid.
- Authentication Data contains a clear text eight-character reused password.
- Virtual Address contains the IP address of the virtual router used by this group.
- State describes the current state (active, standby, init) of the router sending the message.

Internally, each router in the standby group implements a state machine. Only the active and standby routers send periodic HSRP messages after the protocol has completed the election process.