| 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.
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