Each standby group contains an IP address and a well−known MAC address assigned to the group. The IP
address for the standby group is within the range of IP addresses belonging to the subnet or VLAN to which
the route processor is providing services. The IP address cannot be assigned to any other device in the
network except the standby group interfaces operating in the standby group.
Inter−Switch Link (ISL) links are used to transport VLAN information across the links to the different route
processors used in each standby group. In order to pass HSRP standby group information between links for
multiple VLANs, the interfaces in the group must be configured with ISL. The encapsulation format must be
defined, and an IP address must be assigned to an interface.
Note Refer to Chapter 5 for information on configuring ISL encapsulation and assigning an IP
address to an interface.
HSRP Message Format
All route processors in the HSRP standby group send and receive HSRP messages to one another. The
messages are used to determine the roles of each route processor in the group. User Datagram Protocol (UDP)
utilizing TCP port 17 encapsulates the data in the data portion of the UDP packet. The packet is then sent to
an all−router multicast address with a time to live (TTL) of one hop.
The fields contained in an HSRP message are:
Version—Indicates the HSRP version.• Op Code—Describes the type of message contained in the packet. The three types of messages are as
follows:
Hello—The first and most common message, which is sent to indicate that a route processor
in the standby group is functioning in the group.
♦
Coup—Indicates that a route processor wants to become the active route processor.♦ Resign—Notifies the other standby group members that a route processor no longer can or
will participate as the active route processor.
♦
State—Indicates the current state of the route processor sending the message.♦ Hellotime—Indicates the time period in seconds between hello messages that the sending
route processor sends. The default is three seconds.
♦
Holdtime—Used when sending hello messages. This field indicates the length of time in
seconds that the message should be considered valid. The holdtime should be at least three
times the value assigned to hellotime. The default time is 10 seconds.
♦
Priority—Used to elect the active and standby route processors. The route processor with the
highest priority in the HSRP group wins the election and becomes the active route processor.
♦
Group—Indicates the standby group number. The valid numbers are 0 to 255; 255 groups can
be configured as valid HSRP groups.
♦
Authentication data—Contains an eight−character clear text password that is continuously
reused.
♦
Virtual address—Contains the IP address of the router that is used by the HSRP group.♦
•
Just as in Spanning−Tree Protocol (STP), a route processor goes through different states before it becomes
fully functional. In the next section, we’ll look at these different states and how skipping one or more of the
states affects HSRP.
The HSRP States
A route processor such as a router can transition through six HSRP states. The definitions of the states are
included here:
Initial state—All route processors begin in this state. In this starting state, HSRP is not running. The
route processor can be found in this state after the power cycles or if a configuration change is applied
•
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