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Catalyst 2940 Switch Software Configuration Guide
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Chapter5 Clustering Switches Planning a Switch Cluster
Note The cluster standby group is an HSRP group. Disabling HSRP di sable s t he c luste r stand by group.
The switches in the cluster standby group are ranked according to HSRP priorities. The switch with the
highest priority in the group is the active command switch (AC). The switch with the next highest
priority is the standby command switch (SC). The other switches in the cluster standby group a re the
passive command switches (PC). If the active command switch and the standby command switch become
disabled at the same time, the passive command switch with the highest priority becomes the active
command switch. For the limitations to automatic discovery, see the “Automatic Recovery of Cluster
Configuration” section on page 5-11. For information about changing HSRP priority values, refer to the
standby priority interface configuration mode command in the Cisco IOS Release 12.1 documentation
set. The HSRP commands are the same for changing the priority of cluster standby gro up m embers and
router-redundancy group members.
Note The HSRP standby hold time interval should be greater than or equal to 3 times the hello time interval.
The default HSRP standby hold time interval is 10 seconds. The default HSRP standby hello time
interval is 3 s econds . For more info rmati on about th e standby hold time and hell o time inte rvals, refer
to the Cisco IOS Release 12.1 documentation set on Cisco.com.
These connectivity guidelines ensure automatic discovery of the switch cluster, cluster candidates,
connected switch clusters, and neighboring edge devices. These topics also provide more detail abo ut
standby command switches:
•Virtual IP Addresses, page 5-9
•Other Considerations for Cluster Standby Groups, page 5-9
•Automatic Recovery of Cluster Configuration, page 5-11
Virtual IP Addresses
You need to assign a unique virtual IP address and group number and name to the cluster standby group.
This information must be configured on the management VL AN on the a ctive command sw itch . T he
active command switch receives traffic destined for the virtual IP address. To manage the cluster, you
must access the active command switch through the virtual IP address, not th rough the command-switch
IP address. This is in case the IP address of the active command switch is different from the virtual IP
address of the cluster standby group.
If the active command switch fails, the standby command switch assumes ownership of the virtual IP
address and becomes the active command switch. The passive switches in the cluster standby group
compare their assigned priorities to determine the new standby command switch. The passive standby
switch with the highest priority then becomes the standby command switch. When the previously active
command switch becomes active again, it resumes its role as t he acti v e command s witch, a nd the cu rrent
active command switch becomes the standby command switch again. For m ore i n forma tio n about I P
address in switch clusters, see the “IP Addresses” section on page5-12.
Other Considerations for Cluster Standby Groups
These requirements also apply:
•Standby command switches must meet these requirements:
–
When the command switch is a Catalyst3550 switch, all standby command switches must be
Catalyst 3550 switches.