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Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter6 Clustering Switches
Planning a Switch Cluster
HSRP and Standby Cluster Command Switches
The switch supports Hot Standby Router Protocol (HSRP) so that you can configure a group of standby
cluster command switches. Because a cluster command switch manages the forwarding of all
communication and configuration information to all the cluster member switches, we strongly
recommend the following:
•For a cluster command switch stack, a standby cluster command switch is necessary if the entire
switch stack fails. However, if only the stack master in the command switch stack fails, the switch
stack elects a new stack master and resumes its role as the cluster command switch stack.
•For a cluster command switch that is a standalone switch, configure a standby cluster command
switch to take over if the primary cluster command switch fails.
A cluster standby group is a group of command-capable switches that meet the requirements described
in the “Standby Cluster Command Switch Characteristics” section on page6-3. Only one cluster standby
group can be assigned per cluster.
Note The cluster standby group is an HSRP group. Disabling HSRP disables the cluster standby 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 cluster command switch (AC). The switch with the next highest
priority is the standby cluster command switch (SC). The other switches in the cluster standby group are
the passive cluster command switches (PC). If the active cluster command switch and the standby cluster
command switch become disabled at the same time, the passive cluster command switch with the highest
priority becomes the active cluster command switch. For the limitations to automatic discovery, see the
“Automatic Recovery of Cluster Configuration” section on page6-13. For information about changing
HSRP priority values, see the “Configuring HSRP Priority” section on page42-8. The HSRP standby
priority interface configuration commands are the same for changing the priority of cluster standby
group members and router-redundancy group members.
Note The HSRP standby hold time interval should be greater than or equal to three times the hello time
interval. The default HSRP standby hold time interval is 10seconds. The default HSRP standby hello
time interval is 3seconds. For more information about the standby hold time and standby hello time
intervals, see the “Configuring HSRP Authentication and Timers” section on page42-10.
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 about
standby cluster command switches:
•Virtual IP Addresses, page6-12
•Other Considerations for Cluster Standby Groups, page6-12
•Automatic Recovery of Cluster Configuration, page6-13