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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 5 Clustering Switches
Planning a Switch Cluster
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 VLAN on the active command switch. The
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 through 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 the active command switch, and the current
active command switch becomes the standby command switch again. For more information about IP
address in switch clusters, see the “IP Addresses” section on page 5-13.
Automatic Recovery of Cluster Configuration
The active command switch continually forwards cluster-configuration information (but not
device-configuration information) to the standby command switch. This ensures that the standby
command switch can take over the cluster immediately after the active command switch fails.
Automatic discovery has these limitations:
• This limitation applies only to clusters that have Catalyst 2950 and Catalyst 3550 command and
standby command switches: If the active command switch and standby command switch become
disabled at the same time, the passive command switch with the highest priority becomes the active
command switch. However, because it was a passive standby command switch, the previous
command switch did not forward cluster-configuration information to it. You must therefore rebuild
the cluster.
• This limitation applies to all clusters: If the active command switch fails and there are two or more
switches (not including the active command switch) in the cluster standby group, the new command
switch does not discover any Catalyst 1900 and Catalyst 2820 member switches. You must re-add
these member switches to the cluster.
When the previously active command switch resumes its active role, it receives a copy of the latest
cluster configuration from the active command switch, including members that were added while it was
down. The active command switch sends a copy of the cluster configuration to the cluster standby group.
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