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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter5 Clustering Switches Planning a Switch Cluster
Virtual IP Addresses
You need to assign a unique virtual IP address and group number and nam e to the cluster standby group.
This information must be configured on a specificVLAN or rou ted port 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 virtua l IP addr ess, n ot thr oug h th e
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 swi tch assum es owne rs hip o f the virt ual 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 page5-15.
Automatic Recovery of Cluster Configuration
The active command switch continually forwards cluster-configuration informa tion (bu t not
device-configuration information) to the standby command switch. This ensures th at the stan dby
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 Catalyst2950 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. The active command
switch only forwards cluster-configuration information to the standby command switch. You must
therefore rebuild the cluster.
•This limitation applies to all clusters: If the active command switch fails and there are more than
two switches in the cluster standby group, the new command switch does not discover any
Catalyst 1900, Catalyst 2820, and Catalyst 2916M XL member switches. You must re-add these
member switches to the cluster.
•This limitation applies to all clusters: If the active command switch fails and becomes active again,
it does not discover any Catalyst 1900, Catalyst 2820, and Catalyst 2916M XL membe r 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 switc h, including members that were added while it was
down. The active command switch sends a copy of the cluster configuration to th e cluster standby group.