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5-11
Catalyst 2940 Switch Software Configuration Guide
78-15507-02
Chapter 5 Clustering Switches
Planning a Switch Cluster
Figure 5-6 VLAN Connectivity Between Standby-Group Members and Cluster Members
Catalyst 1900, Catalyst 2820, Catalyst 2900 XL, Catalyst 2950, and Catalyst 3500 XL member
switches must be connected to the cluster standby group through their management VLANs.
For more information about VLANs in switch clusters, see these sections:
–
“Discovery Through the Same Management VLAN” section on page 5-5
–
“Discovery Through Different Management VLANs” section on page 5-6
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 2940, Catalyst 2950, Catalyst 2955, 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 member switches.
You must again 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.
Management
VLAN 16
Catalyst 2900 XL or
Catalyst 3500 XL
switch
Catalyst 3550
multilayer switch
Catalyst 3550 standby
command switch
VLAN 16
VLAN 9
VLAN 9
Catalyst 3550 active
command switch
Catalyst 3550 passive
command switch
Member switches
Management
VLAN 9
Catalyst 3550
switch
Catalyst 2940
switch
VLAN 9
VLANs 9,16 VLANs 9,16
Management
VLAN 16
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