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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 6 Clustering Switches
Planning a Switch Cluster
Figure 6-5 Discovery of Newly Installed Switches
HSRP and Standby Cluster Command Switches
The switch uses 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“NoteThe CMP-NAT-ACL access list is created when a device is configured as the command
switch. Configuring any other access list on the switch can restrict access to it and affect the discovery
of member and candidate switches.” section on page 6-3. Only one cluster standby group can be assigned
per cluster.
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 page 6-11.
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 10 seconds. The default HSRP standby hello
time interval is 3 seconds.
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, page 6-10
Command device
New (out-of-box)
candidate device
AP
Device A Device B
VLAN 9
VLAN 9
VLAN 16
VLAN 16
New (out-of-box)
candidate device
AP
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