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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter5 Clustering Switches
Planning a Switch Cluster
HSRP and Standby Command Switches
The switch supports Hot Standby Router Protocol (HSRP) so that you can configure a group of standby
command switches. Because a command switch manages the fo rwar ding of a ll c omm un icat ion a nd
configuration information to all the member switches, we strongly recommend that you confi gure a
cluster standby command switch to take over if the primar y com ma nd swit ch fa il s.
A cluster standby group is a group of command-capable switches that meet the requirements described
in the “Standby Command Switch Characteristics” section on page 5-3. Only one cluster standby g roup
can be assigned per cluster.
Note • When the command switch is a Catalyst 3550 switch, all standby command switches must be
Catalyst 3550 switches or Catalyst 2950 switches running Cisco IO S Rel ease 12.1(6)EA2 or
later.
•When the command switch is a Catalyst 2950 switch running Cisco IOS Re le ase 12.1(6)EA2 or
later, all standby command switches must be Catalyst 2950 switches running Ci sco IOS
Release 12.1(6)EA2 or later.
•When the Catalyst 2950 command switch is running Cisco IOS Release 12.0(5)WC2 or earlier, the
standby command switches can be these switches also running Ci sco IOS Release 12.0(5)WC2 or
earlier: Catalyst 2900 XL, Catalyst 2950, andCatalyst 350 0 X L sw it ches.
Note The cluster standby group is an HSRP group. Disabling HSR P disa ble s t he clust er stan dby gr oup .
The switches in the cluster standby group are ranked according to HSRP prioritie s . The switch with the
highest priority in the group is the active command switch (AC). The switch with the ne xt highe st
priority is the standby command switch (SC). The other switches in the cluster standby group are the
passive command switches (PC). If the active command switch and the standby command switch become
disabled at the same time, the passive command switch with the highest priority becomes the active
command switch. For the limitations to automatic discovery, see the “Automatic Recovery of Cluster
Configuration” section on page 5-13. For information about changing HSRP priority values, see the
“Configuring HSRP Priority” section on page 23-6. The 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 3 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 hello time intervals,
see the “Configuring HSRP Authentication and Timers” section on page 23-8.
These connectivity guidelines ensure automatic discovery of the switch cluster, cluster candidates,
connected switch clusters, and neighboring edge devices. These topics also provid e more det ail about
standby command switches:
•“Virtual IP Addresses” section on page 5-13
•“Automatic Recovery of Cluster Configuration” section on page 5-13
•“Considerations for Cluster Standby Groups” section on page 5-14