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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter5 Clustering Switches
Understanding Switch Clusters
Understanding Switch Clusters
A switch cluster is a group of connected Catalyst switches that a re manage d as a sing le entity. In a switch
cluster, 1switch must be the command switch and up to 15 switches can be member switches. The total
number of switches in a cluster cannot exceed 16switches. The command switch is the single point of
access used to configure, manage, and monitor the member switches. Cluster members can belong to
only one cluster at a time.
The benefits of clustering switches include:
•Management of Catalyst switches regardless of their interconnection media and their physical
locations. The switches can be in the same location, or they can be distributed across a Layer 2 or
Layer 3 (if your cluster is using a Catalyst3550 multilayer switch as a Layer 3 router between the
Layer2 switches in the cluster) network. Cluster members are connected through at least one VLAN
in common with the command switch according to the connectivity guidelines described in the
“Automatic Discovery of Cluster Candidates and Members” section on page 5-4. This section
includes some management VLAN considerations for the Catalyst1900, Catalyst2820,
Catalyst 2900 XL, Catalyst 2950, and Catalyst3500 XL switches. For complete information about
these switches in a switch-cluster environment, refer to the software configuration guide for that
specific switch.
•Command-switch redundancy if a command switch fails. One or more swit che s ca n be desi gnat ed
as standby command switches to avoid loss of contact with cluster members. A cluster stan dby
group is a group of standby command switches.
•Management of a variety of Catalyst switches through a single IP address. This conserves on IP
addresses, especially if you have a limited number of them. All communication with the switch
cluster is through the command switch IP address.
For other clustering benefits, see the “Advantages of Using CMS and Clustering Switches” section on
page 1-6.
Refer to the release notes for the list of Catalyst switches eligible for switch clustering, including which
ones can be command switches and which ones can only be member switches, and the required softw are
versions.

Command Switch Characteristics

A Catalyst 3550 command switch must meet these requirements:
•It is running Cisco IOS Release 12.1(4)EA1 or later.
•It has an IP address.
•It has Cisco Discovery Protocol (CDP) version 2 enabled (the defa ult ).
•It is not a command or member switch of another cluster.
Note • We strongly recommend that the highest-end, command-capable switch in the cluster be the
command switch:
•If your switch cluster has a Catalyst3550 switch, that switch should be the command switch.
•If your switch cluster has Catalyst 2900XL, Catalyst 2950, and Catalyst 3500 XL switches, the
Catalyst 2950 should be the command switch.
•If your switch cluster has Catalyst 1900, Catalyst2820, Ca talyst 2900 XL, and Catalyst 3500 XL
switches, either the Catalyst2900 XL or Catalyst 3500XL should be the command switch.