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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter14 Configuring STP
Configuring Spanning-Tree Features
Disabling Spanning Tree
Spanning tree is enabled by default on VLAN 1 and on all newly created VLAN s up to the spanning-tree
limit specified in the “Supported Spanning-Tree Instances” section on page 14-9. Disable spanning tree
only if you are sure there are no loops in the network topology.
Caution When spanning tree is disabled and loops are present in t he t op ology, excessive traffic and indefinite
packet duplication can drastically reduce network performance.
Beginning in privileged EXEC mode, follow these steps to disable spanning tree on a per-VLAN basis.
This procedure is optional.
To re-enable spanning tree, use the spanning-tree vlan vlan-id global configuration command.
Configuring the Root Switch
The switch maintains a separate spanning-tree instance for each active VLAN configured on it. A bridge
ID, consisting of the switch priority and the switch MAC address, is associated with each instance. For
each VLAN, the switch with the lowest bridge ID becomes the root switch for that VLAN.
To configure a switch to become the root for the specified VLAN, use the spanning-tree vlan vlan-id
root global configuration command to modify the switch pri orit y fr om the defau lt value (32768) to a
significantly lower value. When you enter this command, the switch checks the switch priority of the
root switches for each VLAN. Because of the extended system ID support, the switch sets its own
priority for the specified VLAN to 24576 if this value will cause this switch to become the root for the
specified VLAN.
If any root switch for the specified VLAN has a switch priority lower than 24576, the switch sets its own
priority for the specified VLAN to4096 less than the lowest switch priority. (4096 is the value of the
least-significant bit of a 4-bit switch priority value as shown in Table14-1 on page 14-4.)
Note The spanning-tree vlan vlan-id root global configuration command fails if the value necessary to be the
root switch is less than 1.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 no spanning-tree vlan vlan-id Disable spanning tree on a per-VLAN basis.
For vlan-id, you can specify a single VLAN identified by VLAN ID
number, a range of VLANs separated by a hyphen, or a series of VLANs
separated by a comma. The range is 1 to 1005 when the SI is installed and
1 to 4094 when the EI is installed.
Step3 end Return to privileged EXEC mode.
Step4 show spanning-tree vlan vlan-id Verify your entries.
Step5 copy running-config startup-config (Optional) Save your entries in the configuration file.