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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 13 Configuring STP
Configuring Spanning-Tree Features
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 priority from the default 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 to 4096 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 Table 13-1 on page 13-4.)
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.
Before Cisco IOS Release 12.1(9)EA1, entering the spanning-tree vlan vlan-id root global
configuration command on a Catalyst 2950 switch (no extended system ID) caused it to set its own
switch priority for the specified VLAN to 8192 if this value caused this switch to become the root for
the specified VLAN. If any root switch for the specified VLAN has a switch priority lower than 8192,
the switch sets its own priority for the specified VLAN to 1 less than the lowest switch priority.
These examples show the effect of the spanning-tree vlan vlan-id root command with and without the
extended system ID support:
• For Catalyst 2950 and Catalyst 2955 switches with the extended system ID (Cisco IOS
Release 12.1(9)EA1 and later), if all network devices in VLAN 20 have the default priority of
32768, entering the spanning-tree vlan 20 root primary command on the switch sets the switch
priority to 24576, which causes this switch to become the root switch for VLAN 20.
Command Purpose
Step 1 configure terminal Enter global configuration mode.
Step 2 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 4094.
Step 3 end Return to privileged EXEC mode.
Step 4 show spanning-tree vlan vlan-id Verify your entries.
Step 5 copy running-config startup-config (Optional) Save your entries in the configuration file.