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11-12
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 11 Configuring STP
Configuring Spanning-Tree Features
Disabling STP
STP is enabled by default on VLAN 1 and on all newly created VLANs up to the spanning-tree limit
specified in Table 11-3. Disable STP only if you are sure there are no loops in the network topology.
Caution When STP is disabled and loops are present in the topology, excessive traffic and indefinite packet
duplication can drastically reduce network performance.
Beginning in privileged EXEC mode, follow these steps to disable STP on a per-VLAN basis:
To re-enable STP, 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 11-1 on page 11-4.)
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 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.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
no spanning-tree vlan vlan-id Disable STP on a per-VLAN basis.
For vlan-id, the range is 1 to 4094 when the enhanced software image (EI)
is installed and 1 to 1005 when the standard software image (SI) is
installed. Do not enter leading zeros.
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.