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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 11 Configuring STP
Configur ing Spanning-Tree F eatures

Disabling STP

STP is e nabl ed by d efau lt on V LAN 1 a nd on al l newly cre ated V LAN s u p to the sp anni ng- tree lim it
specified in Table 11-3. Disable STP onl y if you are sure there are no loops in the network top ology.
Caution When STP is disable d and loops ar e present in the topology, excessive traffic and indefinite packet
duplica tio n ca n drast ica lly r ed uce net work pe rf orma nc e.
Beginning in privileged EXEC mo de, follow these steps to di sable STP on a per-VLAN basis:
To re-e nabl e STP, use the spanning- tree vlan vlan-id global configur ation comm and.

Configuring the Root Switch

The switch m aintains a separate sp anning-tree in stance for eac h acti ve VLAN conf igured on it. A b ridge
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 g l obal c onfigura tio n com man d t o modi fy the sw it ch pr iori ty fr om the de fau 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 an y root switch for the spe cified VLAN has a switch priority lo wer than 24576, the switch sets its o wn
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 Th e spanning-tree vlan vlan-id root global conf iguration command fa ils if the v alue necessary to be the
root switch is less than 1.
Before Release 12.1(9)EA1, entering the spanning-tree vlan vlan- id root glo bal configurat ion
command on a Catalyst 2 950 switch (no extended system ID) ca use d it to set its own switch prio ri ty for
the specified VLAN to 8192 if th is value caused this swi tch to bec o me th e r oot f or th e specified VLAN.
If any ro ot switc h for th e specified VLAN has a switc h pr iority lower than 8192, the switch sets its o w n
priority for the specified VLAN to 1 less than the lowest switch priority.
Command Purpose
Step 1 configure terminal Enter g lo bal c onfigurat ion m ode.
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 install ed and 1 to 1005 when the stan dard software image (SI) is
installe d. D o not en ter le ad ing z eros .
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.