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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 9 Configuring STP
Configuring Basic STP Features
To configure a switch to become the root, the switch priority can be modified from the default
value (32768) to a significantly lower value so that the switch becomes the root switch for the specified
VLAN. Use the spanning-tree vlan vlan-id root global configuration command to alter the switch
priority. When you enter this command on a switch, it checks the switch priority of the current root
switch for each VLAN and sets its own switch priority for the specified VLAN to 8192 if this value
causes 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.
For example, if all switches in the network have the switch priority for VLAN 100 set to the default value
of 32768, entering the spanning-tree vlan 100 root primary global configuration command on a switch
sets the switch priority for VLAN 100 to 8192, causing the switch to become the root switch for
VLAN 100.
Note The root switch for each instance of STP should be a backbone or distribution switch. Do not configure
an access switch as the spanning-tree primary root.
Use the diameter keyword to specify the network diameter (that is, the maximum number of switch hops
between any two end stations in the network). When you specify the network diameter, the switch
automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of
that diameter, which can significantly reduce the convergence time. You can use the hello keyword to
override the automatically calculated hello time.
Note We recommend that you avoid manually configuring the hello time, forward-delay time, and
maximum-age time after configuring the switch as the root switch.
Beginning in privileged EXEC mode, follow these steps to configure a switch as the root switch:
To return the switch to its default setting, use the no spanning-tree vlan vlan-id root global
configuration command.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
spanning-tree vlan vlan-id root primary
[diameter net-diameter [hello-time seconds]]
Configure a switch as the root switch.
For vlan-id, the range is 1 to 1005. Do not enter leading zeroes.
(Optional) For diameter net-diameter,specifythemaximum
number of switches between any two end stations. The range
is 2 to 7.
(Optional) For hello-time seconds, specify the interval in
seconds between the generation of configuration messages by
the root switch. The range is 1 to 10 seconds; the default is 2
seconds for STP.
Step 3
end Return to privileged EXEC mode.
Step 4
show spanning-tree Verify your entries.
Step 5
copy running-config startup-config (Optional) Save your entries in the configuration file.
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