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18-10
Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 18 Configuring STP and MST
Configuring STP
Configuring the Root Bridge
A Catalyst 4000 family switch maintains an instance of spanning tree for each active VLAN configured
on the switch. A bridge ID, consisting of the bridge priority and the bridge MAC address, is associated
with each instance. For each VLAN, the switch with the lowest bridge ID becomes the root bridge for
that VLAN. Whenever the bridge priority changes, the bridge ID also changes, resulting in the
recomputation of the root bridge for the VLAN.
To configure a switch to become the root bridge for the specified VLAN, use the spanning-tree vlan
vlan-ID root command to modify the bridge priority from the default value (32,768) to a significantly
lower value. The bridge priority for the specified VLAN is set to 8192 if this value causes the switch to
become the root for the VLAN. If any bridge for the VLAN has a priority lower than 8192, the switch
sets the priority to 1 less than the lowest bridge priority.
For example, assume that all the switches in the network have the bridge priority for VLAN 100 set to
the default value of 32,768. Entering the spanning-tree vlan 100 root primary command on a switch
sets the bridge priority for VLAN 100 to 8192, causing this switch to become the root bridge for
VLAN 100.
Note The root switch for each instance of spanning tree 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 Layer 2 network diameter (the maximum number of bridge
hops between any two end stations in the network). When you specify the network diameter, a switch
automatically picks an optimal hello time, forward delay time, and maximum age time for a network of
that diameter. This action can significantly reduce the spanning tree convergence time.
Use the hello-time 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 bridge.
To configure a switch as the root switch, perform this task:
This example shows how to configure a switch as the root bridge for VLAN 10, with a network diameter
of 4:
Switch# configure terminal
Switch(config)# spanning-tree vlan 10 root primary diameter 4
Switch(config)# end
Switch#
This example shows how the configuration changes when a switch becomes a spanning tree root. This
configuration is the one before the switch becomes the root for VLAN 1:
Switch# show spanning-tree vlan 1
Command Purpose
Step 1
Switch(config)# [no] spanning-tree vlan vlan_ID
root primary [diameter hops [hello-time seconds]]
Configures a switch as the root switch.
Use the no keyword to restore the defaults.
Step 2
Switch(config)# end
Exits configuration mode.