Cisco Catalyst 4500 series Administration Manual page 648

Configuring STP
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.
The root switch for each instance of spanning tree should be a backbone or distribution switch. Do not
Note
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.
We recommend that you avoid manually configuring the hello time, forward delay time, and maximum
Note
age time after configuring the switch as the root bridge.
To configure a switch as the root switch, perform this task:
Command
Step 1
Switch(config)# [no] spanning-tree vlan vlan_ID
root primary [diameter hops [hello-time seconds]]
Step 2
Switch(config)# end
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
VLAN1 is executing the ieee compatible Spanning Tree protocol
Bridge Identifier has priority 32768, address 0030.94fc.0a00
Configured hello time 2, max age 20, forward delay 15
Current root has priority 32768, address 0001.6445.4400
Root port is 323 (FastEthernet6/3), cost of root path is 19
Topology change flag not set, detected flag not set
Number of topology changes 2 last change occurred 00:02:19 ago
Times: hold 1, topology change 35, notification 2
Software Configuration Guide—Release IOS XE 3.5.0SG and IOS 15.2(2)SG
22-10
from FastEthernet6/1
hello 2, max age 20, forward delay 15
Chapter 22
Purpose
Configures a switch as the root switch.
Use the no keyword to restore the defaults.
Exits configuration mode.
Configuring STP and MST
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