Configuring The Root Bridge - Cisco 4500M Software Manual

Chapter 14 Understanding and Configuring STP
To enable the extended system ID, perform this task:
Command
Step 1
Switch(config)# spanning-tree extend system-id
Step 2
Switch(config)# end
Step 3
Switch# show spanning-tree vlan vlan_ID
When you enable or disable the extended system ID, the bridge IDs of all active STP instances are
Note
updated, which might change the spanning tree topology.
This example shows how to enable the extended system ID:
Switch# configure terminal
Switch(config)# spanning-tree extend system-id
Switch(config)# end
Switch#
This example shows how to verify the configuration:
Switch# show spanning-tree summary | include extended
Extended system ID is enabled.

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 will become the root bridge
for that VLAN. Whenever the bridge priority changes, the bridge ID also changes. This results 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 will cause 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
will set 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.
OL-6696-01
Purpose
Enables the extended system ID.
Disables the extended system ID.
Note You cannot disable the extended system ID on
chassis that support 64 MAC addresses or when
you have configured extended range VLANs (see
"Table 14-4Spanning Tree Default Configuration
Values" section on page
Exits configuration mode.
Verifies the configuration.
Software Configuration Guide—Release 12.2(25)EW
Configuring STP
14-6).
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