Configuring The Root Bridge - Cisco 7609 Configuration Manual

Chapter 15 Configuring STP and IEEE 802.1s MST
This example shows how to enable the extended system ID:
Router# configure terminal
Router(config)# spanning-tree extend system-id
Router(config)# end
Router#
This example shows how to verify the configuration:
Router# show spanning-tree summary | include Extended
Extended system ID is enabled.

Configuring the Root Bridge

Cisco 7600 series routers maintain a separate instance of STP for each active VLAN. A bridge ID,
consisting of the bridge priority and the bridge MAC address, is associated with each instance. For each
VLAN, the network device with the lowest bridge ID becomes the root bridge for that VLAN.
To configure a VLAN instance to become the root bridge, enter the spanning-tree vlan vlan_ID root
command to modify the bridge priority from the default value (32768) to a significantly lower value.
When you enter the spanning-tree vlan vlan_ID root command, the router checks the bridge priority of
the current root bridges for each VLAN. When the extended system ID is disabled, the router sets the
bridge priority for the specified VLANs to 8192 if this value will cause the router to become the root for
the specified VLANs. When the extended system ID is enabled, the router sets the bridge priority for the
specified VLANs to 24576 if this value will cause the router to become the root for the specified VLANs.
If the extended system ID is disabled and if any root bridge for the specified VLANs has a bridge priority
lower than 8192, the router sets the bridge priority for the specified VLANs to 1 less than the lowest
bridge priority.
If the extended system ID is enabled and if any root bridge for the specified VLANs has a bridge priority
lower than 24576, the router sets the bridge priority for the specified VLANs to 4096 less than the lowest
bridge priority. (4096 is the value of the least significant bit of a 4-bit bridge priority value; see
Table 15-2 on page
The spanning-tree vlan vlan_ID root command fails if the value required to be the root bridge is less
Note
than 1.
The spanning-tree vlan vlan_ID root command can cause the following effects:
•
•
The root bridge for each instance of STP should be a backbone or distribution switch. Do not configure
Caution
an access switch as the STP primary root.
78-14064-04
15-3.)
If the extended system ID is disabled, and if all network devices in VLAN 100 have the default
priority of 32768, entering the spanning-tree vlan 100 root primary command on the router sets
the bridge priority for VLAN 100 to 8192, which causes the router to become the root bridge for
VLAN 100.
If the extended system ID is enabled, and if all network devices in VLAN 20 have the default priority
of 32768, entering the spanning-tree vlan 20 root primary command on the router sets the bridge
priority to 24576, which causes the switch to become the root bridge for VLAN 20.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
Configuring STP
15-25