Configuring The Root Bridge - Cisco 6500 Series Software 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

Catalyst 6500 series switches 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 switch checks the bridge priority
of the current root bridges for each VLAN. When the extended system ID is disabled, the switch sets the
bridge priority for the specified VLANs to 8192 if this value will cause the switch to become the root
for the specified VLANs. When the extended system ID is enabled, the switch sets the bridge priority
for the specified VLANs to 24576 if this value will cause the switch 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 switch 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 switch 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:
•
•
Caution The root bridge for each instance of STP should be a backbone or distribution switch. Do not configure
an access switch as the STP primary root.
78-14099-04
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 switch sets
the bridge priority for VLAN 100 to 8192, which causes the switch 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 switch sets the bridge
priority to 24576, which causes the switch to become the root bridge for VLAN 20.
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
Configuring STP
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