You
can use speci?¬?c con?¬?guration commands to help determine which switch will become the
root bridge.
A Cisco Catalyst switch running PVST+ or PVRST+ maintains an instance of spanning
tree for each active VLAN that is con?¬?gured on the switch. A unique BID is associated with
each instance. For each VLAN, the switch with the lowest BID becomes the root bridge for
that VLAN. Whenever the bridge priority changes, the BID also changes. This change
results in the recomputation of the root bridge for the VLAN.
To con?¬?gure a switch to become the root bridge for a speci?¬?ed VLAN, use the command
spanning-tree vlan vlan-ID root primary. With this command, the switch checks the
priority of the root switches for the speci?¬?ed VLAN. Because of the extended system ID
support, the switch sets its own priority to 24576 for the speci?¬?ed VLAN if this value will
cause the switch to become the root for this VLAN. If another switch for the speci?¬?ed
VLAN has a priority lower than 24576, then the switch on which you are con?¬?guring the
spanning-tree vlan vlan-ID root primary command sets its own priority for the speci?¬?ed
VLAN to 4096 less than the lowest switch priority.
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