The switch with the lowest BID will become the root bridge for a
VLAN. Specific configuration commands are used to affect which switch will
become the root bridge.
A Catalyst switch running PVST (Per VLAN Spanning
Tree) maintains an instance of spanning tree for each active VLAN configured on
the switch. A unique Bridge ID is associated with each instance. For each VLAN,
the switch with the lowest BID will become the root bridge for that VLAN.
Whenever the bridge priority changes, the BID also changes. This result is the
re-computation of the root bridge for the VLAN.
To configure a switch to
become the root bridge for a specified VLAN, use the command
spanning-tree vlanvlan-ID root primary.
Assuming the other bridges in the VLAN retain their default STP priority, this
switch will become the root bridge.
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NOTE:
The root bridge for each instance of spanning tree should be a
Building Distribution switch. On a network with a collapsed backbone and
Building Distribution layer, one of the backbone switches should be the root
bridge.
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A secondary root is a switch that may become the root bridge for a
VLAN if the primary root bridge fails. To configure a switch as the secondary
root bridge for the VLAN, use the command spanning-tree vlanvlan-ID root secondary. Assuming the other bridges in the
VLAN retain their default STP priority, this switch will become the root bridge
in the event that the primary root bridge fails. This command can be executed
on more than one switch to configure multiple backup root bridges.