In a PVST+ environment, the extended switch ID ensures each switch has a unique BID for each VLAN.

For example, the VLAN 2 default BID would be 32770 (priority 32768, plus the extended system ID of 2). If no priority has been configured, every switch has the same default priority and the election of the root for each VLAN is based on the MAC address. This method is a random means of selecting the root bridge.

There are situations where the administrator may want a specific switch selected as the root bridge. This may be for a variety of reasons, including the switch is more centrally located within the LAN design, the switch has higher processing power, or the switch is simply easier to access and manage remotely. To manipulate the root bridge election, simply assign a lower priority to the switch that should be selected as the root bridge.