Question

If no bridge priority is configured in PVST, which criteria is considered when electing the root bridge?

1) Bridge ID
2) MAC address
3) IP address
4) Root path cost

Answer 1

In PVST+ with no configured bridge priority, the root bridge is elected based on the Bridge ID, which combines the default bridge priority and the switch's MAC address; the lowest MAC address wins.

If no bridge priority is configured in PVST+, the election of the root bridge is based on the lowest Bridge ID. The Bridge ID is a combination of a configurable bridge priority value and the switch's MAC address; the default priority is 32768. Since the priority would be the same on all switches with default settings, the MAC address becomes the deciding factor.

In such cases, the switch with the lowest MAC address becomes the root bridge. The root path cost is considered when determining the best path from non-root bridges to the root bridge, once the root bridge has been elected, not during the election of the root bridge itself.