Electing the Root Bridge

The network topology is determined by the selection of the root bridge. The topology is based on each bridge‘s best path to root. Root election occurs on each bridge when new information is received from a neighboring bridge in a BPDU, when link is lost on a port connecting a neighboring bridge, or when the bridge‘s priority is administratively changed.

The root is elected by comparing the root IDs received in BPDUs as well as the bridge‘s own bridge ID. The bridge with the lowest ID is chosen as root. The bridge ID is an 8-byte value with the 2 most significant bytes being the bridge priority and the 6 least significant bytes being the bridge MAC address. Root may be forced to a particular bridge by the configuration of bridge priority. Among bridges with the same bridge priority, the one with the lowest MAC address is elected root. If a bridge receives no BPDUs indicating a better bridge ID than its own, it becomes the root bridge.