How many root bridges are elected in a Spanning Tree Protocol instance?

In a Spanning Tree Protocol (STP) instance, only one root bridge is elected within a switched network. The root bridge serves as the central reference point for all path calculations within the spanning tree topology. It is selected based on the bridge ID, which consists of the bridge priority (configurable) and the MAC address of the switch.

When switches participate in the STP process, they use the bridge ID to determine which switch has the lowest value and thus should become the root bridge. This ensures a single point of control in the topology, which is crucial for preventing loops in a network by establishing a loop-free broadcast domain.

Having more than one root bridge (such as two or three, or multiple based on the topology) would undermine the purpose of the Spanning Tree Protocol, which is designed specifically to maintain a single logical path to avoid broadcast storms and ensure efficient data flow. Therefore, it’s essential to have just one root bridge elected in any given STP instance.