When a switch is powered on in STP, it assumes the role of which type of bridge?

When a switch is powered on and begins participation in the Spanning Tree Protocol (STP), it initially assumes the role of a root bridge. This is because, during the STP convergence process, every switch in the network will send Bridge Protocol Data Units (BPDUs) to discover and elect a root bridge based on the lowest bridge ID. The bridge ID is determined from the priority value configured on the switch (which defaults to 32768) combined with the MAC address of the switch.

As the switches exchange BPDUs, they compare their own bridge ID with the BIDs received from others. The switch with the lowest bridge ID becomes the root bridge, ensuring a single point for the root of the spanning tree topology, which helps prevent loops in the network. Once the root bridge is established, the remaining switches will then determine their roles—such as designated bridge and backup bridge—based on their proximity to the root and the accumulated network topology.

Thus, the switch chooses the role of root bridge upon startup as part of the automatic election process that ensures the stability and efficiency of the network.