The lowest bridge ID of the forwarding switch is used next to determine the root port. The root port on a switch is the port that receives the best Spanning Tree Protocol (STP) bridge protocol data unit (BPDU), which indicates the best path to the root bridge based on the best path cost. A root port is always in the forwarding state. Because there is only one best path to the root bridge, a switch cannot have more than one root port. Only the root bridge does not have a root port.

The bridge ID is composed of a 2byte bridge priority and a 6byte Media Access Control (MAC) address. For example, a switch with a bridge priority of 32768 and a MAC address of 1234.5678.9abc would have a bridge ID of 32768.1234.5678.9abc. A switch with a lower priority value would also have a lower bridge ID. If priority values are equal, the switch with the lower MAC address is preferred; in MAC addresses, numbers are lower than letters and the hexadecimal value A is lower than the hexadecimal value F.

The root bridge sends hello packets every two seconds by default. When a switch receives a hello packet, the receiving switch modifies the forwarding switch's bridge ID, port priority, port number, and cost to reach the root bridge before forwarding the hello packet to neighboring switches. The interface that receives the hello packet with the lowest path cost will become the root port. When a switch receives multiple hello packets with the same path cost, it will choose the interface connected to the forwarding switch with the lowest bridge ID. When multiple equalcost paths to a forwarding switch exist, the receiving switch will choose the lowest port priority of the forwarding switch. If all port priorities are equal, the receiving switch will choose the lowest port number of the forwarding switch.