spanning tree root bridge priority

Every spanning tree switched network or broadcast domain has a switch designated as the root bridge. It serves as the reference point for all spanning-tree-enabled switches. The spanning tree algorithm determines which redundant paths to block. It is selected as an election process.

The figure below illustrates the bridge ID (BID) fields. BID is a unique identity when the switch is part of a network. It is a 64-bit field divided into three parts: the 4 4-bit Bridge Priority field, the 12 12-bit Extended System ID field, and the switch’s 48-bit MAC address. The Bridge Priority field is configurable, while the MAC address is unique among all switches. The sum of these two ensures a unique Bridge ID.

Root Bridge election

All switches in the broadcast domain join the election process. When switches complete their booting process, they send out BPDU frames every two seconds containing the switch BID and the root ID. The adjacent switches receive the BPDU frames and read the root ID information from them. If the root ID of the BPDU sending switch is lower than the root ID on the receiving switch, the receiving switch updates its root ID, identifying the adjacent switch as the root bridge.

It doesn’t need to be the adjacent switch. It could be any other switch in the broadcast domain. The switch then sends new BPDU frames with the lower root ID to the adjacent switches. Finally, the switch with the lowest BID is selected as the root bridge for the spanning tree instance.

Now, look at the election process from another angle. Before manually configuring the bridge priority, all the switches have a default priority. Therefore, it is a tie based on priority. The switch with the lowest Mac address will become a root bridge. In the figure below, switch-3 is elected as the Root-Bridge because the bridge priority ID is the default, which is 32769; so, the switches elect the root-bridge based on the MAC address. The switch-3 is the lowest MAC address, so all the root-bridge ports become in the forwarding state, i.e., designated port.