5.1 Basic STP Operations
5.1.4 Bridge identification

A bridge ID (BID) is a single, 8-byte field that is composed of two subfields, as illustrated in the Figure. The low-order subfield consists of a 6-byte MAC address assigned to the switch. Some switches use one of the MAC addresses from the pool of 1024 addresses assigned to every supervisor or backplane. This is a hard-coded number that is designed not to be changed by the user. The MAC address in the BID is expressed in the usual hexadecimal (base 16) format.

The high-order BID subfield is referred to as the bridge priority. The Bridge Priority field is a 2-byte (16-bit) value. This field can have 216 possible values that range from 0 to 65,535. The default bridge priority is the midpoint value, 32,768. Bridge priorities are typically expressed in a decimal (base 10) format. The key point to remember concerning STP cost values is that lower costs are better.