Cisco -- Bridge ID

Bridge ID (BID) is 8 bytes long and contains a bridge priority (2 bytes, that can be changed) along with one of the switch's MAC addresses (6 bytes)

Based on the location of the Root Bridge, the other switches determine which of their ports has the lowest path cost to the Root Bridge. These ports are called Root Ports and each switch (other than the current Root Bridge) must have one.

The switches determine who will have Designated Ports. A Designated Port is the connection used to send and receive packets on a specific segment. By having only one Designated Port per segment, all looping issues are resolved!

Designated Ports are selected based on the lowest path cost to the Root Bridge for a segment. Since the Root Bridge will have a path cost of "0", any ports on it that are connected to segments will become Designated Ports. For the other switches, the path cost is compared for a given segment. If one port is determined to have a lower path cost, then it becomes the Designated Port for that segment. If two or more ports have the same path cost, then the switch with the lowest BID is chosen.

Once the Designated Port for a network segment has been chosen, any other ports that connect to that segment become non-Designated Ports. They block network traffic from taking that path so that it can only access that segment through the Designated Port.