Bridge ID used to select the root bridge in the STP domain and also determine root ports. This ID length is 8 bytes and includes both the priority and the MAC address of the appliance. The default Priority on all tools that run STP version of IEEE is 32768.
To specify the root bridge, the priority of each bridge combined with the MAC address. If two switches or a bridge turns out to have the same priority value, then the MAC address be appraisers to decide who has the lowest ID (which is also the best). Example if there is a switch A and B and both have the same default priority is 32768, then MAC address that will be used for the determination. If the MAC address of the switch A is 0000.0 A 00.1300 and the MAC address of the switch B is A 00.1315 then switch 0000.0 à will become the root bridge.
BPDU are sent by default every 2 seconds, out of all the ports that are active on a bridge and the switch with the lowest bridge ID is selected as the root bridge. We can change the bridge ID with priority how to lose so that he would become the root bridge automatically.