Selecting the Root Bridge

Selecting the Root Bridge
Switches or bridges running STP exchange information with what are called
Bridge Protocol
Data Units (BPDUs)
. BPDUs multicast frames containing port cost and other information. The
bridge ID of each device is sent to other devices using BPDUs.
The
bridge ID
is used to determine the root bridge in the network and to determine the root
port. The bridge ID is eight bytes long and includes the priority and the MAC address of the
device. The priority on all devices running the IEEE STP version is 32768 by default. The lower
the bridge ID, the more likely a device is to become the root bridge.
At startup, switches multicast their ID inside BPDUs. To determine the root bridge, switches
in the network compare the bridge IDs they receive via the BPDUs and their own ID. Whichever
switch has the lowest bridge ID becomes the root bridge. If two switches or bridges have the
same priority value, then the MAC address is used to determine which has the lowest ID.
For example, if two switches—A and B—both use the default priority of 32768, the MAC
address will be used. If switch A’s MAC address is 0000.0c00.1111 and switch B’s MAC address
is 0000.0c00.2222, switch A would become the root bridge.