Switches or bridges running STP exchange information with what is known as Bridge Protocol Data Units (BPDUs). BPDUs send configuration
messages using multicast frames. 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 8 bytes long and includes the priority
and the MAC address of the device. The priority on all devices running
the IEEE STP version is 32,768.
To determine the root bridge, the priorities of the bridge and the MAC
address are combined. If two switches or bridges have the same priority
value, then the MAC address is used to determine which one has the lowest
ID. For example, if two switches, which I’ll name A and B, both use the
default priority of 32,768, then the MAC address will be used. If switch A’s
MAC address is 0000.0c00.1111.1111 and switch B’s MAC address is
0000.0c00.2222.2222, then switch A would become the root bridge.
BPDUs are sent out every two seconds by default.
Selecting the Designated Port
To determine the port or ports that will be used to communicate with the
root bridge, you must first figure out the path cost. The STP cost is an accumulated
total path cost based on the bandwidth of the links.
The IEEE 802.1d specification has recently been revised to handle thenew higher-speed links. The 1900 switches use the original IEEE 802.1d specifications.