Bridge Protocol Data Unit

Bridge Protocol Data Units (BPDUs) are frames that contain information about the Spanning tree protocol (STP). Switches send BPDUs using a unique MAC address from its origin port and a multicast address as destination MAC (01:80:C2:00:00:00, or 01:00:0C:CC:CC:CD for Per VLAN Spanning Tree). For STP algorithms to function, the switches need to share information about themselves and their connections. What they share are bridge protocol data units (BPDUs). BPDUs are sent out as multicast frames to which only other layer 2 switches or bridges are listening. If any loops (multiple possible paths between switches) are found in the network topology, the switches will co-operate to disable a port or ports to ensure that there are no loops; that is, from one device to any other device in the layer 2 network, only one path can be taken.^[1]

If any changes occur in the layer 2 network, such as when a link goes down, a new link is added, a new switch is added, or a switch fails, the switches share this information by transmitting BPDUs, causing the STP algorithm to be re-executed, and a new loop-free topology is then created. STP and BPDUs help speed up convergence. Convergence is a term used in networking to describe the amount of time it takes to deal with changes and get the network back up and running. The default BPDU advertisement time of 2 seconds allows changes to be quickly shared with all the other switches in the network, reducing the amount of time any disruption would create.

There are three kinds of BPDUs:^[2]

• Configuration BPDU, used by Spanning tree protocol to provide information to all switches.
• TCN (Topology Change Notification), tell about changes in the topology.
• TCA (Topology Change Acknowledgment), confirm the reception of the TCN.

By default the BPDUs are sent every 2 seconds.

References