Definition - What does Bridge Protocol Data Unit (BPDU) mean?
A bridge protocol data unit (BPDU) is a data message transmitted across a local area network to detect loops in network topologies. A BPDU contains information regarding ports, switches, port priority and addresses.
BPDUs contain the information necessary to configure and maintain spanning tree topology. They are not forwarded by switches, but the information is used by switches to calculate their own BPDUs for information passing.
When a root receives the TCN BPDU, it transmits a configuration BPDU message on all the ports with topology change flag set to 1. Switches that receive this BPDU on the root port filter the database and generate their own configuration BPDUs on designated ports. This propagates down the tree to the end of the path.