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.
Techopedia explains Bridge Protocol Data Unit (BPDU)
When devices are initially attached to switch ports, they do not start data transmission immediately. Instead, they moves through different states while BPDU processing determines the network topology. A topology change notification (TCN) BPDU informs other switches of port changes. They are injected into the network by a non-root switch and propagated to the root. When a TCN is received, the root switch will set a topology change flag in its normal BPDU. This flag is propagated to all other switches to instruct them to rapidly age out their forwarding table entry
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.