What is Bridge Protocol Data Unit?
A Bridge Protocol Data Unit (BPDU) is a data message that is transmitted over a local area network to detect loops in network topologies. A BPDU contains information about ports, switches, port priority and addresses.
BPDUs contain the information necessary to configure and manage the spanning tree topology. They are not forwarded by switches, but the information is used by switches to calculate their own BPDUs for passing information.
When devices are initially connected to switch ports, they do not start data transfer immediately. Instead, they go through various states while the BPDU processing determines the network topology. A Topology Change Notification (TCN) BPDU informs other switches of port changes. They are injected into the network through a non-root switch and passed on to the root. When a TCN is received, the root switch sets a topology change flag in its normal BPDU. This flag is passed on to all other switches to instruct them to use their forwarding table entry
Switches to age quickly.
When a trunk receives the TCN-BPDU, it sends a configuration BPDU message to all ports with the 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 certain ports. This will spread the tree to the end of the path.