Bridge Protocol Data Units (BPDUs) are frames exchanged between switches to establish and maintain the spanning tree protocol. There are two main types: configuration BPDUs, which are sent to establish the network topology, and topology change BPDUs, which are sent when a topology change is detected to initiate the spanning tree algorithm. The format of an IEEE 802.1D BPDU includes fields for the protocol ID, message type, root and sender bridge IDs, port ID, and timers.