BPDU

A BPDU (Bridge Protocol Data Unit) is a type of network message exchanged by Ethernet switches to detect and prevent Layer 2 bridging loops by implementing the Spanning Tree family of protocols. BPDUs carry topology and configuration information used to elect a root bridge, determine port roles (root, designated, blocking), and calculate loop-free paths through a switched network.

Typical BPDU fields include the root identifier, root path cost, sending bridge identifier, sending port identifier, message age, maximum age, hello time, forward delay, and topology-change flags. In IEEE 802.1D (classic STP) there are two basic BPDU forms: configuration BPDUs, which convey the spanning tree parameters, and topology-change notifications (TCNs), which signal a change in connectivity. IEEE 802.1w (Rapid STP) and IEEE 802.1s (Multiple STP) extend and modify BPDU formats and behaviors to provide faster convergence and support multiple spanning tree instances.

BPDUs are exchanged at Layer 2 using reserved multicast destination MAC addresses and are normally processed locally by switches without being forwarded. Modern implementations often have protective features such as BPDU Guard and Root Guard to block unexpected or malicious BPDU sources; improper BPDU injection or misconfiguration can disrupt network topology and cause outages.