BPDU
Bridge Protocol Data Unit — the messages switches exchange to run the Spanning Tree Protocol. By sending BPDUs they agree on a root bridge and which ports to block to break loops.
Mechanic: switches send BPDUs (normally every two seconds, "hello") on all ports. They contain, among other things, the sender's bridge ID and the cost to the root bridge. From this, the network elects a root bridge (lowest bridge ID) and each switch computes its best path there; redundant ports are blocked → a loop-free tree topology. Topology Change Notification (TCN) BPDUs signal when something has changed so tables can update. Security: an unexpected BPDU from a miswired or malicious device can trigger a root change and disrupt the whole network → which is why BPDU guard (shut the port if a BPDU appears where it shouldn't) and root guard exist. The BPDUs are the "language" STP speaks. Related to BPDU guard and root guard.