BPDU

In STP (Spanning Tree Protocol), a Bridge Protocol Data Unit is a packet that initiates communication between devices. BPDU packets contain information on ports, addresses, priorities, and costs and they ensure that the data ends up where it was intended to go. BPDU messages are exchanged across bridges to detect loops in a network topology. The loops are then removed by shutting down selected bridge interfaces and placing redundant switch ports in a backup, or blocked, state.