BPDU
acronymStands for: Bridge Protocol Data Unit
The frame spanning tree switches exchange to elect a root bridge and decide which ports forward and which block.
Sent to the bridge group address 01-80-C2-00-00-00, every two seconds by default from the root. A BPDU arriving on a port that faces a host is a warning sign, which BPDU Guard turns into a shut port.
BPDUs carry everything the election needs: the sender's bridge ID, a priority and a MAC address, the ID of the bridge it believes is root, and its cost to reach it. The lowest bridge ID wins; Cisco's default priority is 32768, settable from 0 to 61440 in steps of 4096. The root sends configuration BPDUs every hello interval, 2 seconds by default, and the other switches relay them, so a switch that stops hearing them knows the topology has changed.
Because a BPDU can claim to be anything, a port that faces users should never accept one. Cisco's BPDU Guard 'shuts the port down by placing it in the errdisable state' when a BPDU arrives on a port configured for end devices, which stops a stray switch, or a crafted frame, from winning the election. In the rapid version of the protocol, BPDUs also work as keepalives: three missed hellos and a neighbour's information ages out at once.
Also known as: bridge protocol data unit, configuration bpdu, bpdu guard, 01-80-c2-00-00-00