BPDU Bridge Protocol Data Unit
The heartbeat and communication mechanism of the Spanning Tree Protocol (STP). BPDUs are multicast frames transmitted continuously between switches to map the network topology, elect a central leader (the Root Bridge), and identify redundant paths that must be blocked to prevent catastrophic broadcast storms.
The Root Bridge Election
By default, every Cisco switch transmits a Configuration BPDU out of every active port every 2 seconds. When a network first boots up, every switch assumes it is the king and sends out BPDUs claiming to be the Root Bridge. The election is settled mathematically based on the data contained inside the BPDU header:
The primary tie-breaker. The default priority is 32768. Switches compare the priority values in the BPDUs they receive. The switch with the lowest priority becomes the Root Bridge.
If all switches have the default priority, the system uses the burned-in MAC address of the switch hardware as the final tie-breaker. The switch with the lowest numerical MAC address wins the election.
CCNA Exam Gotchas
Oldest Hardware Wins
Because the tie-breaker relies on the lowest MAC address, leaving a network at default settings guarantees that the oldest, slowest, most outdated switch in your enterprise will win the Root Bridge election (since MAC addresses are assigned sequentially during manufacturing). You must manually lower the priority of your powerful Core switches using spanning-tree vlan X root primary.
BPDU Guard
If you configure an access port facing a user PC with spanning-tree portfast, you are bypassing the STP listening/learning phases. To protect the network, you must pair it with spanning-tree bpduguard enable. If a user unplugs their PC and plugs in a rogue switch, the port will receive a BPDU, detect the threat, and instantly err-disable the port.