Spanning Tree BPDU messages are comprised in the following format:
- Contains a 4bit (originally 2bytes) Priority field which can be configured to affect the RBID of the BPDUs being sent influencing the BPDU to be superior or inferior manually
- System ID Extension field is also included which allows the VLAN ID to be placed into what was formerly the last 12 bits of the priority field.
- Contains a 6byte MAC address field, MACs should be unique to each device so it can be a natural tie breaker during the election process. Allowing a single switch to eventually win the election process even when no priority is configured.