What are BPDUs (Bridge Protocol Data Units) and what role do they play in STP (Spanning Tree Protocol)?

BPDUs (Bridge Protocol Data Units) are the control messages switches exchange to build and maintain the spanning tree.

During STP operation, switches send BPDUs to share information about themselves and their connections. BPDUs are used to:

• Elect the root bridge
• Elect root ports (best path to root on each non-root switch)
• Elect designated ports (best port on each segment toward the root)
• Elect alternate (blocked) ports

Each BPDU contains a Bridge ID (BID) that identifies which switch sent it. The BID is critical for STP decisions — it determines the root bridge and influences port roles.

The BID consists of three fields:

1. Bridge Priority — configurable value (default 32768)
2. Extended System ID — identifies the VLAN (Virtual Local Area Network)
3. MAC (Media Access Control) Address — unique hardware address of the switch

The lowest BID wins the root bridge election. This is determined by comparing these three fields in order.

Go deeper:

• BPDUs — Spanning Tree Protocol (Wikipedia) — the BPDU frame layout (bridge ID, root path cost, port ID, timers) and Configuration vs TCN types.