Spanning Tree BPDU Format

Let’s explore the Spanning Tree Protocol BPDU format.

Bridges exchange BPDUs encapsulated into Ethernet frames. There are two types of BPDUs:

  • Configuration BPDU

Spanning Tree BPDU format : what does it look like in an Ethernet Frame?

Let’s see how a BPDU fits into an Ethernet frame. We should know that STP needs the LLC sub-layer because LLC acts as a transport protocol for STP. So in a STP frame, you’ll find LLC.

If you recall from the Ethernet post, the IEEE 802.3 Ethernet frame with LLC sub-layer is composed of:

  • IEEE 802.2 LLC
  • IEEE 802.3 MAC


In the Data field of the Ethernet frame lays the BPDU. The BPDU format is as follows:


© CCIE Practical Studies, Vol II

So When STP process kicks in, it gives its BPDU to the Ethernet protocol which encapsulates the BPDU into an Ethernet frame, in the Data field.

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