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
  • TCN 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
spanning-tree-bpdu-format-1

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

spanning-tree-bpdu-format-2

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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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