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