Spanning Tree Protocol (STP)


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Industrial Communications Handbook

By Mick Crabtree

Chapter 8: Spanning Tree Protocol (STP)



Spanning Tree Protocol (STP) is an algorithm - created by Dr Radia Periman (software designer and network engineer) - which allows the switches to create a loop-free topology over a mesh network. A key element in STP is the generation, exchange and processing of data messages called Bridge Protocol Data Units (BPDUs) that are used, in conjunction with the STP algorithm, to allow switches to identify redundant paths and ensure that there is no loop path in the network. The BPDUs are transmitted using the standardised multicast Layer 2 Media Access Control (MAC) address 01-80-c2-00-00-00.


There are three types of BPDU:

  • Configuration BDPUs (CBDPU) used for spanning tree computation
  • Topology Change Notification (TCU) BDPUs, used to announce changes in the network topology
  • Topology Change Notification Acknowledgement (TCA)