The Spanning Tree Protocol (STP) is a Layer 2 network protocol that prevents looping within a network topology. It was created to avoid the problems that arise when bridges in local area networks (LANs) are interconnected using redundant links to improve resilience should one connection fail. However, this connection configuration creates a switching loop resulting in broadcast radiations and MAC table instability. STP helps to create a loop-free logical topology by blocking certain interfaces.
To control the flow of traffic and prevent looping, bridges exchange bridge protocol data units (BPDUs) via an extended LAN that uses a spanning tree protocol. BPDUs are data messages that provide the bridges with network information thats used to carry out STP operations. The original spanning tree protocol and algorithm were invented in 1985 by Radia Perlman when she was working at Digital Equipment Corporation. Spanning tree protocols were later standardized by the Institute of Electrical and Electronics Engineers (IEEE) .
The following are some key features of STP:
- Bridge ID: A unique identifier assigned to each bridge in the network.
- Path Cost: The cost of sending a packet through a particular path in the network.
- Root Bridge: The bridge with the lowest Bridge ID in the network.
- Designated Port: The port on a bridge that is responsible for forwarding traffic to the root bridge.
- Non-Designated Port: The port on a bridge that is not responsible for forwarding traffic to the root bridge.
- STP States: STP has five states: Blocking, Listening, Learning, Forwarding, and Disabled.
The first STP standard was published by the IEEE in 1990 as 802.1D, based on the algorithm designed by Perlman. Subsequent versions were published in 1998 and 2004, incorporating various extensions. However, spanning tree is an older protocol with a longer convergence time, and improper use or implementation can contribute to network disruptions. Modern networks can make use of all connected links by using protocols that inhibit, control, or suppress the natural behavior of logical or physical loops.
