How Spanning Tree Works

The STP algorithm creates a spanning tree of interfaces that forward frames. The tree structure of forwarding interfaces creates a single path to and from each Ethernet link, just like you can trace a single path in a living, growing tree from the base of the tree to each leaf.


STP was created before LAN switches even existed. In those days, Ethernet bridges used STP. Today, switches play the same role as bridges, implementing STP. However, many STP terms still refer to bridge. For the purposes of STP and this chapter, consider the terms bridge and switch synonymous.

The process used by STP, sometimes called the spanning-tree algorithm (STA), chooses the interfaces that should be placed into a forwarding state. ...

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