5.1 Basic STP Operations
5.1.1 Usefulness of STP

In its most basic definition, the Spanning-Tree Protocol (STP) is a loop-prevention protocol. It is a technology that allows bridges to communicate with each other to discover physical loops in the network. The protocol then specifies an algorithm that bridges can use to create a loop-free logical topology. In other words, STP creates a tree structure of loop-free leaves and branches that spans the entire Layer 2 network. The actual mechanics of how the bridges communicate and how the STP algorithm works is the subject of this chapter.

Loops may occur in a network for a variety of reasons. Usually loops in networks are the result of a deliberate attempt to provide redundancy. However, loops can also occur by configuration error. The Figure shows a typical switched network and how loops can be intentionally used to provide redundancy. Loops can be absolutely disastrous in a bridged network for two primary reasons: broadcast loops and bridge-table corruption.