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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.
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