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Spanning Tree Protocol (STP)
Once upon a time a company called Digital Equipment Corporation (DEC) was purchased
and renamed Compaq. But before that happened, DEC created the original version of Spanning
Tree Protocol, or STP. The IEEE later created its own version of STP called 802.1D. The
bad news is that by default, Cisco switches run the IEEE 802.1D version of STP, which isn’t
compatible with the DEC version. The good news is that Cisco is moving toward another
industry standard in its newer switches called 802.1w. I’ll cover that STP version in this section
as well, but first, let’s define some important STP basics.
STP’s main task is to stop network loops from occurring on your layer 2 network (bridges
or switches). It vigilantly monitors the network to find all links, making sure that no loops
occur by shutting down any redundant links. STP uses the spanning-tree algorithm (STA)
to first create a topology database and then search out and destroy redundant links. With STP
running, frames will be forwarded only on the premium, STP-picked links.
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