Wikipedia
The Spanning Tree Protocol (STP) is a network protocol that builds a loop-free logical topology for Ethernet networks The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them Spanning tree also allows a network design to include backup links providing fault tolerance if an active link fails As the name suggests, STP creates a spanning tree that characterizes the relationship of nodes within a network of connected layer-2 bridges, and disables those links that are not part of the spanning tree, leaving a single active path between any two network nodes STP is based on an algorithm that was invented by Radia Perlman while she was working for Digital Equipment Corporation
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