Spanning Tree

Spanning Tree Protocol (STP, IEEE 802.1D) keeps a network with multiple redundant connections between switches from looping. If two switches are connected by more than one path, data packets would circle endlessly without loop protection and bring the network down. STP automatically works out a loop-free tree across all the connections and puts any extra paths into standby.

In practice, a redundantly cabled network shows only one active path carrying traffic between switches at any time, despite the extra physical links. If the active connection fails, STP automatically switches over to one of the previously blocked paths, though that takes a certain amount of time; faster variants such as Rapid Spanning Tree, RSTP, cut that time down considerably. Without this automatic switchover, a failed connection would have to be replaced manually.

The classic failure case is a loop created when someone accidentally cables two switch ports directly together while Spanning Tree is disabled or misconfigured — the network typically collapses within seconds as traffic multiplies uncontrollably. That's why Spanning Tree should stay active on every switch with redundant links, even though it normally works unnoticed in the background.