Spanning Tree Protocol: STP Port States

The Spanning Tree Protocol (STP) has long been a staple of Local Area Networks (LAN) in multiple environments. It is required in environments where multiple layer 2 paths exist between the various layer 2 devices within the LAN. Each of these ports that connect these devices together pass through a number of different states (Port states) that allow STP enough time to determine the status of the network and in what final state the port should be placed. This article takes a look at these port states, what happens during each of them, and why they are needed for STP operation.

What are STP Port States?

The main purpose of STP is to prevent switching loops throughout a LAN. The way it does this is by controlling the redundant links that connect into the same network segment. Each network segment is only allowed to have a single designated port that is used to forward traffic onto it. All other access points into the same segment are enabled, but in a blocking state that disallows traffic flow. If a failure should occur on the forwarding port, one of the blocking ports will be transitioned into forwarding state to continue to allow access to the network segment.

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