Cisco Basics: Spanning Tree

Spanning Tree is a network protocol designed to prevent network loops. A loop exists at Layer 2 of the OSI model where there are multiple paths between any two devices. If you draw out a network diagram consisting of multiple switches, and can connect any two devices in more than one way there is potentially a loop in your network.

Loops cause problems when it comes to broadcast traffic. When a device sends a broadcast frame a switch will see it and forward the frame out of all ports except the one it received it on. With loops in the network this means that the frame will be broadcast indefinitely, as a switch will see the frame more than once.

Spanning Tree was introduced to prevent switching loops, but also allows for some redundancy in the design of a network. Spanning Tree will have a view of a network and calculates the most efficient paths between switches. Should a connection between switches fail a spare path that would have previously created a loop will become available in around a minute.

Traditionally when you plug a switch into a network it will immediately begin forwarding data, with spanning tree a device first waits to receive special frames known as BPDU’s. Each switch port will go through a series of states whilst waiting for these frames.
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