Aswitch that is configured to use the spanning tree protocol will place a port in mode if sending traffic to it may cause a switching loop.

Blocking Mode    

Explanation:

Spanning Tree Protocol is used to allow path redundancy in the network without creating cycles/circles also called loops.

When two parts of the switched network are connected via two or more Layer 2 switches this result in a loop.

This affects the performance of the network as the result of broadcast packets flooding.

STP puts one port of the switch to forwarding mode and the rest of the ports within the same part of the network to the blocking mode to avoid broadcast packet flooding. STP puts all the ports that are allowing redundant paths to blocking mode and the one port that is left after this is placed in forward mode.

Spanning Tree Algorithm is used by STP to determine the optimal path  of switch to the network.

Bridge Protocol Data Units are used to share the information about the optimal path determined by the spanning tree algorithm with other switches.

This information helps STP to eliminate the redundant paths.

So this is how STP allows only one active path to the destination while blocking all other paths to avoid switching loop.

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