What is STP?

Spanning Tree Protocol (STP) are network protocol designed to the prevent layer 2 loops. It’s standardized as IEEE 802.D protocol.  The STP blocks some ports on the switches with the redundant links to prevent broadcast storms and the ensure loop-free topology. With STP in place, you can have redundant links between the switches in order to the provide redundancy.

To the better understand importance of STP and the how STP prevents broadcast storms on network with redundant links, consider following example are:

SW1 sends broadcast frame to the SW2 and SW3. Both switches receive the frame and forward frame out every port, except the port frame was received on. So SW2 forwards frame to the SW3. The SW3 receives that frame, and forwards it to the SW1. SW1 then again forwards frame to the SW2! The same thing also the happens in opposite direction. Without STP in the place, these frames are would loop forever. STP prevents loops by the placing one of switch ports in the blocking state.

So, our topology is above could look like this are:

In this topology above, STP has placed one port on the SW3 in blocking state. That port will be no longer process any frames except STP messages. If the SW3 receives a broadcast frame from the SW1, it will be not forward it out port connected to the SW2