Spanning Tree Protocol - STP

The old adage 2 is 1 and 1 is none holds true to the networking field in which redundancy is always good to avoid waking up in the middle of the night. 

So, say we have 3 switches in a network as below.


In case SW1 goes down or SW2 goes down, both of our servers go down. In order to make this redundant, very simply, we can connect SW1 to SW2.




But this brings about an issue, in case one of the virtual machines on our servers sends out a broadcast message, this creates a 'broadcast storm' in our network effectively bringing our entire network down. So the smarty pants at Cisco invented a way to prevent this from happening by giving the Cisco devices smarts to shutdown one of its ports.

But how does the Cisco device know about the loops?

It figures it out through BPDUs (Bridge Protocol Data Units) which are transmitted every 2 seconds. Based on this, it does the following calculations to figure out which port to block.







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