## SPANNING-TREE PROTOCOL:

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 Defination:

 

 It’s a Loop Free Path to reach everywhere in the topology. 

 Explanation:

 A Spanning-Tree protocol helps in preventing the loop, when there are redundant-links between the switches.  And this loop is created due to the switch’s nature of sending a broadcast or multicast frame which it received on its interface to all other interfaces except the one on which it received it.

Confused ............ ???

I will explain it with a diagram.

NOW HERE STP IS RUNNING.

Suppose a Broadcast packet is being generated by PC0 and sent to Switch-A on its interface Fa0/3, as this being a broadcast packet, it is sent on All the ports EXCEPT THE PORT ON WHICH IT RECEIVED IT (ie. Fa0/3) . Switch-B will be receiving this packet within a frame on the PORT Fa0/1 and after looking in to the packet, it will broadcast it on all other ports EXCEPT the port on which it received it.

## But 2 incidents occurring here are very interesting to note:

1. Switch-A though has two redundant links to Switch-B, but uses only one Fa0/1 to forward its frame.

2. Switch-B will broadcast the frame on all the ports Except the port on which it received it and on Fa0/2.

##  WHAT DID STP DO ???

1.       STP PREVENTED THE LOOP.

2.       STP PREVENTED THE MULTIPLE COPIES OF THE FRAME TO REACH THE DESTINATION.

##  A MISCONCEPTION:

Some people thinks that a frame will be discarded on the basis of its TTL value. But just to clear the fog……..

- TTL VALUE IS A NETWORK LAYER CONCEPT.

- ONLY A ROUTER CAN DECREMENT A TTL VALUE.

- SO A TTL VALUE HAS NOTHING TO DO WITHIN A SWITCHED NETWORK.

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