stp loop not understanding

[laksh]

Hi

I have three switchs a , b, c and pc1 is connected to switch a and pc2 is connected to switch c now stp is running in all the swtichs, PC1 pings to pc2, source mac address learned for pc1 and pc2 and now how the switch is flooding the frame in all the switch when it already learned the mac address that I am not getting the answer.

Can someone please answer the question.

Thanks,
Lakshmikant

 

[Gryz]

How are the 3 switches connected amongst themselves ? In a line: A-B-C ? Or in a triangle ?

 

[laksh]

it is connected in traiangle I am not getting how the loop is created once the PC1 ping to PC2 after that why the loop is form, please explain

 

[MtnMan]

The first frame is a broadcast (ARP request). Broadcasts are "flooded" by switches.

 

[Gryz]

The three switches run a protocol called "Spanning Tree Protocol" (stp).
https://en.wikipedia.org/wiki/Spanning_Tree_Protocol

Basically each switch sends out periodic hello-message. (Called BPDUs). And they listen to the hello messages of other switches. So they will learn to which switches they are connected. Then the switches will "elect" a root bridge. Based on who the root-bridge is, they will put some of their own ports in "forwarding mode" and others in "blocked mode". A switch will never send packets and never receive packets on ports that are in blocked mode. The result is that the network of links over which packets can go, is now a tree-structure. With all loops/redundancy removed. And therefor, when a PC is transmitting packets, those packets will never loop.

That is the general idea behind STP.
Read the wiki page to understand more details.

Or else, your question need to be more specific. I have no idea what you are asking exactly.