Switching loop question

Look at the below diagram

STP is not running on the network and there is a loop between switch B and switch C.

Why does the loop between switch B and switch C affect switch A?

The loop will cause constant changes to B and C's mac address table, how come these changes affect switch A?

There will be no MAC address changes between A and B because there is no loop and only 1 cable.

Yet, a switching loop in a layer 2 network will effect entire network, I am not grasping why because only mac address changes would be registered on the switches in the loop (at least in this simple network).

Thanks

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Comments

ColbyG

Bosefus wrote: »

Yet, a switching loop in a layer 2 network will effect entire network, I am not grasping why because only mac address changes would be registered on the switches in the loop (at least in this simple network).

Thanks

Broadcasts.

Forsaken_GA

To expand further on that... the looped pair will keep generating broadcast packets ad infinitum, which will eventually take down every single switch in broadcast domain. Broadcast storms are bad juju.

ColbyG

Forsaken_GA wrote: »

To expand further on that... the looped pair will keep generating broadcast packets ad infinitum, which will eventually take down every single switch in broadcast domain. Broadcast storms are bad juju.

I hoped maybe he'd do some research on his own, lol.

Forsaken_GA

Should probably also mention that a broadcast storm basically corrupts your mac address tables.

In the diagram you have above, if a PC connected to switch 1 sent out a broadcast, that broadcast would eventually loop back to switch 1 from switch 2 with the same source mac address. Since switches have to assume that machines will move and use the most recent information they have, when switch 2 sends that broadcast back to switch 1, switch 1 will then decide that the best way to reach the PC that broadcasted is via switch 2 rather than it's own directly connected port.

As I said, bad juju. You never want to see this happen in a production network.

Bosefus

ColbyNA wrote: »

Broadcasts.

ARP would make up the brunt of the broadcasts correct?

Thanks

jovan88

Out of first hand experience, switching loops will take down the entire subnet, doesn't matter how many switches there are.

Good times....

Bosefus

Ya we had this happen at work yesterday, which got me to thinking which is the reason for the post.

When I was in cisco net acad and we went over loops we were just taught that loops create broadcast storms but did not go to deep into why, just that stp stops it at which point we learned about stp, how to configure, troubleshoot, etc...

tech-airman

Bosefus wrote: »

Ya we had this happen at work yesterday, which got me to thinking which is the reason for the post.

When I was in cisco net acad and we went over loops we were just taught that loops create broadcast storms but did not go to deep into why, just that stp stops it at which point we learned about stp, how to configure, troubleshoot, etc...

Bosefus,

Do you recall that there's at least _two_ kinds of broadcasts?

Bosefus

If you are referencing protocols that broadcast ARP and DHCP are the first two that come to mind right off the top of my head.

If you are referring to the OSI model then there could be L2 and L3 broadcasts.