Fighting a multicast storm

flash2200 · ‎11-02-2007

Hi all,

I'd like to know if this statement is true...

"A multicast storm occurs when STP fails (for whatever reason) to do it's job of blocking ports on redundant uplinks hence allowing loops to form. Therefore in the event of a multicast storm one quick way to stop it would be to manually remove one of the two uplinks from each edge switch stack to the core switch stack in effect manually breaking the loop."

This sounds right to me but I'd like some further opinions. Also, on a more general note, what else can one do if one experiences a multicast storm to isolate the source of the problem?

TIA

Jon Marshall · ‎11-02-2007

Hi

If STP fails then yes there is the potential for not just multicast but also braodcast storms within your network so the above is a fair statement.

As to what else you can do, some switches support broadcast/multicast storm control which works on thresholds. Attached is a document for the 4500 switch which explains this in more detail - note that the 4500 is not the only switch with these features.

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/37sg/configuration/guides/bcastsup.html

HTH

Jon

flash2200 · ‎11-02-2007

Thanks for this Jon. I'll check out the link as well.

I'm just curious if people agree that you could just remove one of the redundant uplinks (Ie: Literally yank out one of the two uplink cables on each edge switch stack.) to solve the problem? (at least temporarily)

Jon Marshall · ‎11-02-2007

Well you could but to be honest it might be too late by then. I have seen a broadcast storm in action and you have no chance of getting to the switches remotely to shut ports down.

As for physically pulling them out, i've never been near enough to do that unfortunately :)

Jon