You really do have Multicast in your network.

I hear people say all the time that they do not have any multicast devices in their network so they do not need to configure it. This is absolutely incorrect! Apple devices use all kinds of multicast to make life easier for users. Airplay, Apple TV, and others were created to reach out on the single VLAN and connect to other like devices. Since it was designed for home use, it works great there, but in the enterprise, this is a different story. Well, I do not have any Apple devices you say. Do you have Windows devices? They use UPnP or Universal Plug and Play which uses multicast under the covers to do the same thing as Apple, make life easier. On everyone but you!

OK, how do I fix this? Can you cut it off? The short answer is yes, but you may not want to. This is all useful technology, it just needs to be handled correctly. The first thing you need to do is see if IGMP Snooping is configured on your Layer 2 switching and wireless infrastructure. There will be some show command to verify if it is on or not. If it is not on, enable it. IGMP Snooping controls multicast traffic at Layer 2 by stopping the switch from flooding it all ports in a broadcast domain. Traffic is only sent to port that are joined to a valid multicast group. You can see how this can significantly reduce the traffic on your network. If you need to route multicast across Layer 3 boundaries, you will need to look at a Bonjour gateways and/or PIM.

The purpose of this article is not to teach you to configure these things, but to let you know that thinking something (multicast) doesn’t exist in your network is killing it. I have seen CPU on a switch go from a constant 20% CPU utilization to 1% utilization just by turning on IGMP Snooping. We all need to know our environment. Download Wireshark and run it on your network for a few minutes. It will show you all the things going on in your network. Even the things you do not want to see or didn’t know where there. Once again, my goal is to help every admin become the “Maytag Man” (if you are too young to know the reference, Google it). I want us all to be drinking coffee and letting everyone thing we are magicians behind the curtain as the whole thing runs like a well-oiled machine.

David Ellis
Systems Engineer
Ruckus Networks/Brocade

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