Multicast Issue

[mklaphek]

Hi. I have a Catalyst 2950 with four servers on it; three of them stream multicast and one reeives it. As long as the switch is not connected to any other part of the network, this works fine. However, as soon as I connect it to the rest of the network (using a trunk port, which connects to another switch), all but one of the muticast streams die. It's almost immediate, too -- disconnect from the network, they all come up. Connect the switch back to the network and they die.

I disabled IGMP snooping on this switch and now all three streams work when connected to the network.

The basic topology is this:

[4 Servers] -- [Cat 2950] --trunk-- [Cat 3550] -- trunk -- [Cat 6509/MSFC with HSRP]

All servers are in the same VLAN. The router config is pretty simple (nothing fancy, just enabling multicast routing globally and sparse-dense mode on the interfaces that we want to receive multicast streams from).

Any ideas why this works the way it does? Any help is appreciated.

Thanks!

Mike

[wong34539]

I am not aware of your issue, but i hope the following URL will help you more ,

http://www.cisco.com/en/US/tech/tk828/technologies_tech_note09186a00800ae957.shtml

[Richard Burts]

Mike

Would I be correct in assuming that when you say that there are 3 servers streaming multicast, that they are sending multicast with 3 different multicast group addresses? And when you say that one server receives multicast, is it receiving multicast for 1 of these multicast addresses? When you connect the switch to the network and 2 streams stop, is the surviving stream the one to which the receiving server is listening?

If my string of assumptions is correct, then I believe that the explanation is that when the switch is not connected to the network it knows that it is isolated (based on not receiving any IGMP general querries from the layer 3 interface) and processes the 3 multicast streams. When you do connect to the network the switch recognizes that there is now a router port reachable, monitors which streams have an active receiver, and does not process streams for which it has recorded no receiver.

And turning off igmp snooping means that all multicast frames are forwarded to all ports.

HTH

Rick

[mklaphek]

Rick,

Thanks for the reply. The one receiver I mentioned is configured to receive all three multicast streams at once (it's a Linux box with a custom app on it and yes they all have different multicast addresses). So, the receiver wants to listen to all three, and if it's connected to the switch by itself, it can.

IGMP is blocking this for some reason, but I don't fully understand why. Also, is it common for a receiver to receive multiple multicasts simultaneously?

Thanks again -- any additional insight is appreciated.

Mike