Multicast communications between 4 6509 switches?

Unanswered Question
Jul 23rd, 2007

We have a couple servers trying to communicate with multicast thru 4 x IOS 6509's. After several captures we see that multicast leaves both access switches but reaches the Distribution switches and doesn't pass the info to the appropriate access switches. After reading some documentation we tried enabling no ip igmp snooping on both Dist Switches it still doesn't work. Any ideas...Thx!

I have this problem too.
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Jon Marshall Mon, 07/23/2007 - 08:49


Coudl you give a few more details.

1) How are the access switches connected to the distribution switches - Layer 2 trunks, or L3 routed links.

2) Are you trying to do multicast within the same vlan or are you trying to cross vlans.

A more detailed explanation of your topology would help.


jcmattos1 Mon, 07/23/2007 - 08:54

We have two Dist switches fully meshed L2 topology to 2 Access Switches. The servers are communicating over the same vlan. We are seeing multicast traffic reaching the Dist from either ends. Not sure why this would not work. Any Ideas?

Jon Marshall Mon, 07/23/2007 - 09:07

Okay, just to clarify

Server1 connected to access layer switch1.

Server2 connected to access layer switch2

Both access layer switches connected to distribution switches via L2 trunk links. L2 trunk link connecting 2 distribution switches.

So if you turn off IGMP snooping on all switches the multicast should become a broadcast although without an IGMP querier it will be flooded to all ports anyway.

And the traffic never gets from one access-layer switch to the other ?


Jon Marshall Mon, 07/23/2007 - 10:38

Nice and succint :)

Okay i'm presuming there is normal IP connectivity between the 2 servers.

As amit suggested you can enable IGMP snooping but if you do you will need to make one of distribution switches an IGMP querier which you can do on the 6500, otherwise the multicast will still get flooded.

However this is not addressing the real problem. With or without IGMP snooping/IGMP querying your muticast adverstisemetn should be flooded.

What is the multicast address you are using ?

Is one of the servers the multicast source and one the receiver ?

Could you psot relevent bits of config ie. server ports, trunk links between switches


Amit Singh Mon, 07/23/2007 - 09:54

what are your access switches models? Are they Cisco switches ? As pointed out by Jon that disabling IP IGMP snooping will simply turn the traffic into broadcast,I would suggest you enable IGMP snooping feature on all the switches to have multicast travel across entire layer 2 domain.

Please see the link on the same for more info:


-amit singh


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