Equivalent <b>ip igmp join-group</b> on layer-2 switchport ?

Unanswered Question
Sep 28th, 2007

I would like to monitor a multicast group on a VLAN. I have a snoop machine connected to an access-port on the VLAN. At the moment I can see the broadcasts, some non-IP multicasts, and the occasional flooded unicast packet.

What I would like to do is generate an IGMP join so that I can see the particular multicast group. But my snoop machine cannot do that. Is there any way I can do it on the switchport, much in the same way as you would do ip igmp join-group on a layer 3 interface?

How about can I force my snooping port to be an IGMP-snooping router port but just for the group I am interested in? Would that disrupt my multicast traffic, and how do I do it on a 4506 platform?

Kevin Dorrell


I have this problem too.
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nambi_gct Fri, 09/28/2007 - 06:07

Hi Kevin,

If there is any other multicast receiver connected in that switch, you can create a span session with destination as your snoop machine port.

Or you can use the vlan itself as span source session to replicate traffic to your snoop machine.

Did you find any issues with this approach?



Kevin Dorrell Fri, 09/28/2007 - 07:27

I did consider using the VLAN as a span source, but I chickened out because the VLAN is carrying up to about 200 Mbps on this switch and the monitor port is 100 Mbps. (That shouldn't be blocking on a 4506 should it? And does the access-list filter actually work when the source is a VLAN? Can someone reassure me?)

Unfortunately there are not any multicast receivers on the switch. My permanent monitor port is on the distribution switch, which carries only downlink trunks to the access layer, the external connections, and a few management connections.

I guess I will have to find out where the multicast receivers are and go and monitor there.

Thanks for the suggestions anyway.

Kevin Dorrell



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