how to display multicast IGMP join groups?

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very basic setup. two servers on the SAME vlan 11 on the SAME switch (C6500-sup720 IOS 12.2(18)SXF13)

it's a layer 2 vlan. switch doesn't have a layer 3 SVI "int vlan11" on this vlan

Both hosts are publishing to a multicast address (verified by wireshark) but they're not receiving data. I suspect IGMP join is not set up correctly on the application itself...

but is there any way on Cat6500 to display multicast groups and show that "your host is not igmp joining the multicast group at all!"

"sh ip igmp groups" would only display multicast groups layer 3-routed. nothing from vlan 11

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Giuseppe Larosa Tue, 05/26/2009 - 13:55
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Hello Eric,

this looks like an IGMP snooping issue.

Verify if for vlan 11 igmp snooping is active.

IGMP snooping tries to optimize multicast traffic forwarding to only necessary ports by listening to IGMP reports.

But for all this to work a multicast router should query for hosts (IGMP querier).

If igmp snooping is enabled on vlan 11 you have two choices:

disable igmp snooping for vlan11

(no multicast optimization forwarding would occur)

or you need to setup one device to behave as the igmp querier to send out periodic IGMP queries


only one device has to be configured as IGMP querier on the broadcast domain

Hope to help


ignore the previous post...

can anyone verify this. I am a little confused.

1. if igmp snooping is enabled on the vlan

2. if there is NO multicast router present running IGMP; just a few multicast hosts on the same vlan

3. what is the default behavior?

switch will flood to all ports on the same vlan

OR drop all multicast data for lack of igmp mrouter?

thanks for the help

Giuseppe Larosa Tue, 05/26/2009 - 21:57
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Hello Eric,

if no IGMP querier is present and IGMP snooping is enabled for the L2 Vlan broadcast domain the default is to drop multicast traffic.

This is the reason I've suggested your issue could be caused by this.

There have been other threads on similar issues, and only in one case the problem was caused by a sw develepoment error (missing to send the right IGMP reports) but hosts don't send IGMP reports if not queried (they can send one unsolicited when they join a group but not periodically every N secs this is the point)

Hope to help


Harold Ritter Wed, 05/27/2009 - 10:50
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The switch will indeed drop the packets for lack of IGMP querier.


thanks a lot!!!

Here is a twist.

we have an identical setup at another location.

but server A and Server B are separated on two trunked switches (C2960), on the SAME vlan.

igmp snooping is enabled for this vlan and there is NO igmp querier.

But multicast seems to be working as confirmed by packet captures.

does it make sense?


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