Multicast source and clients on the same vlan

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I configured a VLAN in a 6500 for multicast traffic. There are sources and clients. I enabled IGMP SNOOPING and Multicast Routing. I enabled PIM in the interface VLAN too. I see that the SW is flooding the multicast traffic in all ports in the VLANS, regardless if the port has clients.

is this the normal behavior?


I have this problem too.
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Jon Marshall Thu, 01/03/2008 - 13:05


If you do a "sh ip igmp membership" what do you see ?

Some multicast addresses ie. the reserved multicast addresses will always be flooded eg 224.0.0.x and even with IGMP snooping and an IGMP querier they will still be sent out all ports.


I used the IP and with the same results. I did the show ip igmp groups and saw the last client (the only one) int the GE8/2, but the SW was flooding the multicast traffic to the GE8/4 too. (All in VLAN 1). Behind GE8/4 there was a router (but it wasnt joined to that GROUP). I will test with the SHOW that you suggest, to see what show.

Thanks !

Kevin Dorrell Fri, 01/04/2008 - 05:45

Maybe the switch thought that the router on G8/4 was a multicast router. The switch will always flood all multicasts to a multicast router, regardless of IGMP membership. You can see if it thinks it is a multicast router by doing a show ip igmp snooping mrouter.

How does the switch know whether it has a multicast router on the port? By default, it detects it by the PIM messages. But there is a way to configure it statically, or to get it to snoop for DVMRP. The command is ip igmp snooping vlan n mrouter ...

Does that help?

Kevin Dorrell


No. The neighbor router doesnt run PIM. I see that the problem is when the SW is the IGMP Querier (I test with 3560 and 6500). When the SW is the DR (or querier) in a VLAN, it flood the multicast traffic in all port for that VLAN. I tested this with a SOURCE and a CLIENT connected to the same SW int the same VLAN. I tested the feature ip igmp snooping querier too (and disabling ip multicast-routing and PIM) and it works. But when i used PIM it didnt works


chschroe Sat, 01/05/2008 - 14:59

If the source and the receiver are in the same VLAN, there is very little you can do to contain flooding within that VLAN. Multicast is, effectively, a broadcast packet within a L2 domain.

IGMP is mostly about interested VLANs, and it is a L3 multicast activity destined to be picked up by the router and forwarded along to the RP or the source in the case of SSM.

Is this the only place in the network where the multicast traffic is present? Is it being routed along to elsewhere? What mode of pim are you using?

Multicast is really more about efficient delivery of one stream to hosts spread across diverse geographies, not necessarily connected to other VLANs on the same device.

That being said, if there are no interested parties out other VLANs on the device, that are running PIM and IGMP+snooping, it won't flood out to those.

The thing is that when the sources and receivers are on the same layer 2 network, the router is not responsible for forwarding on igmp join requests. It gets it, but it knows the source is back out that same interface so it doesn't really participate all that much.

The hardware in your NIC is designed to cull multicast packets out that are intended for a group it is interested in. This hardware acceleration is a pretty handy feature that spares your upper level stacks from getting any work.

Back in the days of CGMP, this was easier to deal with. All of these features we are discussing are Layer 3 multicast routing dependent.

Try IGMPv3 and set the mrouter statically to your VLAN interface. That is at least a possibility, albeit I don't think it will really work.

Let me know if this helps you any or if I've gotten your idea wrong.


we have one source y a few client in one VLAN. In the same vlan there are some PC. These PCs are recieving the multicast traffic. There is no multicast clients outside that VLAN.

if I configured IP IGMP SNOOPING QUERIER, the SW sent multicast traffic correctly, but if I configured IP MULTICAST-ROUTING and IP PIM in the VLAN, multicast traffic is flooded.

i think that the better choice is to configure PIM + IGMP SNOOPING. Am I wrong?

When sources and recievers are in the same vlan, will the SWITCH flood the traffic?


Kevin Dorrell Mon, 01/07/2008 - 05:54

As far as I am aware, if you enable IGMP snooping on the switch, it will block all IP multicast to any port, unless either it sees an IGMP query from the connected device (specific to the multicast group), or it detects a multicast router (in which case it opens that port to all multicasts).

Do a show ip igmp snooping groups on the switch. that will tell you which groups the switch knows about, and which ports it is forwarding each one to.

Note: IGMP Snooping will filter only genuine IP multicast frames, i.e. those with a geniune multicast MAC address (except the very low numbers) that have Ethertype 0x0800. Any other Ethertype will be flooded.

Kevin Dorrell


chschroe Mon, 01/07/2008 - 14:00

Ok, having PIM and IGMP snooping on with no other clients in any other VLANs doesn't really serve a purpose. The multicast traffic doesn't need to be routed, so the router will not really participate... it won't send joins back to the interface it got them from. Turning on querier is the right solution for you.

IGMP snooping querier is for networks where there is no igmp querier (a router). This is basically your situation, even if you have PIM on, the traffic isn't being routed (there's nowhere for it to route to) so the router never participates.

Generally, yes, if all sources and receivers are in the same VLAN and the router isn't participating in any way, the traffic will just be flooded, yes. At that point, multicast is basically broadcast.

Turning on querier and off multicast routing should give you results.



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