Unanswered Question
Aug 31st, 2009

In networks where a Layer 2 switch interconnects several routers, such as an Internet exchange point (IXP), the switch floods IP multicast packets on all multicast router ports by default, even if there are no multicast receivers downstream.IGMP snooping will not help but what is not clear that if there is no igmp message received on ports connected to routers why still switch will forward the traffic to router port and there is no downstream recievrs as well.

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I have this problem too.
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Paolo Bevilacqua Mon, 08/31/2009 - 10:20

Because a multicast routing protocol will tell to the router what to send where even if no IGMP.

Note there is no multicast in the internet, eg at IXP - no multicast.

tahseen12 Mon, 08/31/2009 - 10:36

this means if i remove pim from interface peer router will not recieve also switch will just forward even it is enabled with igmp snooping.

What if i remove pim from lan interface of the router.

mlund Mon, 08/31/2009 - 23:52


When you enable pim on a interface, that interface will automaticly do an igmp join to all groups. That's why a switch will flood all mcast traffic to all router-ports.

To overcome this situation you can enable RGMP, wich is designed for a router only (IXP). More info in this link.


Peter Paluch Tue, 09/01/2009 - 00:07


Can you be please more specific what you mean by writing: "When you enable pim on a interface, that interface will automaticly do an igmp join to all groups." As far as I know, until you use the "ip igmp join" command on a router interface, the router does not send IGMP joins. Actually, that is why the RGMP had to be invented.

Best regards,


tahseen12 Tue, 09/01/2009 - 02:22

This is incorrect it will not do automatic join when u enable pim then what is the use of ip igmp join-group and static-group

Moreover my question if both routers in the lan has upstream interface(wan) to source they why the traffic is copied back throgh L2 switch by the DR to the peer router interface which ultimately gets dropped due to RPF check failure and igmp snoop in the switch doesn't help.

mlund Tue, 09/01/2009 - 05:59


I was a little bit too quick, and unclear, in my previous answer.

The router don't do a join, instead the switch will detect that a router exist on the port when the router sends out either a igmp querier or a pim hello.

Because of that the switch adds the router port to the outgoing portlist for all mcast groups. As a result of this the router will receive all mcasttraffic that excists in the switch.

This can be viewed by "sh ip igmp sno mro" and "sh ip igmp sno gro"

To overcome this rgmp can be used, then the switch will use rgmp-info to decide what ports to send mcast-traffic to, and routers that don't have receivers can signal to the switch that they don't want to receive that traffic.



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