IGMP propagation between layer 3 and layer 2 switches

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Mar 17th, 2009
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Is there a way to get a host on a L2 switch to do IGMP joins to across a dot1q trunk to a multicast source on the same vlan. The source side has a vlan interface created the host side is strictly a L2 vlan.

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izackvail Tue, 03/17/2009 - 21:43
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I'm not sure what you mean by "The source side has a vlan interface created" but if the source and the host are on the same vlan the host will receive the multicast traffic. If the switch supports snooping it will prune the traffic off unnecessary interfaces based on the join. If not it will flood the traffic out all interfaces.

Giuseppe Larosa Wed, 03/18/2009 - 04:06
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IGMP messages are used between receivers and routers.

if the source and the receivers are in the same vlan if igmp snooping is disabled it works.

To be noted that if IGMP snooping is enabled a L3 multicast router (the vlan interface indeed) has to be enabled for multicast so that it will send out IGMP queries and the receiver will answer with IGMP reports.

IGMP snooping requires some device to send IGMP queries on the vlan.

Some devices like C6500 can use a special feature called IGMP querier that allows a switch to perform IGMP queries to make IGMP snooping to work

Hope to help


izackvail Wed, 03/18/2009 - 08:08
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IGMP Snooping allows the switch to process the IGMP reports locally. If there is a local router it will listen for IGMP reports to determine if they should continue to forward traffic but they aren't required for IGMP snooping to function.


Take a look at this document.

izackvail Wed, 03/18/2009 - 08:32
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It will work it just won't work for long. The host will send an unsolicited IGMP report when it starts up but if there is no device on the local segment to send queries the switch will eventually time out that entry.


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