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IGMP Snooping questions



I have a couple of questions on the behaviour of IGMP snooping on Cisco 2960 and 3750:


  • What happens if I enable IGMP Snooping but don't have a querier on the VLAN? "show ip igmp snooping groups" will then not show anything.

Is multicast then dropped or is it flooded? I heard that switches realize that IGMP Snooping without a querier can not work so they just flood. Is this correct?

  • What happens to packets that are switched that have a Unicast destination IP and a Multicast Destination MAC? Checkpoint ClusterXL is doing it this way.

I saw in our lab that they are flooded. This happens with or without valid IGMP Snooping groups. So even if there is a Multicast Group that belongs to this destination Multicast MAC and the correct ports are members of it the packets are still flooded. Is this because the switch doesn't like Unicast IP with Multicast MAC? I know that ARP Tables are not filled this way (see Microsoft NLB static ARP entries). But what do switches with IGMP snooping do? I think they should restrict the traffic to the ports that send IGMP reports for this group. But this doesn't seem to work. It is still flooded although IGMP snooping is enabled and the Groups are listed on "show ip igmp snooping groups".

  • Is "show mac address table multicast" supposed to show IGMP based Multicast entries? It is described it older Cisco documentations but is it not working on our newer switches. We only see static multicast entries or none at all. Multicast and IGMP still works correctly.
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Two more questions:How does

Two more questions:

  • How does IGMP Snooping filter traffic? What happens when the Switch gets a Multicast that does not belong ot a IGMP learned group. Does it drop it or does it flood it?
  • How does the switch forward the multicast packets? Does it use a L3 Lookup (Destination IP == Multicast Group) or does it look at the Multicast Destination MAC and map a multicast group to it? (on 6500 you can configure the IGMP snooping lookup method for each VLAN!?)
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