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What is Meant by RFC 4541 when it says...

4) All non-IPv4 multicast packets should continue to be flooded out to all remaining ports in the forwarding state as per normal IEEE bridging operations.

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This part of the RFC discusses what should happen to ipv6 multicasts (or any other ethernet multicast mac address) that might share the same mac address with the ipv4 multicast IGMP join received by the switch.

Section 1 makes this clear...

Note that the IGMP snooping function should apply only to IPv4 multicasts. Other multicast packets, such as IPv6, might be suppressed by IGMP snooping if additional care is not taken in the implementation as mentioned in the recommendations section. It is desired not to restrict the flow of non-IPv4 multicasts other than to the degree which would happen as a result of regular bridging functions. Likewise, MLD snooping switches are discouraged from using topological information learned from IPv6 traffic to alter the forwarding of IPv4 multicast packets.

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IGMP snooping is a feature that lets a switch save bandwidth by only sending multicast packets to downstream ports where some client has asked for the multicast stream; by default, a layer-2 switch would need to send Ethernet multicast frames out all downstream ports, even the ones where nobody cared about the stream.

That line in the RFC explains that if a switch uses IGMP snooping, it has to be careful to only filter out unwanted Ethernet frames that contain IPv4 multicast packets, not all Ethernet multicast frames, since some other layer-3 protocol that uses Ethernet multicast (like IPv6 multicast) might get filtered out incorrectly.

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