I believe this is not off-topic, as IGMP now is a part of almost every decent Layer-2 switch. My question is is there a default behavior for the switch when IGMP is enable and there is a steady multicast traffic from a router, but there are no Join messages coming from hosts connected to the switch. Should the switch flood this mcast traffic or block until it hears some Join packets from clients? I can't fogure it out from rfc4541 and rfc1112.

Thanks in advance for any valuable comments!

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IGMP snooping at layer 2 switches is only done IN ADDITION TO the normal multicast processing at routers. So if there are no members of a given multicast group in that IP subnet, the multicast router won't forward the multicast at all. Therefore, the layer 2 switch won't need to worry about forwarding the multicast packets.

If there are members in the IP subnet, then the IGMP snooping switch will only forward the multicast to the switch ports that connect to the group members, rather than flooding to all ports.

The whole point of IGMP is to not forward multicast packets at all, unless there are hosts in the IP subnet who respond to the all host or group-specific queries transmitted by the multicast router. (All host queries are flooded by layer 2 switches, because all host queries are transmitted on a multicast group in which multicast-capable hosts are always members, without having to join explicitly.)


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Albert Manfredi

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