When subscribing to a multicast group on a tunnel device such as a GRE tunnel, are you supposed to participate in IGMP/MLD over that tunnel in order to announce your membership in the multicast group? If so, is this merely a SHOULD or is it a MUST? Any RFC citations would be much appreciated!

  • Not all tunnels support multicast. For example, IPSec does not. – Ron Maupin Jan 26 at 4:27
  • Ah, that's good to know, thanks! Do you know other protocols (besides GRE) which do? – joshlf Jan 26 at 4:30
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    The usual implementation is to tunnel multicast in GRE, and then tunnel the GRE inside other tunnel protocols, such as IPSec. – Ron Maupin Jan 26 at 4:32
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    Of course, the tunnel in a tunnel really cuts down on the MTU. – Ron Maupin Jan 26 at 4:33
  • Gotcha, that makes sense. Are there any other tunneling protocols besides GRE that support multicast? – joshlf Jan 26 at 4:39

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