My guess is that this is because IGMP is a host-to-router protocol while PIM is for router-to-router signaling. IGMP joins typically flow from a multicast receiver to its local router. A GRE tunnel emulates a router-to-router connection, so you need PIM running on it for signaling. An IGMP join is really just a first hop host-to-router message and isn't intended to be routed, which is what would be necessary to get it over a GRE tunnel.
HTH, John On Wed, Jan 9, 2013 at 8:51 AM, Brian Christopher Raaen < [email protected]> wrote: > I am trying to set up multicast between a Linux server and Router using > GRE. The GRE tunnel is up fine and I can see traffic go across it, but the > router is not indicating it is receiving the IGMP joins that the server is > sending. I have identical setting with another server attached to > fastethernet0/1 and it is joined to the group fine, but I am not able to > get the server to link to the router via GRE interface. Note that I have > another server behind another router where the two routers do GRE and PIM > and that on is working fine. Is there some reason that IGMP joins would > not work across the GRE link, but another router using PIM would? > > -- > Brian Christopher Raaen > Network Architect > Zcorum >
