IGMP first group report.

Unanswered Question
Nov 6th, 2009

Hi all,

Suppose that no multicast traffic has flowed in a LAN segment. The multicast routing protocol operates in sparse-mode. If a host wants to join a group, it sends a group report to the group address, but how does the local router receives this report if the router is not listing to this address group?


Paulo Roque

I have this problem too.
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Jon Marshall Fri, 11/06/2009 - 10:33


If the router is running PIM on an interface that also enables IGMP. So the router will receive the IGMP join message on the interface and then send a join request to the RP (Rendevous Point) asking to be sent the multicast traffic.


pauloroque Fri, 11/06/2009 - 12:44

Thank you, Jon.

But if the router has not joined the group and does not have the group address assigned to any of its interface, how come does it receive the packet?


Paulo Roque

Jon Marshall Fri, 11/06/2009 - 13:26


The router doesn't need to have joined the group. Once the router receives the IGMP request from the host it then looks up the RP assigned to that group and sends a join request to the RP.

The RP will then begin forwarding traffic to the router and then the router can forward the traffic to the host.


pauloroque Fri, 11/06/2009 - 14:22


I think I can rephrase my question to "how come does the router receive a packet not directly addressed to it?"

For instance, the packet is addressed to the multicast group not to the router. In any other situation this packet would have been forwarded to the destination, next-hop or down the tree.

Any way, I checked the IGMP RFC 1112 and found that "the multicast routers receive all IP multicast datagrams, and therefore need not be addressed explicitly"

And that "all IGMP messages are sent contain the IP Router Alert option (a mechanism whereby routers can intercept packets not addressed to them directly"

Thank you.

Paulo Roque


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