1

I have 3 hosts (h1, h2, h3) on my LAN segment and a router with IGMP enabled.

Now the host h3 wants to join the multicast group (239.0.0.2 say)

  1. Host h3 will send a membership report to 239.0.0.2 (correct/incorrect)
  2. All other hosts (h1, h2) will suppress their Membership report as they are also listening to this group (correct/incorrect)
  3. How does the router receive this request which was initiated in part 1 (because the router is listening ONLY at all router address-224.0.0.2)?
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 14 '17 at 23:02
3

How IGMP specifically works depends on the IGMP version in use. Cisco has some documents related to how the different IGMP versions work in general and specifically on Cisco devices. For instance, IP Multicast Technology Overview, Intradomain Multicast Protocols, Internet Group Management Protocol (IGMP). The real answers are found in the RFCs:

Hosts will send IGMP membership reports to the multicast router, and the multicast router will periodically send out multicast membership queries to see if any hosts on the LAN are still interested in the multicast group traffic. Beyond that, it depends on the IGMP version.

I'm not sure why you think routers only listen to 224.0.0.2; routers are also just hosts on a LAN, and while they must listen to that multicast group, they also must listen to 224.0.0.1, and any other multicast groups to which you configure them to listen. When you configure multicast routing on a router, part of it is setting up IGMP on the interfaces which will be used.

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  • So can i interpret in this way?Is the following understanding correct ? Multicast routers must listen to all multicast traffic for every group because the IGMP control messages also are sent as multicast traffic – fsociety Aug 11 '16 at 17:26

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