I might be missing the obvious but I would like to know how the first-hop routers are aware that multicast sources are sending multicast packets and such packets are eligible for the PIM-register process. The PIM-register process itself is clear, is the step before that is not clear to me. Do routers listen to all the multicast groups (sources send to specific L2 multicast groups) or do first-hop routers have their LAN interface in promiscuous mode?

Could someone be so kind to explain or point me to the right documentation?

Thanks, Alex

1 Answer 1


A router with multicast routing enabled will have its LAN interface configured to listen for the multicast groups it intends to serve. This is time when using a mask with multicast actually means something. Multicast groups are individual addresses, so a mask is normally meaningless, but you can use a mask to specify a range of addresses for the router, e.g.

I think you are also missing that besides PIM, or whichever multicast routing protocol you use, the router will also use IGMP on the LAN (or the old Cisco CGMP). IGMP is the protocol between the multicast hosts and the multicast router. IGMP is used to tell the multicast router that a host is interested in listening to a multicast group, and the router also uses IGMP to see if hosts are still interested in the group traffic, otherwise it stops routing it.

Many people now think IGMP is used for switches, but that is not its purpose. Most modern business switches can eavesdrop on the IGMP messages between the hosts and router (IGMP snooping) to tailor which interfaces receive which multicast traffic. Originally, switches would send all multicast traffic to all switch interfaces, but that is wasteful of link bandwidth, and IGMP snooping is a refinement.

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  • Thanks Ron; I have always thought of IGMP as a way for hosts to inform routers that they wanted to receive multicast traffic. Do you mean that IGMP can be used by sources to inform first-hop routers that they have traffic to send ig refor that specific multicast group? What would be the command on Cisco routers for instance? Again I don't seem to have found anything related to how to instruct first-hop routers to listen for specific multicast groups but the 99% oif the docs focus on the receiving multicast trafic. Jan 9, 2020 at 0:43
  • Without IGMP, a source and router on an IGMP snooping switch would never communicate because the router would not try to join the group, so the switch would never send the group traffic to the switch interface where the router is connected. In any case, you must configure the router interface for the multicast groups you want it to use on that interface. You can allow just the Organization-Local scope and ignore all other groups, or you can allow any combination of groups, ignoring all others.
    – Ron Maupin
    Jan 9, 2020 at 1:15
  • So are you saying that first-hop routers in sources' LAN act as a host willing to receive certain traffic and hence send out IGMP JOIN and such JOIN messages are processed by the sources? Sorry I'm trying to understand and I hope my reasoning is not wrong. Jan 9, 2020 at 10:07
  • Right in RFC, 2236, it says, "Routers that are members of multicast groups are expected to behave as hosts as well as routers, and may even respond to their own queries." There is also the behavior of a multicast router coming up on a LAN that will send a general IGMP query the the Link-Local (all such multicasts are not allowed to be stopped by a switch) multicast group for all hosts, to which all the multicast hosts respond. Read the two current multicast RFCs for details.
    – Ron Maupin
    Jan 9, 2020 at 15:22
  • Indeed the sentence you mention seems to clarify. Pity that I haven't found any single doc that elaborates a bit more on this aspect of multicast. Even in certain Ciscopress books on the topic, that normally I trust as to depth and detail of explanatio, I could not find it. I should then search for the code of certain opensource applications to see indeed how the source treats the JOIN messages from such routers Jan 10, 2020 at 14:06

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