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So my question is, will using IGMP alone(without IGMP snooping, PIM...etc) making router forwarding multicast traffic?

I'm working on a buggy switch, but I don't know about networking too, so I'm not sure the problems is my understanding or this router.

I'm now testing IGMP, the setup is pretty simple:
A <-> Router <-> B

A and B are PC or NB whatever, they both run iperf, A as sender, B as receiver, they join/send certain multicast group. (224.0.55.55)

Router running a default single vlan 1, with all interfaces on it.

After enable IGMP on router and its vlan 1, I can observe from wireshark that router query and B report membership, using show commands on router also confirm the membership.

However then when A pump multicast traffic, I can't see router forwarding traffic to B.

From what I survey from google, most IGMP usage are bind to other protocols, there's hardly any test case about IGMP alone, or the query/report process simply what all IGMP do? itself just maintain membership?

BTW the IGMP snooping works fine, but need running VLC to actively report its membership, after that I can see router forwarding multicast traffic.

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  • 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 can provide and accept your own answer.
    – Ron Maupin ♦
    Dec 25, 2018 at 8:55

2 Answers 2

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Routers do not forward multicast unless you have explicitly enabled multicast routing. That requires something like PIM. IGMP is a link-local protocol.

Multicast is a form of broadcast, and you cannot normally send broadcasts across routers. In order to be able to route multicast, you need some controls to prevent it from being sent where it is not wanted. IGMP tells a multicast router (one with multicast routing enabled) to send traffic for a specific multicast group to the LAN where the IGMP multicast request was made. IGMP is between a host and a multicast router.

PIM is a multicast routing protocol. There is dense mode (DM) and sparse mode (SM) PIM. Even with PIM-DM that starts by flooding multicast traffic everywhere, PIM-DM will stop sending multicast traffic to interfaces where it has not been requested. PIM-SM doesn't even request the multicast traffic until a host requests it from the multicast router.

IGMP snooping is simply a further refinement to not send multicast traffic where it is not wanted. It is not a protocol, but a feature to let a switch listen in on the IGMP traffic between a host and a multicast router in order to determine specific switch interfaces that want the traffic for a multicast group, and not send the multicast traffic to switch interfaces that have not explicitly requested the traffic. In the absence of IGMP snooping, multicast traffic, like broadcast traffic, is sent to every other switch interface, whether the host on an interface wants it or not, and that is very wasteful of network resources.

By the way, the multicast group you are using, 224.0.55.55, is registered for a specific purpose (the range 224.0.54.0 to 224.0.57.255 is registered to Arne Hvidsten for Get - BCN), and you should not be using it for a different purpose; you run the risk of messing up legitimate uses. We have the Organization-Local IPv4 multicast scope, 239.0.0.0/8 for locally defined multicast.

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  • In my setup, there's only one vlan, so there shouldn't be routing issue, I suppose? And since IGMP work between router and host, I think it should make router forward multicast after learning membership?
    – moeCake
    Jun 21, 2018 at 2:31
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    Based on your little diagram, A <-> Router <-> B, you have two completely separate networks. Routers route between networks, not from a network back to the same network. The router would need to be configured to route multicast. You could improve your question by adding the router model and configuration, then I could edit my answer to explain how to do it.
    – Ron Maupin ♦
    Jun 21, 2018 at 2:39
  • Since A,B are simple hosts do send/receive, even if they are on different vlan on router, is there still routing involve in this case when they are directly connect?
    – moeCake
    Jun 21, 2018 at 2:58
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    Traffic that is sourced and destined to the same network does not pass through a router. For unicast, a host will mask the source and destination addresses with the host mask to determine if the destination is on the same network. If it is, then the traffic is sent directly to the destination layer-2 address, otherwise it is sent the the configured gateway address. Multicast is assumed to be on the same network, and only a multicast configured router can send it to a different network.
    – Ron Maupin ♦
    Jun 21, 2018 at 3:02
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As I understand it and from my experience setting up a multi-site Cisco VoIP system which used multicast for music on hold as well as informacast paging, PIM is required for multicast to traverse routers / extend groups from one ethernet segment to another.

To the second part of your question, I believe IGMP is just the protocol that is running on the individual hosts that's responsible for managing group membership, whether that involves telling devices upstream (e.g. a switch in pim sparse mode) that it wants the stream, or making the host IP stack know that those packets should or should not be discarded if it's an unmanaged switch / a switch in dense mode.

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