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Forum Replies

  1. I have one question related to a scenario similar to the topology shown early in this article with the multicast router.

    Assume you still have the source, various L2 switches as shown, and the router R1, however the receivers for the multicast receivers are actually on the far side of R1. Assume there are one or more routers connected to R1 that are not shown. The receivers we are concerned about are connected to those routers. A loopback on R1 is configured as the RP for the multicast group(s) in question.

    How does SW1 know to forward the multicast traffic out

    ... Continue reading in our forum

  2. Hello Vanya,

    Your traffic should only be flooded if your switches don’t have a “router port” for multicast in your topology. This might depend on your switch model / IOS version though. Most switches will drop membership reports and flood your multicast traffic.

    Adding snooping querier to your interfaces adds a “router port” to the topology so it solves the problem of flooding, not causing it.

    Do you have multiple VLANs? Is it enabled on each VLAN?

    interface vlan 1
     ip igmp snooping querier

    There is no need to enable PIM on any interfaces in your case, yo

    ... Continue reading in our forum

  3. For IGMP snooping without a router, it states that “H2 will not receive anything. SW2 will receive the IGMP report from H2 but doesn’t know a router port, so it doesn’t forward the IGMP report towards SW1.” But in the paragraph above it states “switches are flooding all multicast traffic now, treating it like broadcast traffic.” If it’s flooding all ports, then it would seem to me that SW2 would receive the traffic and then in turn H2 would. Please explain why it doesn’t quite work that way. Thank you.

  4. Hello Mike,

    It depends on the switch / IOS that you use. In my case, I used 3560s and those drop the IGMP reports and just flood multicast traffic so in that case, H2 does receive the traffic.

    Other switches might accept the IGMP report and not broadcast multicast traffic when they don’t have a router port. In that case, H2 will never receive anything since SW2 doesn’t know where the router port is…it can’t forward the IGMP report to SW1.


  5. Hello Laz,
    thanks for your reply and your answers. I already studied your mentioned lesson before my post. Main difference between the lessons architecture and mine was, that I had two separate Subnet, whereas in the lesson all equipments were located in the same subnet.

    Best regards, Florian

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