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Forum Replies

  1. Thanks , great learning again.

    If you have to tell just theoretically- what exactly is the role of multicast-router in first example. what would that be ? because in our earlier examples video server was behind router but in our first example its behind the switch. my concept will get clear with this.

  2. 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

  3. 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

  4. 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.

  5. 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.


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