Notable Replies

  1. Hi Rene,thanks for the explanation

    This behavior is the same for sparse mode as well?

  2. Hi Mungi,

    In this example, R4 is a host device where we used IGMP join to request multicast traffic on the interface.

    The PIM assert mechanism is only triggered when there are two routers on the same multi-access segment. When you connect R4 to R2 and R3 directly, you will be using two different segments so we don’t have to use PIM assert.

    Whether R4 will receive multicast traffic will depend if you configured IGMP join. If you do this on both interfaces then it will receive multicast traffic on both interfaces.


  3. Hi Jose,

    It’s very likely to see PIM assert in dense mode since we flood everything. It’s unlikely to encounter in sparse mode but it does exist. Let me give you an example. Take a look at the “pim-sparse-assert.png” image in the attachment for the topology.

    On top, we have the RP, connected to R1/R2. At the bottom we have R3/R4 with a receiver each.

    R1/R2/R3/R4 are connected to the same multi-access segment through the switch.

    • R3 uses R1 to get to the RP.
    • R4 uses R1 to get to the RP.

    Let’s say our receivers both join group This is what happens:

    R3 sends the PIM join to R1.
    R4 sends the PIM join to R2.

    R1 and R2 will both start forwarding traffic on their links to the switch and we get duplicate traffic. The assert mechanism will be used on the links that connect to the switch so that R1 or R2 will stop forwarding.

    Hope this helps!


  4. QUOTE
    Now when R1 starts streaming multicast traffic towards R2 and R3 they will both forward multicast packets to R4 resulting in duplicate traffic.
    Can you integrate the lesson on PIM DR with this lesson on the Assert mechanism?
    Consider that R3 is the PIM DR on Fa0/1 (As shown early on in the PIM DR lesson).
    So as I understand it then R3 as the DR will be sending the PIM Join upstream when the receiver joins the MC group (not R2).
    In this lesson you relate a state where both R2 and R3 are forwarding the same MC traffic to R4. This as you say causes the assert mechanism to come into play. But for both R2 ans R3 to be forwarding the MC traffic in question, would both R2 and R3 not have had to send a join upstream to R1?
    I hope this makes sense. Thanks for any reply

  5. Hi Djan,

    The DR has two roles:

    - Send PIM register packets from source to RP.
    - Forward PIM join/prune packets towards the RP.

    The forwarder does something else. It ensures that downstream multicast traffic (from source to receiver) is only forwarded on a multi-access segment by one router, not two (otherwise we get duplicate packets).


Continue the discussion forum.networklessons.com

21 more replies!