Multicast PIM Assert Explained

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 R2 to get to the RP.

Let’s say our receivers both join group 239.1.1.1. 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!

Rene

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