Multicast PIM Assert Explained

I am having some issues understanding this statement -

“Now when R1 starts streaming multicast traffic towards R2 and R3 they will both forward multicast packets to R4 resulting in duplicate traffic.”

  1. Why will R1 start streaming simultaneously towards both R2 and R3?
  2. Why will R2 and R3 both forward multicast packets towards R4?

Can you explain this in a little more detail?

Hello Dhananjay

Multicast routing has two modes. Dense and Sparse mode. In dense mode, even if it is not requested, multicast traffic will be sent out all interfaces except for the incoming interface. In sparse mode, multicast traffic will only be transmitted to routers that request it. In both cases, if all four routers are configured with multicast and all four request multicast traffic, then R1 will send multicast traffic to both R2 and R3 and these will forward it to R4. You can find out more about this specific functionality at this lesson.

This is the behaviour of mutlicast if all routers request multicast traffic.

I hope this has been helpful!


@lagapides thanks for that explanation

Hello Rene,

Thanks for the great explanation

PIM DR is used in PIM-Sparsh mode while PIM ASSERT is used in PIM-DENSE mode.

Considering PIM DR is used to find router in the same broadcast domain for join message towards RP and same DR is used to forward traffic as per RPF check.

While with PIM ASSERT only for DENSE Mode where only need to avoid duplicate packet transmissions.

Please confirm if my understanding is right?

Hello Prashant,

You almost got it. The purpose of the DR is to forward PIM register packets from a source to the RP and to forward join/prune packets from receivers to the RP. We only use a DR in PIM sparse mode.

The purpose of PIM assert is to ensure that when you have two or more routers on a multi-access network, that only one router forwards the multicast packets. It is typically used in PIM dense mode but it’s not exclusive to dense mode…it’s also needed sometimes in sparse mode. Here’s an example for that:

Hi Rene,
Sometimes PIM DR and PIM forwrder do the same role? I meant when you told don’t confuse this 2 features I’am confuse.

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).


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