PIM Dense mode duplicate traffic

[cosmingrosu]

Hello,

I have some uncertainties related to PIM Dense Mode operation:

Imagine the following topology:

The purple arrows indicate the paths of the multicast traffic stream, from the source (SRS).

The assumption is that all the routers are configured with PIM Dense Mode. R1 and R2 both receive the traffic, and they both forward the stream on the link between them. I understand that an Assert process will take place and one winner will be elected, R1 for the sake of this example.

According to the RFC, when R2 becomes an assert loser, it must send a PIM Prune to the assert winner R1. My first question is, what is the point of this PIM Prune? As an assert loser, R2 prunes its interface and won't forward traffic on the link between R1 and itself. What is the logic behind sending a Prune message towards the Assert winner R1?

Now, moving forward, my second question: what will R1 do? Will it keep sending multicast traffic although there is no other receiver on that link? Will it stop sending traffic on the link and prune its interface? 

The relevant information I could decipher form the RFC is:
- the RFC makes no difference between a ptp or multiaccess link. If there is multicast traffic received on a OIL interface, the assert will be triggered.
- as I said before, the loser of the assert sends a PIM Prune towards the assert winner.
- a router keeps its interface in the OIL if there are PIM neighbors or receivers. R1 does have R2 as neighbor on that link, event after the assert takes place so for all I could find, R1 should keep sending traffic on the link for nothing. 

[Joseph W. Doherty · Accepted solution]

To your first question, R2 is informing R1 it doesn't need the multicast stream from R1.  In concept, this is similar to what R3 will inform R1, when it doesn't need the multicast stream, too.

To your second question, if R1 doesn't "see" hosts or routers wanting multicast stream, it should (eventually) prune.  (PIM-DM is a [cyclic] flood-and[then]-prune distribution process.)

[cosmingrosu]

Thank you, your answer pointed me towards the solution that I was searching

I verified the behaviour and it seems that there will be an Assert process and a winner will be chosen. The loser of the Assert process will send a PIM Prune towards the winner (link between R1 and R2). Now, here are three situations:
there are only the two routers on the segment: In this case the winner receives the PIM Prune message and it stops sending the traffic.
there is a third router (downstream) on the segment: In which case, both the winner and the third router receive the PIM Prune message, and the third router responds to it by sending a PIM Join to override the PIM Prune sent by the loser.
there is an IGMP receiver on the segment: in which case the winner won't stop forwarding traffic, I assume that because of the IGMP active client.

[paul driver]

Hello
Here there is no requirement for a assert message between R1-R2 as they do not share the same network towards a receiver thus do not forward traffic to each other.
Initially mc will be sent out to all interfaces (dense mode) and then pruned away, in this case R2 will become the forwarder towards the receiver , R1 will prune itself from the shared tree if it has no need to forward any mc .