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