PIM SM designated router

[simone.colpi]

I read this about DR in sparse mode:

"Since the DR is used to forward PIM join messages to the RP"

So I have a question.

Because DR and Forwarder can be different in a lan segment (The forwarder have the shortest path toward the rp while the DR have the highest ip), how can be this if it always is the DR the leaf of the tree sourced from the RP the forwarder should be pruned from the tree?

[AMediaFilm]

For example.

R1 is RP

R2 -- DR

R3 -- source's router

R4 -- receiver's router

R1, R2 and R3 in LAN1

R2, R3 and R4 in LAN2.

JOIN (*,G) will keep traveling R4->R2->R1 path to RP.

JOIN(S,G) will travel R4->R3 path right to the source network so R3 will become a forwarder.

RP tree and SP tree are two different trees. RPT root is RP and SPT root is source's router. As soon as R3 receives first multicast packet it joins as leaf to SPT for the source of received packet and prunes from RPT for this source when SPT to source built and traffic flows over new path.

In case ssm join received by R4 it might omit RPT phase and start directly from SPT.