In this phase, the RP joins back toward the source using the shortest path tree. Although having the RP join back toward the source removes the encapsulation overhead, it does not completely optimize the forwarding paths. For many receivers the route via the RP can involve a significant detour when compared with the shortest path from the source to the receiver.
To obtain lower latencies, a router on the receiver’s LAN, typically the DR, may optionally initiate a transfer from the shared tree to a source-specific shortest-path tree (SPT). To do this, it issues an (S,G) Join toward S. This instantiates state in the routers along the path to S. Eventually this join either reaches S’s subnet or reaches a router that already has (S,G) state. When this happens, data packets from S start to flow following the (S,G) state until they reach the receiver.
At this point the receiver (or a router upstream of the receiver) receives two copies of the data - one from the SPT and one from the RPT. When the first traffic starts to arrive from the SPT, the DR or upstream router starts to drop the packets for G from S that arrive via the RP tree. In addition, it sends an (S,G) prune message toward the RP. The prune message travels hop-by-hop instantiating state along the path toward the RP indicating that traffic from S for G should not be forwarded in this direction. The prune message is propagated until it reaches the RP or a router that still needs the traffic from S for other receivers.
By now, the receiver receives traffic from S along the shortest-path tree between the receiver and S. In addition, the RP is receiving the traffic from S, but this traffic is no longer reaching the receiver along the RP tree. As far as the receiver is concerned, this is the final distribution tree.