SPT Cutover

Instead of continuing to use the SPT to the RP and the RPT toward the receiver, a direct SPT is created between the source and the receiver in the following way:

Once the receiver's DR receives the first multicast packet from the source, the DR sends a PIM join message to its RPF neighbor (see Figure 18).
The source's DR receives the PIM join message, and an additional (S,G) state is created to form the SPT.
Multicast packets from that particular source begin coming from the source's DR and flowing down the new SPT to the receiver's DR. The receiver's DR is now receiving two copies of each multicast packet sent by the source—one from the RPT and one from the new SPT.
Figure 18: Receiver DR Sends a PIM Join Message to the Source
To stop duplicate multicast packets, the receiver's DR sends a PIM prune message toward the RP router, letting it know that the multicast packets from this particular source coming in from the RPT are no longer needed (see Figure 19).
Figure 19: PIM Prune Message Is Sent from the Receiver's DR Toward the RP Router
The PIM prune message is received by the RP router, and it stops sending multicast packets down to the receiver's DR. The receiver's DR is getting multicast packets only for this particular source over the new SPT. However, multicast packets from the source are still arriving from the source's DR toward the RP router (see Figure 20).
Figure 20: RP Router Receives PIM Prune Message
To stop the unneeded multicast packets from this particular source, the RP router sends a PIM prune message to the source's DR (see Figure 21).
Figure 21: RP Router Sends a PIM Prune Message to the Source DR
The receiver's DR now receives multicast packets only for the particular source from the SPT (see Figure 22).
Figure 22: Source's DR Stops Sending Duplicate Multicast Packets Toward the RP Router