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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 1).
- 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 1: 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 2).
Figure 2: 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 3).
Figure 3: 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 4).
Figure 4: 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 5).
Figure 5: Source’s DR Stops Sending Duplicate Multicast Packets Toward the RP Router
