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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 20).
- 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 20: 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 21).
Figure 21: 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 22).
Figure 22: 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 23).
Figure 23: 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 24).
Figure 24: Source's DR Stops Sending Duplicate
Multicast Packets Toward the RP Router
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