Dell SmartFabric OS10 User Guide Release 10.5.3

PIM-SM source registration

An RPT is a unidirectional tree, permitting traffic to flow down from the RP to the receiver in one direction. For multicast traffic to reach the receiver from the source, another branch of the distribution tree that is called the shortest-path tree, must be built from the source DR to the RP.

The shortest-path tree is created in the following way:

1. The source becomes active, sending out multicast packets on the LAN to which it is attached. The source DR receives the packets and encapsulates them in a PIM register message, which it sends to the RP router.
2. When the RP router receives the PIM register message from the source, it sends a PIM join message back to the source.
3. The source DR receives the PIM join message and begins sending traffic down the SPT toward the RP router.
4. Once the RP router receives traffic, it sends a register stop message to the source DR to stop the register process.
5. The RP router sends the multicast traffic down the RPT toward the receiver.

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:

1. Once the receiver DR receives the first multicast packet from the source, the DR sends a PIM join message to its RPF neighbor.
2. The source DR receives the PIM join message, and an additional (S, G) state is created to form the SPT.
3. Multicast packets from that particular source begin coming from the source DR and flowing down the new SPT to the receiver DR. The receiver DR is now receiving two copies of each multicast packet that is sent by the source - one from the RPT and one from the new SPT.
4. To stop duplicate multicast packets, the receiver 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.
5. The RP router receives the PIM prune message, and it stops sending multicast packets down to the receiver DR. The receiver 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 DR toward the RP router.
6. To stop the unneeded multicast packets from this particular source, the RP router sends a PIM prune message to the source DR.
7. The receiver DR now receives multicast packets only for the particular source from the SPT.