PIM Sparse-Mode Source Registration

The 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 called the SPT needs to be built from the source's DR to the RP.

The SPT 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's DR receives the packets and encapsulates them in a PIM register message, which it sends out to the RP router (see Figure 17).
2. When the RP router receives the PIM register message from the source, it sends a PIM join message back to the source.

   Figure 17: PIM Register Message and PIM Join Message Exchanged

   

3. The source's DR receives the PIM join message and begins sending traffic down the SPT toward the RP router (see Figure 18).
4. Once traffic is received by the RP router, it sends a register stop message to the source's DR to stop the register process.

   Figure 18: Traffic Sent from the Source to the RP Router

   

5. The RP router sends the multicast traffic down the RPT toward the receiver (see Figure 19).

   Figure 19: Traffic Sent from the RP Router Toward the Receiver

   

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