The following steps describe how a source sends multicast packets to a group:
A source directly attached to a VLAN bridges the multicast data to the DR. The DR for the VLAN (the router with the highest IP address) encapsulates each packet in a register message and sends a unicast message directly to the RP router to distribute to the multicast group.
If a downstream group member chooses to receive multicast traffic, the RP router sends a join or prune message toward the source DR and forwards the data down the RP tree after it obtains the data natively.
After the receiver DR obtains the first packet, it switches to the shortest-path tree (SPT) and continues receiving data through the SPT path.
If no downstream members want to receive multicast traffic, the RP router sends a register-stop message (for the source) to the DR.
The DR starts the register suppression timer after it receives the first register-stop message. During the register suppression timeout period (the default is 60 seconds), the following events occur:
The DR for the source sends a probe packet to the RP router before the register suppression timer expires. The probe packet prompts the RP router to determine whether new downstream receivers joined the group.
If no new receivers joined the group, the RP router sends another register-stop message to the DR for the source, and its register suppression timer restarts.
After the RP router no longer responds with a register-stop message to the source DR probe message, the register suppression timer expires and the DR sends encapsulated multicast packets to the RP router. The RP router uses this method to tell the DR that new members joined the group.
The RP sends a register-stop message to the DR immediately after it receives the first multicast data packet.