IP multicast sources do not use a signaling mechanism to announce their presence. Sources just send their data into the attached
network, as opposed to receivers that use Internet Group Management Protocol (IGMP) to announce their presence. If a source
sends traffic to a multicast group configured in PIM sparse mode (PIM-SM), the Designated Router (DR) leading toward the source
must inform the rendezvous point (RP) about the presence of this source. If the RP has downstream receivers that want to receive
the multicast traffic (natively) from this source and has not joined the shortest path leading toward the source, then the
DR must send the traffic from the source to the RP. The PIM registering process, which is individually run for each (S, G)
entry, accomplishes these tasks between the DR and RP.
The registering process begins when a DR creates a new (S, G) state. The DR encapsulates all the data packets that match
the (S, G) state into PIM register messages and unicasts those register messages to the RP.
If an RP has downstream receivers that want to receive register messages from a new source, the RP can either continue to
receive the register messages through the DR or join the shortest path leading toward the source. By default, the RP will
join the shortest path, because delivery of native multicast traffic provides the highest throughput. Upon receipt of the
first packet that arrives natively through the shortest path, the RP will send a register-stop message back to the DR. When
the DR receives this register-stop message, it will stop sending register messages to the RP.
If an RP has no downstream receivers that want to receive register messages from a new source, the RP will not join the shortest
path. Instead, the RP will immediately send a register-stop message back to the DR. When the DR receives this register-stop
message, it will stop sending register messages to the RP.
Once a routing entry is established for a source, a periodic reregistering takes place between the DR and RP. One minute
before the multicast routing table state times out, the DR will send one dataless register message to the RP each second that
the source is active until the DR receives a register-stop message from the RP. This action restarts the timeout time of the
multicast routing table entry, typically resulting in one reregistering exchange every 2 minutes. Reregistering is necessary
to maintain state, to recover from lost state, and to keep track of sources on the RP. It will take place independently of
the RP joining the shortest path.