In this phase, register-encapsulation of data packets is performed. However, register-encapsulation of data packets is unsuitable for the following reasons:
Encapsulation and de-encapsulation can be resource intensive operations for a router to perform depending on whether or not the router has appropriate hardware for the tasks.
Traveling to the RP and then back down the shared tree can cause the packets to travel a relatively long distance to reach receivers that are close to the sender. For some applications, increased latency is unwanted.
Although register-encapsulation can continue indefinitely, for these reasons, the RP will switch to native forwarding. To do this, when the RP receives a register-encapsulated data packet from source S on group G, it will initiate an (S,G) source-specific join toward S. This join message travels hop-by-hop toward S, instantiating the (S,G) multicast tree state in the routers along the path. The (S,G) multicast tree state is used only to forward packets for group G if those packets come from source S. Eventually the join message reaches S's subnet or a router that already has the (S,G) multicast tree state, and packets from S start to flow following the (S,G) tree state toward the RP. These data packets can also reach routers with the (*,G) state along the path toward the RP, and if this occurs, they can take a shortcut to the RP tree at this point.
While the RP is in the process of joining the source-specific tree for S, the data packets will continue being encapsulated to the RP. When packets from S also start to arrive natively at the RP, the RP receives two copies of each of these packets. At this point, the RP starts to discard the encapsulated copy of these packets and sends a register-stop message back to S's DR to prevent the DR unnecessarily encapsulating the packets. At the end of phase 2, traffic will be flowing natively from S along a source-specific tree to the RP and from there along the shared tree to the receivers. Where the two trees intersect, traffic can transfer from the shared RP tree to the shorter source tree.
A sender can start sending before or after a receiver joins the group, and therefore, phase two may occur before the shared tree to the receiver is built.