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 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 normally switches to native forwarding. To do this, when the RP receives a register-encapsulated data packet from source S on group G, it normally initiates an (S,G) source-specific join toward S. This join message travels hop-by-hop toward S, instantiating (S,G) multicast tree state in the routers along the path. (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 (S,G) multicast tree state, and then packets from S start to flow following the (S,G) tree state toward the RP. These data packets can also reach routers with (*,G) state along the path toward the RP - if so, they can short-cut onto the RP tree at this point.
While the RP is in the process of joining the source-specific tree for S, the data packets 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 it sends a register-stop message back to S’s DR to prevent the DR unnecessarily encapsulating the packets. At the end of phase two, traffic is 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.