In this phase, a multicast receiver expresses its interest in receiving traffic destined for a multicast group. Typically, the receiver does this using IGMP or MLD, but other mechanisms may also serve this purpose. One of the receiver’s local routers is elected as the DR for that subnet. When the expression of interest is received, the DR sends a PIM join message toward the RP for that multicast group. This join message is known as a (*,G) join because it joins group G for all sources to that group. The (*,G) join travels hop-by-hop toward the RP for the group, and in each router it passes through, the multicast tree state for group G is instantiated. Eventually the (*,G) join either reaches the RP or reaches a router that already has (*,G) join state for that group. When many receivers join the group, their join messages converge on the RP and form a distribution tree for group G that is rooted at the RP. This is known as the RP tree and is also known as the shared tree because it is shared by all sources sending to that group. Join messages are resent periodically, as long as the receiver remains in the group. When all receivers on a leaf-network leave the group, the DR sends a PIM (*,G) prune message toward the RP for that multicast group. However, if the prune message is not sent for any reason, the state eventually times out.
A multicast data sender starts sending data destined for a multicast group. The sender’s local router (the DR) takes those data packets, unicast-encapsulates them, and sends them directly to the RP. The RP receives these encapsulated data packets, removes the encapsulation, and forwards them onto the shared tree. The packets then follow the (*,G) multicast tree state in the routers on the RP tree, being replicated wherever the RP tree branches, and eventually reaching all the receivers for that multicast group. The process of encapsulating data packets to the RP is called registering, and the encapsulation packets are known as PIM register packets.
At the end of phase one, multicast traffic is flowing encapsulated to the RP, and then natively over the RP tree to the multicast receivers.