3.1. Phase One: RP Tree

In Phase One, multicast receivers express interest in receiving traffic destined for a multicast group. The local router on the receiver's subnet becomes the Designated Router (DR). Upon receiving the receiver's interest, the DR sends a PIM Join message towards the RP for that group. This Join is called a (*,G) Join because it joins the tree for all sources sending to group G.

Key Concepts:

• (*,G) Join: Propagates hop-by-hop towards the RP, instantiating tree state at each router
• RP Tree (RPT): Distribution tree rooted at the RP, also called shared tree because it's shared by all sources
• Register: Senders' DR encapsulates data packets and unicasts them to the RP
• Register Packets: Encapsulated packets sent from source DR to RP

At the end of Phase One, multicast traffic flows encapsulated to the RP, then natively down the RP tree to receivers.

note

For complete protocol details and state machine specifications, see Section 3.1 of RFC 7761.