3.3. Phase Three: Shortest-Path Tree

While having the RP join back towards the source eliminates encapsulation, it doesn't optimize the forwarding path. Routing via the RP may involve significant detours.

SPT Switchover Process:

1. SPT Join: Receiver's DR sends (S,G) Join towards source S to build shortest-path tree (SPT)
2. State Instantiation: (S,G) state created along path to S
3. Duplicate Traffic: Receiver temporarily receives packets from both SPT and RPT
4. (S,G,rpt) Prune: DR sends Prune to RP to stop receiving S's traffic on shared tree
5. Optimized Path: Traffic flows along shortest path between receiver and source

At the end of Phase Three, receivers get traffic along the shortest-path tree between receiver and S. The RP receives traffic from S but no longer forwards it down the RP tree to this receiver.

note

For SPT switchover thresholds and configuration, see Section 3.3 of RFC 7761.