NG-MVPN support P2MP RSVP-TE and P2MP LDP LSPs as selective provider multicast service interface (S-PMSI). S-PMSI is used to avoid sending traffic to PEs that participate in multicast VPN, but do not have any receivers for a given C-multicast flow. This allows more-BW efficient distribution of multicast traffic over the provider network, especially for high bandwidth multicast flows. S-PMSI is spawned dynamically based on configured triggers as described in S-PMSI trigger thresholds section.
In MVPN, the head-end PE firstly discovers all the leaf PEs via I-PMSI A-D routes. It then signals the P2MP LSP to all the leaf PEs using RSVP-TE. In the scenario of S-PMSI:
The head-end PE sends an S-PMSI A-D route for a specific C-flow with the Leaf Information Required bit set.
The PEs who are interested in the C-flow respond with Leaf A-D routes.
The head-end PE then signals the P2MP LSP to all the leaf PEs using RSVP-TE.
Also, because the receivers may come and go, the implementation supports dynamically adding and pruning leaf nodes to and from the P2MP LSP.
When the tunnel type in the PMSI attribute is set to RSVP-TE P2MP LSP, the tunnel identifier is <Extended Tunnel ID, Reserved, Tunnel ID, P2MP ID>, as carried in the RSVP-TE P2MP LSP SESSION Object.
The PE can also learn via an A-D route that it needs to receive traffic on a particular RSVP-TE P2MP LSP before the LSP is actually setup. In this case, the PE needs to wait until the LSP is operational before it can modify its forwarding tables as directed by the A-D route.
Because of the way that LDP normally works, mLDP P2MP LSPs are setup without solicitation from the leaf PEs towards the head-end PE. The leaf PE discovers the head-end PE via I-PMSI or S-PMSI A-D routes. The tunnel identifier carried in the PMSI attribute is used as the P2MP FEC element. The tunnel identifier consists of the head-end PE’s address, along with a Generic LSP identifier value. The Generic LSP identifier value is automatically generated by the head-end PE.