Which operation is performed by an interested PE upon receiving an MDT Join TLV?

In Next-Generation Multicast VPN (NG-MVPN), an MDT Join TLV is carried within a BGP C-multicast route advertisement. It is sent by a Provider Edge (PE) router that has a downstream receiver for a specific customer source and group (C-S, C-G). This message signals to the source-PE that it should forward this multicast stream. To receive this traffic, the interested PE (the one that sent the MDT Join TLV) must join the specific multicast distribution tree in the provider core, known as a Selective-PMSI (S-PMSI) or Data-MDT. This is a source-specific tree. Therefore, the PE sends a PIM (S, G) Join into the provider core. The PIM Join is a standard IP control plane packet and is not encapsulated in GRE; GRE is a data plane encapsulation used for the actual multicast traffic.

A. PIM Join messages are control plane packets and are not encapsulated in GRE. GRE is a data plane encapsulation for the multicast traffic itself.
C. A (, G) PIM Join is used to build the shared tree for the Default-MDT, not the source-specific tree (Data-MDT) triggered by an MDT Join TLV.
D. A (, G) PIM Join is incorrect because the MDT Join TLV is for a specific source, necessitating a source-specific (S, G) join in the provider core.

1. RFC 6513: Multicast in MPLS/BGP IP VPNs, Section 9.1, "S-PMSI Instantiated by PIM". This section states: "A PE that receives and accepts the S-PMSI advertisement, and that needs to receive the C-traffic carried over the specified P-tunnel, acts as a PIM router that needs to join the P-multicast tree. It sends a PIM Join message upstream." This confirms a PIM Join is sent. The context of S-PMSI implies a source-specific tree, hence an (S,G) join.

2. RFC 6514: BGP Encodings and Procedures for Multicast in MPLS/BGP IP VPNs, Section 5.1, "Source Tree Join C-multicast route". This section details the route that carries the MDT Join TLV. The purpose is to signal interest in a specific (S,G) channel, which leads to the creation of a source-specific P-tunnel that the interested PE must join.

3. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR Triple Play Service Delivery Architecture Guide, Release 20.10.R1, Section: "NG-MVPN Data MDT". The documentation explains: "When an egress PE receives the C-multicast route with the data-MDT address, it will initiate a PIM (S,G) join toward the ingress PE, where S is the ingress PE and G is the data-MDT address." This explicitly confirms an (S,G) PIM join is sent by the interested (egress) PE.