The value of vRtrPimNgIfP2mpLdpTreeJoin specifies whether to join a
P2MP LDP tree towards a multicast source.
When the value is 'true', a multicast join received on an interface is
processed to join a PIM multicast tree and also copied over to LDP to
join the in-band signaled P2MP tree for the same multicast flow. LDP
P2MP tree is setup either to RP (ASM group) or source (SSM group). The
route to the RP or the source of the multicast node is looked up from
the RTM. The next-hop address for the route to RP or source is set as
the root of LDP P2MP tree. When it is 'false', the P2MP LDP tree is
disjoined from the multicast source.