Overview of the Problem

How to detect the RPF upstream among multiple addresses
if neighboring address != RPF upstream address, the downstream router cannot build join/prune or graft

Not typical for IPv4, but quite possible to happen for IPv6
PIM neighboring address should be linklocal
RPF upstream address can be global
manual configuration, MBGP
RP address may share a global prefix with downstream routers
Need a solution to deploy IPv6 multicast using PIM