When the root node listens to the MP-BGP SPMSI AD routes and determines that a set of (S,G)s are interested in an SPMSI, it sends an update message to the PCE with the <tree ID, root ID> of the SPMSI AD route and the leaf that is interested in joining this SPMSI. The PCE uses this information to build a P2MP policy for that specific tree ID and downloads it to be used for an SPMSI.