When an IGMP receiver joins a multicast group, PIM on the leaf router first joins the shared tree. After the first packet is received on the shared tree, the router uses the source address information in the packet to immediately switch over to the shortest path tree (SPT). If you enable PIM Infinite Threshold Policy for IPv4 and IPv6, multicast traffic follows the shared tree path through a Rendezvous Point (RP) instead of switching immediately over to the SPT.