When receiving a multicast packet, a switch performs Reverse Path Forwarding (RPF) check on the packet and searches for the optimal route to the destination. The outbound interface of the route is the RPF interface and the next hop of the route is the RPF neighbor. A multicast forwarding entry can be created only after PIM is enabled on the RPF interface and the RPF neighbor. In this way, a complete multicast distribution tree can be set up along the forwarding path of the packet.
          It is recommended that you enable PIM on all non-edge interfaces in the PIM domain because all these interfaces have a chance to become an RPF interface or RPF neighbor. On the switch directly connecting to a multicast source, the RPF interface for packets sent from the multicast source is the interface connected to the multicast source. Therefore, PIM must be enabled on this interface.