Explanation/Reference:
You can filter Protocol Independent Multicast (PIM) register messages sent from the designated router (DR) or to the rendezvous point (RP).
Note: In a shared tree, the root of the distribution tree is a router, not a host, and is located somewhere in the core of the network. In the primary sparse mode multicast routing protocol, Protocol Independent Multicast sparse mode (PIM SM), the core router at the root of the shared tree is the rendezvous point (RP).
References: http://www.juniper.net/documentation/en_US/junos15.1/topics/example/ospf-designated- router-election-configuring.html