Explanation/Reference:
A priority of 1 means the routing device has the least chance of becoming a designated router. Here R1 has priority 1, while R4 has priority 5. R4 will be elected the designated router and the rendezvous point (RP) for both address ranges.
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