Multicast DR and DF

Designated Forwarder uses Assert Messages to forward the traffic to a shared segment.

• The router generating an Assert with the lowest Admin Distance is elected the forwarder.  If the Admin Distances are the same then,
• The lowest unicast routing metric is used to break a tie. If metrics are the same,
• The device with the highest IP Address will be elected as the PIM Forwarder. 

Note the Assert Looser will prune it's interface(s) for that segment; flags P on show ip mroute command and Assert Winner will have the A flag set/displayed.
Also note the NBMA networks may be more prone to multicast flow breakage if the wrong router is elected/assigned the DR role; Hub and Spoke frame-relay is an example when Spoke becomes DF, thus Hub stops sending multicast traffic to other Spokes.

R1#sh ip mroute 239.6.6.6 155.1.108.10
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
       L - Local, P - Pruned, R - RP-bit set, F - Register flag,
       T - SPT-bit set, J - Join SPT, M - MSDP created entry,
       X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
       U - URD, I - Received Source Specific Host Report,
       Z - Multicast Tunnel, z - MDT-data group sender,
       Y - Joined MDT-data group, y - Sending to MDT-data group,
       V - RD & Vector, v - Vector
Outgoing interface flags: H - Hardware switched, A - Assert winner
 Timers: Uptime/Expires
 Interface state: Interface, Next-Hop or VCD, State/Mode

(155.1.108.10, 239.6.6.6), 00:01:35/00:01:29, flags: T
  Incoming interface: Serial0/0/0.1, RPF nbr 155.1.0.5
  Outgoing interface list:
    GigabitEthernet0/0, Forward/Sparse-Dense, 00:01:35/00:00:00, A
R1#
R4#sh ip mroute 239.6.6.6 155.1.108.10
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
       L - Local, P - Pruned, R - RP-bit set, F - Register flag,
       T - SPT-bit set, J - Join SPT, M - MSDP created entry,
       X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
       U - URD, I - Received Source Specific Host Report,
       Z - Multicast Tunnel, z - MDT-data group sender,
       Y - Joined MDT-data group, y - Sending to MDT-data group,
       V - RD & Vector, v - Vector
Outgoing interface flags: H - Hardware switched, A - Assert winner
 Timers: Uptime/Expires
 Interface state: Interface, Next-Hop or VCD, State/Mode

(155.1.108.10, 239.6.6.6), 00:00:15/00:02:48, flags: PT
  Incoming interface: Serial0/0/1, RPF nbr 155.1.45.5
  Outgoing interface list:
    GigabitEthernet0/1, Prune/Sparse-Dense, 00:00:14/00:02:45
R4#

Designated Router uses PIM Hello messages, every 30 seconds, to elect a router on the shared segment to do the IGMP Query for the segment towards the upstream PIM neighbors.  

• If the router, after 105 seconds, hears no response, it declares itself as DR. 
• On hearing from other routers, higher IP address determines the DR.

Note when source of the multicast is also the DR of that segment, in that case DR can not register the source with the RP so, the RPF check may fail!