A source's Designated router (DR) is responsible for sending Register messages to a Rendezvous Point (RP), and a receiver's DR is responsible for sending Join messages to an RP. The routers elect a DR by exchanging Hello messages. The router with the highest priority wins the election. If the router have the same priority, the router with the largest IP address wins the election.
In PIM-SM, a DR needs to be elected on a shared network segment to process multicast source registration and multicast group join requests. DR election is based on DR priorities and IP addresses. The routers exchange Hello messages to elect a DR.
If all routers support Hello messages carrying DR priorities, the interface with the highest DR priority is selected as the DR. If the routers have the same priority, the interface with the largest IP address is elected as the DR.
If one or more routers do not support Hello message carrying the DR priority, the interface with the largest IP address is elected as the DR.
You can set a DR priority either globally or on an interface.
Global configuration: takes effect on all interfaces.
Interface-specific configuration: takes precedence over the global configuration. If an interface-specific configuration is not available, the interface uses the global configuration.