The designated router is a router on broadcast multiaccess media that is responsible for maintaining the topology table for the segment.
If routers are connected to a broadcast segment, one router on the segment is assigned the duty of maintaining adjacencies with all the routers on the segment. This router is known as the designated router and is elected by the use of the Hello protocol. The hello packet carries the information that determines the DR and the BDR. The election is determined by either the highest IP address or this command:
ip ospf priority number
All other routers need only peer with the designated router, which informs them of any changes on the segment.
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