OSPF (Open Shortest Path First) uses the concept of Designated Router (DR) and Backup Designated Router (BDR) to avoid excessive adjacencies.
In a multi-access network like Ethernet, OSPF would require a full adjacency with each router in the network, which can be inefficient and time-consuming. To avoid this, OSPF elects a DR and a BDR for the network. All routers on the network form a full adjacency with the DR and BDR only, and not with each other. This reduces the number of adjacencies and helps to optimize the network.
So, OSPF uses the DR and BDR concept to avoid excessive adjacencies.