In OSPF, the election of the designated router (DR) and backup designated router (BDR) is based on the OSPF network type. The network types include:
Broadcast: This is the default network type for Ethernet interfaces. In a broadcast network, OSPF elects a DR and BDR. All other routers become DROTHERs. To prevent the election of a DR/BDR, you can configure the interface as non-broadcast (NBMA) using the ip ospf network non-broadcast command.
Point-to-Point: This network type is used for point-to-point links (e.g., serial links). There is no DR/BDR election in point-to-point networks.
Point-to-Multipoint: This network type is used for point-to-multipoint links (e.g., Frame Relay). There is no DR/BDR election in point-to-multipoint networks.
Point-to-Multipoint Non-Broadcast: Similar to point-to-multipoint, but explicitly configured as non-broadcast. There is no DR/BDR election in point-to-multipoint non-broadcast networks.
Loopback: Loopback interfaces are always treated as point-to-point.
Virtual-Link: Used for OSPF virtual links.
Sham-Link: Used for OSPF sham links.
Demand Circuit: Used for demand circuits (e.g., ISDN).
Non-Broadcast: Explicitly configured as non-broadcast. There is no DR/BDR election in non-broadcast networks.
Reference:
Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR) v1.0 Cisco Learning Network Store - SPCOR Training