OSPF routers exchange Hello packets to establish neighbor relationships.

Comprehensive and Detailed In-Depth Explanation:

OSPF uses Hello packets to establish and maintain neighbor relationships. The method of sending Hello packets depends on the network type :

Broadcast networks (e.g., Ethernet) use multicast (224.0.0.5) to discover and maintain OSPF neighbors.

Point-to-Point (P2P) networks (e.g., PPP links) also use multicast (224.0.0.5) for Hello packet exchange.

Non-Broadcast Multi-Access (NBMA) networks (e.g., Frame Relay) require manual neighbor configuration and use unicast Hello packets instead of multicast.

Point-to-Multipoint (P2MP) networks can send Hello packets either unicast or multicast , but it depends on configuration.

???? Reference: Huawei HCIA-Datacom Study Guide, OSPF Neighbor Discovery and Hello Packet Mechanism.