How OSPF DBD packets are unicast, how does it know neighbor interface IP?

Question

In OSPF DBD packets are send as unicast packets even in braodcast network types right ?

Question here is, how does the OSPF router know the neighbor's interface address in the LAN ?

Only information it has exchanged already is HELLO packets, which has neighbor's Router IDs and the interface address of DR and BDR alone. Also the subnet mask of the LAN.

Answer 1

1. The Hello packets are sourced from the router's unicast address on that link, providing a unicast destination for the receiver.
2. The receiving router knows to begin formation of a new adjacency by seeing itself in the Hello packet's header. It sees its own router ID in the "Neighbor" field of the Hello packet, and begins the exchange if it is not already aware of the neighbor.

From https://www.ietf.org/rfc/rfc2328.txt: (search for "Hello Protocol" and "Receiving Hello Packets")

The Hello Protocol is responsible for establishing and maintaining neighbor relationships. It also ensures that communication between neighbors is bidirectional. Hello packets are sent periodically out all router interfaces. Bidirectional communication is indicated when the router sees itself listed in the neighbor's Hello Packet.

and

If the receiving interface connects to a broadcast, Point-to-MultiPoint or NBMA network the source is identified by the IP source address found in the Hello's IP header.

and

If a matching neighbor structure cannot be found, (i.e., this is the first time the neighbor has been detected), one is created. The initial state of a newly created neighbor is set to Down.

Answer 2

The router receiving hello receives the packet from a unicast ip address. This allows the the router to arp for the neighboring router's interface address and use that for DBD packets.