Before the OSPF routers can form a neighbor relationship, which state must they complete?

For OSPF (Open Shortest Path First) routers to form a neighbor relationship, they must first complete the Exchange state. In the OSPF neighbor formation process, routers go through several states as they establish and maintain communication with one another.

During the Exchange state, routers share their link-state information with each other. Each router sends Link State Update (LSU) packets containing their Link State Advertisements (LSAs) to neighbors. This exchange is crucial, as it enables routers to synchronize their link-state databases, ensuring that they have a consistent view of the network topology. Only after successfully completing the Exchange state can the routers move on to the next phase, further solidifying their neighbor relationship and eventually reaching the Full state.

The other states that OSPF routers go through include Init, where a router detects its neighbors, and Loading, where it downloads additional database information from neighbors. However, establishing the Exchange state is key to initializing the neighbor relationship.