Like BGP neighbor adjacency states, OSPF has its own too. OSPF neighbor adjacency is not a bit straightforward. You might be expecting it should be in “FULL” state for the neighbor adjacency to be established, but you shouldn’t be assuming that it needs to stop at this state at all times.
OSPF IN DOWN STATE
Hello packets are very important parameters in establishing adjacency in any routing protocol not only in OSPF. Now, if no hello packets have been received from the neighbor and the dead timer interval has expired, OSPF is in DOWN state. The first OSPF neighbor state is “DOWN” state. It usually happens on Non-Broadcast MultiAccess (NBMA) networks and Non-Broadcast Point-to-Multipoint networks where neighbor is manually configured.