Which three OSPF states are involved when two routers are establishing an adjacency?

When two routers are establishing an adjacency in OSPF (Open Shortest Path First), they undergo a series of states to ensure proper communication and synchronization of routing information. The three relevant OSPF states in the adjacency formation process are Init, Two-way, and Down.

In the Down state, the router has not yet established any communication with its neighbor. This is the initial state for any OSPF interface that is not yet operationally connected to a neighbor.

Once the routers begin the process of neighbor discovery, they move to the Init state. In this state, a router has received a Hello packet from a neighboring router, which is a preliminary step in establishing an adjacency. It indicates that the router is aware of the presence of the neighbor, although the adjacency has not yet been fully formed.

Finally, the Two-way state signifies that both routers have exchanged Hello packets and agree to maintain an adjacency. In multi-access networks, reaching the Two-way state is crucial as it ensures that the routers recognize each other as potential neighbors and can communicate effectively.

Thus, the states Init, Two-way, and Down are integral in the process of adjacency establishment in OSPF, delineating the evolution from no communication to the recognition of neighbors.