Adjacencies are established with some subset of the router's neighbors. Routers connected by point-to-point networks and virtual links always become adjacent. On multi-access networks, all routers become adjacent to both the Designated Router and the Backup Designated Router.
The adjacency-forming decision occurs in two places in the neighbor state machine. First, when bidirectional communication is initially established with the neighbor, and secondly, when the identity of the attached network's (Backup) Designated Router changes. If the decision is made to not attempt an adjacency, the state of the neighbor communication stops at 2- Way.
An adjacency should be established with a bidirectional neighbor when at least one of the following conditions holds: