OSPF uses LSAs to become adjacent with each other.
OSPF routers become adjacent when each router has the same link-state database. Following is a brief summary of the states an interface passes through before the router becomes adjacent to another router on that interface:
• Down: No information has been received from anyone on the segment.
• Attempt: On non-broadcast multiaccess clouds such as Frame Relay, this state indicates that no recent information has been received from the neighbor. An effort should be made to contact the neighbor by sending Hello packets at the reduced rate poll interval.
• Init: The interface has detected a Hello packet coming from a neighbor, but bi-directional communication has not yet been established.
• Two-way: Bi-directional communication exists with a neighbor. The router has seen itself in the Hello packets coming from a neighbor. At the end of this stage, the DR and BDR election would have been done. At the end of the two-way stage, routers decide whether to proceed in building an adjacency or not. The decision is based on whether one of the routers is a DR or BDR or the link is a point-to-point or a virtual link.
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