Final answer:
To establish adjacencies on an OSPF network, routers must share the same Area ID, Subnet Mask, match Hello and Dead intervals, and have consistent Authentication credentials. The type of network (e.g., broadcast, non-broadcast) also affects the process.
Step-by-step explanation:
Requirements for OSPF Adjacencies:
To establish adjacencies on an OSPF network, several conditions must be met. Firstly, both routers must agree on certain parameters such as Area ID, Subnet Mask, Hello and Dead Intervals, and Authentication. Routers on the same network need to be configured to be in the same OSPF area and must have matching OSPF interface parameters. The network types, such as point-to-point, broadcast, non-broadcast, point-to-multipoint, or virtual links, can influence the adjacency formation.
Moreover, OSPF routers need to exchange 'Hello' packets at regular intervals, as specified by the Hello interval, to establish and maintain adjacencies. If a router does not receive a 'Hello' packet within the specified Dead interval from a neighbor OSPF router, the adjacency is considered broken. For a secure OSPF design, Authentication may be required where routers establish adjacencies only with other routers that have matching authentication credentials.