When a FortiGate unit interface is connected to an OSPF area, that unit can participate in OSPF communications. FortiGate units use the OSPF Hello protocol to acquire neighbors in an area. A neighbor is any router that is directly connected to the same area as the FortiGate unit, and ideally is adjacent with a state of Full. After initial contact, the FortiGate unit exchanges Hello packets with its OSPF neighbors regularly to confirm that the neighbors can be reached.

The number of routes that a FortiGate unit can learn through OSPF depends on the network topology. A single unit can support tens of thousands of routes if the OSPF network is configured properly.