Checking the state of OSPF neighbors
In OSPF each router sends out link state advertisements to find other routers on its network segment, and to create adjacencies with some of those routers. This is important because routing updates are only passed between adjacent routers. If two routers you believe to be adjacent are not, that can be the source of routing failures.
To identify this problem, you need to check the state of the OSPF neighbors of your FortiGate unit. Use the CLI command get router info ospf neighbor all to see all the neighbors for your FortiGate unit. You will see output in the form of:
FGT1 # get router info ospf neighbor
OSPF process 0:
Neighbor ID Pri State Dead Time Address Interface
10.0.0.2 1 Full/ - 00:00:39 10.1.1.2 tunnel_wan1
10.0.0.2 1 Full/ - 00:00:34 10.1.1.4 tunnel_wan2
The important information here is the State column. Any neighbors that are not adjacent to your FortiGate unit will be reported in this column as something other than Full. If the state is Down, that router is offline.