An administrator wants to use BFD to detect failures in an OSPF neighbor relationship faster than OSPF hello timers. They configure 'config router ospf' and 'set bfd enable'. However, BFD sessions are not coming up. What is a possible reason?
Correct. The interface-level BFD enablement is required for BFD sessions to form over OSPF, and it is a common omission.
Why this answer
On FortiGate, enabling BFD under the OSPF process alone does not activate BFD on the interface. The interface itself must also have 'set bfd enable' under 'config system interface' to allow BFD session establishment. Option C is incorrect because while BFD generally requires both ends to enable it, the question specifically asks for a reason on the local FortiGate's configuration; the neighbor's configuration is outside the administrator's control for this scenario, and the most common oversight is the interface-level setting.
Exam trap
The trap here is that candidates assume enabling BFD under the OSPF process alone is sufficient, overlooking the separate interface-level BFD configuration required on FortiGate, unlike some other vendors where a single command enables BFD across all OSPF interfaces.
How to eliminate wrong answers
Option B is wrong because OSPF network type 'point-to-multipoint' does not inherently prevent BFD sessions; BFD can operate over point-to-multipoint links as long as the interface has BFD enabled. Option C is wrong because the question asks for a reason why BFD sessions are not coming up on the local FortiGate; while BFD requires both sides to be enabled, the local configuration is the focus, and the neighbor not having BFD enabled would cause sessions to fail, but the question implies the local side is misconfigured. Option D is wrong because setting the BFD transmit interval too high would only affect detection speed, not prevent BFD sessions from coming up entirely.