Which BFD mode is used to detect forwarding path failures between two FortiGates that are directly connected?
Default mode with periodic hello packets.
Why this answer
Asynchronous mode is the default BFD mode used between two directly connected FortiGates to detect forwarding path failures. In this mode, both peers periodically send control packets at negotiated intervals, and if a peer stops receiving these packets within the detection multiplier, the session is declared down. This allows rapid detection of link or forwarding failures without relying on echo packets or on-demand triggers.
Exam trap
The trap here is that candidates confuse 'echo mode' with the default mode because echo mode can be used to reduce overhead, but asynchronous mode is the standard and mandatory mode for BFD operation between directly connected peers.
How to eliminate wrong answers
Option A is wrong because synchronous mode is not a BFD mode; BFD uses asynchronous, demand, and echo modes, and synchronous mode is unrelated to BFD operation. Option B is wrong because echo mode is an optional BFD mechanism where one end sends echo packets that the other end loops back, but it is not the primary mode for detecting forwarding path failures between directly connected FortiGates; it is used to reduce control packet overhead. Option C is wrong because demand mode is a BFD mode where control packets are sent only when there is a change or on demand, not periodically, and it is typically used in scenarios where reducing control traffic is critical, not for default direct-connect failure detection.