NSE7 Advanced Networking and SD-WAN Practice Question
You are troubleshooting BFD on a FortiGate SD-WAN deployment. BFD is configured on two WAN interfaces (wan1, wan2) with a minimum transmit interval of 100 ms and a multiplier of 3. The network experiences occasional jitter causing packet loss. After a brief outage, the BFD session does not recover. Which setting should be adjusted to improve BFD resilience without significantly increasing failover time?
⚠ Common exam trap
The trap here is that candidates mistakenly increase the transmit interval (Option C) thinking it reduces jitter sensitivity, but that actually increases failover time for all failures, whereas adjusting the multiplier provides resilience against intermittent loss without proportionally increasing detection time for sustained outages.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Increase the BFD multiplier to 4 or higher.
Increasing the BFD multiplier (from 3 to 4 or higher) allows the session to tolerate more lost BFD control packets before declaring a failure. This directly addresses the jitter-induced packet loss without changing the detection timing for sustained outages, as the multiplier only affects the number of missed packets required to trigger a failure. The minimum transmit interval remains at 100 ms, so the base detection time (multiplier × interval) increases only slightly, preserving fast failover for true link failures.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Disable BFD and rely on route timers.
Why it's wrong here
Disabling BFD removes sub-second liveness detection entirely, so SD-WAN failover falls back to slower route or link-state timers, contradicting the requirement to avoid significantly increasing failover time. It tempts as a stability workaround, and would be correct where BFD flapping causes more disruption than slow failover.
- ✗
Enable BFD on the management interface.
Why it's wrong here
BFD on the management interface monitors a path unrelated to the wan1 and wan2 SD-WAN members, so it cannot detect their jitter-induced failures or restore those sessions. It tempts because enabling BFD somewhere appears to add redundancy, and would be correct when the management path itself needs liveness detection.
- ✗
Increase the BFD minimum transmit interval on both interfaces.
Why it's wrong here
Raising the transmit interval slows detection, so failover takes longer — the opposite of the goal. The multiplier governs how many missed packets trigger a down state, so increasing it absorbs jitter while keeping the 100 ms interval and failover timing intact.
- ✓
Increase the BFD multiplier to 4 or higher.
Why this is correct
Raising the multiplier from 3 to 4+ tolerates more consecutive missed BFD packets before declaring the peer down, absorbing jitter-induced loss so sessions recover. Transmit interval changes would alter detection timing more drastically, so the multiplier is the precise resilience lever.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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