NSE7 Advanced Networking and SD-WAN Practice Question
An administrator configures BFD on a BGP session between two FortiGates. After enabling BFD, the BGP session flaps intermittently. What is the most likely cause?
⚠ Common exam trap
It's easy for candidates to assume BFD is always beneficial and that lower intervals are better, but the question tests the understanding that overly aggressive BFD timers can cause instability due to false positives, not that BFD is incompatible or that higher intervals cause flapping.
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
✓
The BFD failure detection intervals are too low, causing false positives
When BFD is enabled on a BGP session, the BFD failure detection intervals (typically the minimum transmit and receive intervals) determine how quickly a link failure is detected. If these intervals are set too low, BFD may generate false positives due to transient network jitter or minor packet loss, causing the BGP session to flap as BFD triggers a session reset even though the underlying link is stable. This is the most likely cause because the symptom appeared immediately after enabling BFD, and the default or misconfigured intervals can be too aggressive for the network conditions.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The BFD failure detection intervals are too low, causing false positives
Why this is correct
Low intervals cause premature detection of failure.
- ✗
BFD is incompatible with BGP and should not be used together
Why it's wrong here
BFD is commonly used with BGP.
- ✗
BGP hold timer is shorter than BFD detection time
Why it's wrong here
That would not cause flapping; BFD would detect first.
- ✗
The BFD minimum transmit and receive intervals are set too high
Why it's wrong here
High intervals would cause slower detection, not flapping.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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