The answer is a firewall between the peers dropping UDP port 500 packets. This causes frequent DPD retransmissions because Dead Peer Detection relies on periodic IKE keepalives sent over UDP 500 to verify the remote peer is still alive; when a firewall silently drops these packets, the local FortiGate never receives a response, triggering repeated DPD retransmissions until the tunnel is declared dead. On the Fortinet NSE 4 exam, this scenario tests your understanding of IPsec VPN troubleshooting, specifically how network congestion, a firewall blocking IKE traffic, or the remote peer going down all manifest as DPD failures. A common trap is assuming DPD retransmissions always mean the peer is down, when in reality a stateful firewall or asymmetric routing can cause the same symptoms. Remember the mnemonic “DPD = Drop, Peer, Down” to recall the three causes: a firewall Drop, network congestion (Peer slow), or the remote peer Down.
NSE4 Authentication and VPN Practice Question
This NSE4 practice question tests your understanding of authentication and vpn. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A FortiGate administrator is troubleshooting an IPsec VPN that is dropping traffic intermittently. The administrator runs 'diagnose vpn ike log' and sees many 'DPD' messages. Which THREE conditions could cause frequent DPD (Dead Peer Detection) retransmissions? (Choose three.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
High network latency causing DPD timeouts
DPD failures indicate the remote peer is not responding. Common causes: network congestion, a firewall blocking IKE packets, or the remote peer going down.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
High network latency causing DPD timeouts
Why this is correct
Latency can cause DPD to time out.
Related concept
Read the scenario before looking for a memorised answer.
✓
The remote peer is rebooting or unstable
Why this is correct
If the peer goes down, DPD will detect it.
Related concept
Read the scenario before looking for a memorised answer.
✗
Mismatched IKE version
Why it's wrong here
IKE version mismatch would prevent Phase 1 from establishing.
A firewall between the peers dropping UDP port 500 packets
Why this is correct
DPD uses IKE packets; if UDP 500 is dropped, DPD fails.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the NSE4 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which NSE4 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Authentication and VPN — This question tests Authentication and VPN — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: High network latency causing DPD timeouts — DPD failures indicate the remote peer is not responding. Common causes: network congestion, a firewall blocking IKE packets, or the remote peer going down.
What should I do if I get this NSE4 question wrong?
Identify which NSE4 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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This NSE4 practice question is part of Courseiva's free Fortinet certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the NSE4 exam.
Question Discussion
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