Quick Answer
The answer is to enable Dead Peer Detection (DPD) on the Phase 1 interface. This resolves intermittent IPsec phase 2 negotiation failure because DPD actively probes the peer’s liveness, allowing the FortiGate to renegotiate both Phase 1 and Phase 2 security associations (SAs) before they expire, thereby preventing the state mismatch that occurs when one peer’s SA times out while the other still considers it valid. On the Fortinet NSE 4 Network Security Professional exam, this scenario tests your understanding of IPsec SA lifecycle management and the common trap of assuming the issue is a firewall rule or routing problem rather than a peer state mismatch. A helpful memory tip: DPD stands for “Dead Peer Detection,” but think of it as “Don’t Phase-out Discrepancies”—it keeps both peers synchronized so Phase 2 negotiations don’t fail intermittently.
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 company with multiple remote sites uses IPsec VPNs. One site reports intermittent connectivity. The administrator checks the logs and sees 'IPsec phase 2 negotiation failed' messages. Which configuration change is most likely to resolve the issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Enable Dead Peer Detection (DPD) on the Phase 1 interface.
DPD detects peer failure and triggers renegotiation.
Change the encryption algorithm from AES256 to 3DES.
Why wrong: Algorithm mismatch would cause constant failure.
Increase the Phase 2 lifetime.
Why wrong: Longer lifetime may mask the problem but not fix intermittent negotiation.
Enable NAT traversal.
Why wrong: NAT-T is for NAT environments, not intermittent drops.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Enable Dead Peer Detection (DPD) on the Phase 1 interface.
Intermittent IPsec phase 2 negotiation failures often occur when one peer's Phase 2 security association (SA) expires while the other peer still considers it valid, causing a mismatch. Enabling Dead Peer Detection (DPD) on the Phase 1 interface allows the FortiGate to actively probe the peer's liveness and renegotiate Phase 1 and Phase 2 SAs before they expire, preventing the state mismatch that leads to intermittent failures.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Enable Dead Peer Detection (DPD) on the Phase 1 interface.
Why this is correct
DPD detects peer failure and triggers renegotiation.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Change the encryption algorithm from AES256 to 3DES.
Why it's wrong here
Algorithm mismatch would cause constant failure.
Increase the Phase 2 lifetime.
Why it's wrong here
Longer lifetime may mask the problem but not fix intermittent negotiation.
Enable NAT traversal.
Why it's wrong here
NAT-T is for NAT environments, not intermittent drops.
Common exam traps
The trap here is that candidates often mistake intermittent phase 2 failures for a cryptographic or NAT issue, but the real cause is typically a mismatch in SA state between peers, which DPD is specifically designed to detect and recover from.
Detailed technical explanation
DPD uses RFC 3706 keepalive messages (R_U_THERE) to detect peer unavailability; when a peer fails to respond, the FortiGate can delete stale SAs and trigger renegotiation. In multi-site VPNs with asymmetric traffic patterns, one peer may have its Phase 2 SA idle-timeout triggered while the other still expects traffic, causing a 'half-open' state that DPD resolves by forcing both sides to re-establish SAs. Real-world scenarios often involve firewalls with different SA lifetime configurations or one peer behind a load balancer that resets idle timers.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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Authentication and VPN — study guide chapter
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FAQ
Authentication and VPN — This question tests Authentication and VPN — Read the scenario before looking for a memorised answer..
The correct answer is: Enable Dead Peer Detection (DPD) on the Phase 1 interface. — Intermittent IPsec phase 2 negotiation failures often occur when one peer's Phase 2 security association (SA) expires while the other peer still considers it valid, causing a mismatch. Enabling Dead Peer Detection (DPD) on the Phase 1 interface allows the FortiGate to actively probe the peer's liveness and renegotiate Phase 1 and Phase 2 SAs before they expire, preventing the state mismatch that leads to intermittent failures.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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.
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