Quick Answer
The answer is Aggressive mode. This is the correct choice because when both sites have dynamic public IP addresses, the IPsec VPN peers cannot rely on a fixed IP for identity verification, and Aggressive mode uses fewer exchanges—only three packets instead of six—allowing the connection to be established even when the IP addresses change. On the Fortinet NSE 4 exam, this question tests your understanding of IKE phase 1 negotiation modes and their appropriate use cases; a common trap is assuming Main mode is always preferred for security, but it requires static IPs for proper operation. Remember that Aggressive mode trades some security for flexibility with dynamic IPs, while Main mode demands fixed addresses. A useful memory tip: think “Aggressive for agile IPs” to recall that this mode adapts to changing addresses quickly.
This NSE4 practice question tests your understanding of authentication and vpn. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
An administrator needs to configure a site-to-site IPsec VPN where both sites have dynamic public IP addresses. Which IKE mode should be used?
IKEv2 without mode
Why wrong: IKEv2 also supports dynamic IPs but the question asks for IKE mode; IKEv2 does not have main/aggressive mode.
Dial-up mode
Why wrong: Dial-up VPN is for client-to-site, not site-to-site.
Main mode
Why wrong: Main mode requires fixed IP addresses on both sides.
Aggressive mode
Aggressive mode can be used with dynamic IP addresses and uses fewer exchanges.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Aggressive mode
Aggressive mode is used when one or both peers have dynamic IP addresses because it uses fewer exchanges and can work with dynamic IPs. Main mode requires fixed IPs and is more secure.
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.
IKEv2 without mode
Why it's wrong here
IKEv2 also supports dynamic IPs but the question asks for IKE mode; IKEv2 does not have main/aggressive mode.
Dial-up mode
Why it's wrong here
Dial-up VPN is for client-to-site, not site-to-site.
Main mode
Why it's wrong here
Main mode requires fixed IP addresses on both sides.
Aggressive mode
Why this is correct
Aggressive mode can be used with dynamic IP addresses and uses fewer exchanges.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
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
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.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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 — 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: Aggressive mode — Aggressive mode is used when one or both peers have dynamic IP addresses because it uses fewer exchanges and can work with dynamic IPs. Main mode requires fixed IPs and is more secure.
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.
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jun 21, 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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