The answer is that the responder is configured for main mode only, but the initiator is sending aggressive mode. This occurs because IKEv1 supports two Phase 1 exchange modes: main mode, which uses six messages for identity protection, and aggressive mode, which uses only three messages but exposes the peer’s identity. When the responder’s VPN policy is set to main mode (the default on FortiGate), it rejects any incoming aggressive mode proposals, logging the “aggressive mode not allowed” error. On the Fortinet NSE 4 exam, this question tests your ability to interpret IKEv1 debug logs and understand Phase 1 mode mismatches—a common trap is confusing the IKE_SA_INIT message (aggressive mode) with the main mode exchange. Remember the memory tip: “Main mode hides, aggressive mode rides”—main mode protects identities by hiding them in later exchanges, while aggressive mode sends them early, making it less secure but faster.
NSE4 Authentication and VPN Practice Question
This NSE4 practice question tests your understanding of authentication and vpn. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
You run the command 'diagnose vpn ike log filter name vpn1' and then 'diagnose vpn ike log filter type phase1'. The log shows: 'IKEv1 exchange:f4470f07:00000000: responder: main mode: received IKE_SA_INIT (aggressive mode not allowed)'. What is the problem?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The responder is configured for main mode only, but the initiator is sending aggressive mode
The log indicates that the initiator sent an IKE_SA_INIT message, which is part of aggressive mode (IKEv1). Since the responder expects main mode, it rejects the aggressive mode proposal.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 initiator is using IKEv2 while the responder uses IKEv1
Why it's wrong here
IKEv2 does not use aggressive mode; the message type would be different.
✓
The responder is configured for main mode only, but the initiator is sending aggressive mode
Why this is correct
The responder rejects the aggressive mode init because its configuration only allows main mode.
Related concept
Static NAT maps one inside address to one outside address.
✗
The pre-shared key is wrong
Why it's wrong here
Wrong PSK would show authentication failures, not mode mismatch.
✗
The phase1 proposal is incompatible
Why it's wrong here
Incompatible proposals would show NO_PROPOSAL_CHOSEN, not mode mismatch.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Trap categories for this question
Command / output trap
Wrong PSK would show authentication failures, not mode mismatch.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
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
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
Authentication and VPN — This question tests Authentication and VPN — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The responder is configured for main mode only, but the initiator is sending aggressive mode — The log indicates that the initiator sent an IKE_SA_INIT message, which is part of aggressive mode (IKEv1). Since the responder expects main mode, it rejects the aggressive mode proposal.
What should I do if I get this NSE4 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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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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