The answer is mismatched Phase 2 selectors (proxy IDs). When an IPsec VPN shows Phase 1 up but Phase 2 down, the IKE SA has been successfully negotiated, meaning authentication and key exchange for the control channel are complete, but the IPsec SA—which carries actual traffic—cannot be established because the local and remote proxy IDs defining the traffic selectors do not align. On the Fortinet NSE 4 exam, this scenario tests your ability to interpret `diagnose vpn ike gateway list` output and distinguish between Phase 1 and Phase 2 failures; a common trap is assuming firewall policies are the root cause when Phase 1 is up, but the real issue is almost always selector mismatch. Remember the memory tip: “Phase 1 shakes hands, Phase 2 needs matching plans”—if the proxy IDs don’t mirror each other, the tunnel stays half-open.
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.
You run the CLI command 'diagnose vpn ike gateway list' and see that an IPsec VPN gateway is in 'up' state with 'initiator' mode, but no Phase 2 selectors are established. What is the most likely cause?
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.
The Phase 2 proposal parameters (encryption, authentication) do not match between peers
Why wrong: While possible, the Phase 2 proposal mismatch typically causes 'no proposal chosen' and Phase 2 fails without establishing. The gateway shows 'up' so Phase 1 is fine.
B
The remote gateway is not responding to IKE packets
Why wrong: If the remote gateway was not responding, Phase 1 would not be up.
C
The local and remote Phase 2 selectors (proxy IDs) do not match
Mismatched proxy IDs prevent Phase 2 negotiation from completing successfully. The Phase 1 can be up but Phase 2 fails to establish.
D
The IPsec interface is down
Why wrong: An interface down would affect Phase 1 as well; the gateway would not be up.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The local and remote Phase 2 selectors (proxy IDs) do not match
The Phase 1 shows up, meaning IKE SA is established. But Phase 2 is down. Common causes include mismatched proxy IDs (Phase 2 selectors), firewall policies not matching, or firewall rules blocking IPsec traffic. The output indicates Phase 1 is up, so the issue is at Phase 2.
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.
✗
The Phase 2 proposal parameters (encryption, authentication) do not match between peers
Why it's wrong here
While possible, the Phase 2 proposal mismatch typically causes 'no proposal chosen' and Phase 2 fails without establishing. The gateway shows 'up' so Phase 1 is fine.
✗
The remote gateway is not responding to IKE packets
Why it's wrong here
If the remote gateway was not responding, Phase 1 would not be up.
✓
The local and remote Phase 2 selectors (proxy IDs) do not match
Why this is correct
Mismatched proxy IDs prevent Phase 2 negotiation from completing successfully. The Phase 1 can be up but Phase 2 fails to establish.
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.
✗
The IPsec interface is down
Why it's wrong here
An interface down would affect Phase 1 as well; the gateway would not be up.
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.
Trap categories for this question
Command / output trap
While possible, the Phase 2 proposal mismatch typically causes 'no proposal chosen' and Phase 2 fails without establishing. The gateway shows 'up' so Phase 1 is fine.
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: The local and remote Phase 2 selectors (proxy IDs) do not match — The Phase 1 shows up, meaning IKE SA is established. But Phase 2 is down. Common causes include mismatched proxy IDs (Phase 2 selectors), firewall policies not matching, or firewall rules blocking IPsec traffic. The output indicates Phase 1 is up, so the issue is at Phase 2.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A FortiGate is configured with an IPsec VPN to a remote site using IKEv1. The VPN tunnel goes down intermittently. The admin runs 'diagnose vpn ike gateway list' and sees 'state=UP' but no Phase2 selectors. What is the most likely cause?
medium
A.The firewall policy allowing IPsec traffic is misconfigured
B.The remote gateway has a different PSK
✓ C.Mismatched Phase2 parameters between the local and remote gateways
D.Dead Peer Detection (DPD) is disabled
Why C: The command shows the IKE gateway is up (Phase1 complete) but no Phase2 selectors, meaning Phase2 has not been established or has failed. Option A is correct because mismatched Phase2 parameters (e.g., encryption, hash, or proxy IDs) are the most common cause of Phase2 failure.
Last reviewed: Jun 21, 2026
Question Discussion
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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.
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