This NSE4 practice question tests your understanding of authentication and vpn. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 runs 'diagnose vpn tunnel list' and sees the following output for an IPsec tunnel: 'status: up', 'incoming: 0 packets', 'outgoing: 100 packets'. Phase 1 and Phase 2 both show state 'up'. What is the MOST likely cause of zero incoming packets?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Phase 2 proposal includes a mismatched proxy ID
If outgoing packets are being sent but no incoming packets, the remote side may have a misconfiguration such as a wrong remote subnet in Phase 2 or a firewall policy blocking return traffic.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 remote gateway is using aggressive mode
Why it's wrong here
Aggressive mode relates to Phase 1 negotiation, not packet flow.
✗
The FortiGate has a static route pointing to the VPN interface
Why it's wrong here
A static route would affect outgoing traffic, not necessarily incoming.
✗
The VPN is configured in policy-based mode
Why it's wrong here
Policy-based mode still allows two-way traffic if policies are correct.
✓
The Phase 2 proposal includes a mismatched proxy ID
Why this is correct
If the remote side expects a different subnet, it may drop incoming packets or not respond.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related NSE4 subnetting questions on CIDR, address ranges, and subnet selection.
Authentication and VPN — This question tests Authentication and VPN — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The Phase 2 proposal includes a mismatched proxy ID — If outgoing packets are being sent but no incoming packets, the remote side may have a misconfiguration such as a wrong remote subnet in Phase 2 or a firewall policy blocking return traffic.
What should I do if I get this NSE4 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related NSE4 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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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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