Question 404 of 1,000

Authentication and VPN →hardMultiple ChoiceObjective-mapped

NSE4 Authentication and VPN Practice Question

This NSE4 practice question tests your understanding of authentication and vpn. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 is configuring a route-based IPsec VPN between two FortiGate devices. After setting up the tunnel and firewall policies, traffic does not flow. The administrator runs 'diagnose vpn tunnel list' and sees the tunnel is up. 'get router info routing-table all' shows routes on both sides. However, pings from the local network to the remote network fail. 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.

Question 1hardmultiple choice

Read the full VPN explanation →

A
The pre-shared key is incorrect
Why wrong: An incorrect PSK would prevent Phase 1 from completing.
B
The firewall policy allowing traffic to the remote subnet has the source and destination interfaces reversed
In a route-based VPN, the policy must be configured with the VPN interface as the destination interface (if traffic flows from internal to VPN) or source interface (if from VPN to internal). Misconfiguration here causes traffic to be dropped.
C
The remote FortiGate's static route points to the wrong local subnet
Why wrong: If routes are correct and appear in the routing table, this is not the issue.
D
The Phase 2 proposal uses different encryption algorithms on each side
Why wrong: If the tunnel is up (Phase 2 status up), the proposals matched. Different algorithms would prevent Phase 2 from establishing.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The firewall policy allowing traffic to the remote subnet has the source and destination interfaces reversed

Option A is correct. For route-based VPN, traffic must be allowed by the policy that has the VPN interface as the destination interface. If the policy's source and destination are reversed (e.g., source internal, destination internal instead of source internal, destination VPN), traffic will be dropped.

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 pre-shared key is incorrect
Why it's wrong here
An incorrect PSK would prevent Phase 1 from completing.
✓
The firewall policy allowing traffic to the remote subnet has the source and destination interfaces reversed
Why this is correct
In a route-based VPN, the policy must be configured with the VPN interface as the destination interface (if traffic flows from internal to VPN) or source interface (if from VPN to internal). Misconfiguration here causes traffic to be dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The remote FortiGate's static route points to the wrong local subnet
Why it's wrong here
If routes are correct and appear in the routing table, this is not the issue.
✗
The Phase 2 proposal uses different encryption algorithms on each side
Why it's wrong here
If the tunnel is up (Phase 2 status up), the proposals matched. Different algorithms would prevent Phase 2 from establishing.

Common exam traps

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 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 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.

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FAQ

Questions learners often ask

What does this NSE4 question test?

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 firewall policy allowing traffic to the remote subnet has the source and destination interfaces reversed — Option A is correct. For route-based VPN, traffic must be allowed by the policy that has the VPN interface as the destination interface. If the policy's source and destination are reversed (e.g., source internal, destination internal instead of source internal, destination VPN), traffic will be dropped.

What should I do if I get this NSE4 question wrong?

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.