Question 505 of 1,000

Authentication and VPN →hardMultiple SelectObjective-mapped

Quick Answer

The answer is incorrect firewall policies, routing issues, or NAT being applied to VPN traffic before it enters the tunnel. When the IPsec VPN tunnel is up but no traffic passes, Phase 1 and Phase 2 have completed successfully, as confirmed by the "IPsec SA up" log, so the problem lies in how traffic is handled outside the encryption process. On the Fortinet NSE 4 exam, this scenario tests your ability to distinguish between tunnel establishment and traffic flow—a common trap is assuming a green tunnel means everything works, when in fact firewall policies must explicitly permit the traffic, routes must direct it into the tunnel interface, and NAT must not alter the source IP before encryption, which would cause an IP address mismatch at the remote end. Remember the mnemonic "FRN" for Firewall, Route, NAT—if the tunnel is up, check these three before touching IKE settings.

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.

A FortiGate administrator is troubleshooting an IPsec VPN between two FortiGates. The tunnel is established, but traffic is not passing. The administrator runs 'diagnose vpn ike log' and sees the following output: IKE: phase 2 negotiation completed IKE: IPsec SA up What THREE possible causes should the administrator investigate?

Question 1hardmulti select

Read the full VPN explanation →

A
Firewall policies on either FortiGate are not allowing traffic between the local and remote subnets
Missing or misconfigured firewall policies will drop traffic even if the tunnel is up.
B
The pre-shared key is incorrect
Why wrong: An incorrect PSK would prevent Phase 1 from completing, but the log shows Phase 2 completed.
C
Routing tables on both FortiGates do not have routes pointing to the remote subnets via the VPN interface
Without proper routes, the FortiGate may not know to send traffic through the tunnel.
D
The IKE mode is set to aggressive mode on one side and main mode on the other
Why wrong: Mode mismatch would cause Phase 1 to fail, not Phase 2 to complete.
E
NAT is being applied to the VPN traffic before it enters the tunnel, causing IP address mismatch
If NAT is applied to traffic that is supposed to be encrypted, the inner IP addresses may change, causing the receiver to drop packets due to mismatch with Phase 2 selectors.

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

✓

Firewall policies on either FortiGate are not allowing traffic between the local and remote subnets

Options A, C, and E are correct. The tunnel is up, so Phase 1 and Phase 2 are fine. Common causes for traffic not passing include incorrect firewall policies, routing issues, or NAT traversal problems if traffic is being NATed before hitting the tunnel.

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.

✓
Firewall policies on either FortiGate are not allowing traffic between the local and remote subnets
Why this is correct
Missing or misconfigured firewall policies will drop traffic even if the tunnel is up.
Related concept
CIDR notation defines the prefix length.
✗
The pre-shared key is incorrect
Why it's wrong here
An incorrect PSK would prevent Phase 1 from completing, but the log shows Phase 2 completed.
✓
Routing tables on both FortiGates do not have routes pointing to the remote subnets via the VPN interface
Why this is correct
Without proper routes, the FortiGate may not know to send traffic through the tunnel.
Related concept
CIDR notation defines the prefix length.
✗
The IKE mode is set to aggressive mode on one side and main mode on the other
Why it's wrong here
Mode mismatch would cause Phase 1 to fail, not Phase 2 to complete.
✓
NAT is being applied to the VPN traffic before it enters the tunnel, causing IP address mismatch
Why this is correct
If NAT is applied to traffic that is supposed to be encrypted, the inner IP addresses may change, causing the receiver to drop packets due to mismatch with Phase 2 selectors.
Related concept
CIDR notation defines the prefix length.

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.

Trap categories for this question

Command / output trap
An incorrect PSK would prevent Phase 1 from completing, but the log shows Phase 2 completed.

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: Firewall policies on either FortiGate are not allowing traffic between the local and remote subnets — Options A, C, and E are correct. The tunnel is up, so Phase 1 and Phase 2 are fine. Common causes for traffic not passing include incorrect firewall policies, routing issues, or NAT traversal problems if traffic is being NATed before hitting the tunnel.

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

What is the key concept behind this question?

CIDR notation defines the prefix length.

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Last reviewed: Jun 21, 2026

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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.