The answer is that the switch port should be configured as a trunk port allowing VLAN 10. This is because a FortiGate VLAN interface, created as a subinterface on a physical port, expects to receive 802.1Q-tagged frames to identify which VLAN the traffic belongs to. When a switch port is set as an access port, it strips the VLAN tag before sending frames untagged, so the FortiGate never sees the VLAN 10 tag and cannot process the traffic for that interface. On the Fortinet NSE 4 Network Security Professional exam, this scenario tests your understanding of how FortiGate handles VLAN tagging versus how a standard switch handles access ports—a common trap is assuming an access port will work because the VLAN ID matches. Remember the key distinction: on a FortiGate, a VLAN subinterface always expects tagged traffic, even if the switch side is untagged. A helpful memory tip is "Tag in, Tag out"—if the FortiGate defines a VLAN ID, the switch must send that ID as a tag, meaning a trunk port is required.
NSE4 System and Network Administration Practice Question
This NSE4 practice question tests your understanding of system and network administration. 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.
An administrator configures a VLAN interface on a FortiGate's physical port with the IP 192.168.10.1/24. The VLAN ID is 10. The administrator connects a switch port configured as an access port (untagged) in VLAN 10. The devices on the switch cannot ping the FortiGate's VLAN interface. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The switch port should be configured as a trunk port allowing VLAN 10
The switch port is configured as an access port, which sends frames untagged. However, the FortiGate's VLAN interface expects to receive 802.1Q-tagged frames because the VLAN is defined as a subinterface on the physical port. For the FortiGate to process traffic for VLAN 10, the switch port must be configured as a trunk (or tagged) port that sends frames with the VLAN 10 tag. An access port strips the tag, so the FortiGate never sees the VLAN 10 traffic.
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 switch port should be configured as a trunk port allowing VLAN 10
Why this is correct
The switch port must send tagged frames (trunk) for the FortiGate VLAN interface to recognize them.
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 FortiGate's physical port is not set to the same VLAN ID
The FortiGate's administrative access is not enabled on the VLAN interface
Why it's wrong here
Administrative access (ping) may be needed, but the primary issue is tagging.
✗
The FortiGate's VLAN interface is not assigned to any VDOM
Why it's wrong here
VDOM assignment is necessary but usually defaults to root; tagging is the immediate issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse access/trunk port behavior with VLAN interface configuration on firewalls, mistakenly thinking an access port is sufficient because the VLAN ID matches, when in fact the FortiGate requires tagged frames for subinterfaces.
Detailed technical explanation
How to think about this question
When a FortiGate VLAN interface is created, it operates as an 802.1Q trunk endpoint — it expects all incoming frames to carry the corresponding VLAN tag. The physical port remains untagged (native VLAN) by default. A common real-world scenario is connecting a FortiGate to a Cisco switch: the switch port must be set to 'switchport mode trunk' and 'switchport trunk allowed vlan 10' to pass tagged frames. If the switch port is an access port, it strips the tag, and the FortiGate drops the untagged frames because they do not match any subinterface.
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.
TExam Day Tips
→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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
System and Network Administration — This question tests System and Network Administration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The switch port should be configured as a trunk port allowing VLAN 10 — The switch port is configured as an access port, which sends frames untagged. However, the FortiGate's VLAN interface expects to receive 802.1Q-tagged frames because the VLAN is defined as a subinterface on the physical port. For the FortiGate to process traffic for VLAN 10, the switch port must be configured as a trunk (or tagged) port that sends frames with the VLAN 10 tag. An access port strips the tag, so the FortiGate never sees the VLAN 10 traffic.
What should I do if I get this NSE4 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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
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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. An administrator configures a VLAN interface on a FortiGate trunk port. The VLAN is allowed on the trunk, but the FortiGate cannot ping the default gateway of that VLAN. Which two items must be verified? (Choose two.)
hard
✓ A.The VLAN interface has an IP address in the correct subnet.
B.A firewall policy allows ICMP from the FortiGate to the gateway.
✓ C.The VLAN interface is administratively up.
D.The trunk port is set to access mode.
✓ E.The VLAN ID matches the switch configuration.
Why A: Option A is correct because the VLAN interface must have an IP address in the correct subnet to communicate with the default gateway. Without a matching subnet, the FortiGate cannot route ICMP packets to the gateway, even if the VLAN is allowed on the trunk.
Last reviewed: Jun 24, 2026
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