The answer is a missing firewall policy in VDOM B to allow return traffic. This is the most likely cause of an inter-VDOM traffic return path issue because inter-VDOM links on a FortiGate operate like separate physical interfaces, requiring explicit firewall policies in each VDOM to permit bidirectional flow. The policy in VDOM A authorizes the outbound session, but the FortiGate’s implicit deny rule in VDOM B drops the return packets unless a corresponding policy exists there. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this concept tests your understanding that inter-VDOM traffic is not automatically stateful across VDOMs; each VDOM independently evaluates its own policies. A common trap is assuming that a single policy in the initiating VDOM is sufficient, but the return path must be explicitly permitted in the destination VDOM. Remember the memory tip: “One VDOM, one policy—two VDOMs, two policies.”
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 is configured with multiple VDOMs. The administrator creates a firewall policy in VDOM A that allows traffic from VDOM A to VDOM B using inter-VDOM links. Users in VDOM A can initiate traffic, but return traffic from VDOM B is not reaching them. 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.
Why wrong: If the link were down, no traffic would pass at all; forward traffic works, so link is up.
B
The session helper is not configured for the application
Why wrong: Session helpers are for specific protocols; general TCP/UDP traffic does not require them.
C
The reverse path forwarding (RPF) check is blocking return traffic
Why wrong: RPF checks source IP; in inter-VDOM, the source IP changes, but RPF is typically not an issue if routing is correct.
D
A firewall policy is missing in VDOM B to allow return traffic
Inter-VDOM communication requires policies in both VDOMs: one for forward traffic and one for return traffic. Without a policy in VDOM B, return packets are dropped.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A firewall policy is missing in VDOM B to allow return traffic
In a multi-VDOM FortiGate, inter-VDOM links require firewall policies in both VDOMs to allow bidirectional traffic. The policy in VDOM A permits the outbound session, but return traffic from VDOM B is implicitly denied unless a corresponding policy exists in VDOM B. Without that policy, the FortiGate drops the return packets, preventing communication.
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 inter-VDOM link is down
Why it's wrong here
If the link were down, no traffic would pass at all; forward traffic works, so link is up.
✗
The session helper is not configured for the application
Why it's wrong here
Session helpers are for specific protocols; general TCP/UDP traffic does not require them.
✗
The reverse path forwarding (RPF) check is blocking return traffic
Why it's wrong here
RPF checks source IP; in inter-VDOM, the source IP changes, but RPF is typically not an issue if routing is correct.
✓
A firewall policy is missing in VDOM B to allow return traffic
Why this is correct
Inter-VDOM communication requires policies in both VDOMs: one for forward traffic and one for return traffic. Without a policy in VDOM B, return packets are dropped.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a single policy on the initiating VDOM is sufficient for bidirectional traffic, forgetting that inter-VDOM links require explicit policies in both VDOMs to allow return traffic.
Detailed technical explanation
How to think about this question
Inter-VDOM links function as virtual point-to-point connections, each with its own VDOM context. Traffic crossing from VDOM A to VDOM B is processed by the firewall policy in VDOM A, but the return traffic enters VDOM B and must be evaluated by VDOM B's policy set. This is analogous to a stateful firewall between two separate security zones; without an explicit policy in VDOM B, the FortiGate treats the return packets as new sessions and drops them. In real-world deployments, administrators often forget to create symmetric policies, leading to asymmetric traffic drops.
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 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: A firewall policy is missing in VDOM B to allow return traffic — In a multi-VDOM FortiGate, inter-VDOM links require firewall policies in both VDOMs to allow bidirectional traffic. The policy in VDOM A permits the outbound session, but return traffic from VDOM B is implicitly denied unless a corresponding policy exists in VDOM B. Without that policy, the FortiGate drops the return packets, preventing communication.
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.
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Question Discussion
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