Quick Answer
The answer is that the policy in VDOM B to allow traffic from VDOM A is missing or misconfigured. This is correct because inter-VDOM traffic requires a forward policy check in both the source and destination VDOMs; the source VDOM (A) handles the outgoing policy, while the destination VDOM (B) must have a separate policy to accept and forward the incoming traffic. The session log error “deny by forward policy check” specifically points to the destination VDOM’s policy being absent or denying the flow, not a routing or source-side issue. On the Fortinet NSE 4 exam, this scenario tests your understanding of inter-VDOM link architecture and the two-policy requirement—a common trap is assuming one policy in the source VDOM suffices. Remember the “two-way rule”: for inter-VDOM traffic, you need a policy out of VDOM A and a policy into VDOM B, just like crossing two borders. A useful memory tip: “Source sends, destination permits—both policies must fit.”
This NSE4 practice question tests your understanding of firewall policies and nat. 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 with multiple VDOMs has a policy that allows traffic from VDOM A to VDOM B. The admin notices that traffic from VDOM A to a specific server in VDOM B is being dropped. The session log shows 'deny by forward policy check'. 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.
The inter-VDOM link is down
Why wrong: Would cause a different error, like 'no route'.
NAT is required for inter-VDOM traffic
Why wrong: NAT is not required; it can be routed.
The source VDOM has exceeded its session limit
Why wrong: Would show 'session limit' error.
The policy in VDOM B to allow traffic from VDOM A is missing or misconfigured
Forward policy check occurs in the destination VDOM.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The policy in VDOM B to allow traffic from VDOM A is missing or misconfigured
Inter-VDOM links require policies in both VDOMs: one in VDOM A for outgoing traffic, and one in VDOM B for incoming traffic. The error 'deny by forward policy check' indicates that the policy in the destination VDOM (VDOM B) is missing or denying the traffic.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
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
Would cause a different error, like 'no route'.
NAT is required for inter-VDOM traffic
Why it's wrong here
NAT is not required; it can be routed.
The source VDOM has exceeded its session limit
Why it's wrong here
Would show 'session limit' error.
The policy in VDOM B to allow traffic from VDOM A is missing or misconfigured
Why this is correct
Forward policy check occurs in the destination VDOM.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
Common exam traps
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Trap categories for this question
Command / output trap
Would show 'session limit' error.
Detailed technical explanation
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
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
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
Firewall Policies and NAT — study guide chapter
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FAQ
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Static NAT maps one inside address to one outside address..
The correct answer is: The policy in VDOM B to allow traffic from VDOM A is missing or misconfigured — Inter-VDOM links require policies in both VDOMs: one in VDOM A for outgoing traffic, and one in VDOM B for incoming traffic. The error 'deny by forward policy check' indicates that the policy in the destination VDOM (VDOM B) is missing or denying the traffic.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 21, 2026
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