The answer is that the FSSO collector agent is listening on an IP address not reachable from the 10.0.1.0/24 subnet due to SD-WAN routing asymmetry. This occurs because the collector agent is bound to 192.168.1.1, the interface tied to ISP1, while SD-WAN load balancing rules force traffic from the 10.0.1.0/24 subnet out through ISP2. As a result, authentication packets sent by these users to the collector agent are routed via ISP2 and never reach the intended listener, causing repeated FSSO authentication prompts despite valid domain credentials. On the Fortinet NSE 4 exam, this scenario tests your understanding of how SD-WAN steering can break FSSO’s dependency on symmetric traffic flow—a common trap where administrators overlook that FSSO control traffic must match the same path as the user’s source IP. A useful memory tip: “FSSO follows the source, not the SD-WAN rule”—always ensure the collector agent’s listening IP is reachable from every subnet that requires authentication.
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. 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 company has a FortiGate 100F with two ISPs (ISP1 and ISP2) for load balancing. They use SD-WAN to direct traffic. The firewall has a policy that allows HTTP and HTTPS traffic from internal users (10.0.0.0/8) to the internet. The policy uses FSSO authentication with an Active Directory domain controller. Recently, users on the 10.0.1.0/24 subnet report that they are prompted for authentication repeatedly, even though they are domain-joined and logged in. Users on other subnets do not have this issue. The administrator checks the FSSO configuration and sees that the collector agent is running and the FortiGate is receiving login events. The FortiGate's policy is configured with source address 10.0.0.0/8 and FSSO group 'Domain Users'. The administrator also notices that the FortiGate's SD-WAN rules are configured to use ISP1 for traffic from 10.0.0.0/8 except for traffic from 10.0.1.0/24, which uses ISP2. The FortiGate's FSSO collector agent is configured to listen on the IP address 192.168.1.1, which is the IP of the interface connected to ISP1. What is the most likely cause of the authentication issue?
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 FSSO group 'Domain Users' does not include the affected users
Why wrong: The issue is subnet-specific, not group-specific.
B
The SD-WAN rule for 10.0.1.0/24 is misconfigured and drops authentication traffic
Why wrong: The SD-WAN rule is for load balancing, not blocking.
C
The domain controller is not reachable from the FortiGate
Why wrong: The administrator confirmed login events are received for other subnets.
D
The FSSO collector agent is listening on an IP that is not reachable from the 10.0.1.0/24 subnet due to SD-WAN routing
The collector agent's IP is on ISP1 interface, but traffic from 10.0.1.0/24 goes via ISP2, so the domain controller may not be able to send login events to that IP.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The FSSO collector agent is listening on an IP that is not reachable from the 10.0.1.0/24 subnet due to SD-WAN routing
The FSSO collector agent listens on 192.168.1.1, which is the IP of the interface connected to ISP1. SD-WAN rules send traffic from 10.0.1.0/24 via ISP2, so authentication packets from these users (e.g., to the collector agent) may be routed through ISP2 and never reach the collector agent at 192.168.1.1, causing repeated authentication prompts. This is a classic SD-WAN routing asymmetry issue where FSSO traffic does not follow the expected path.
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 FSSO group 'Domain Users' does not include the affected users
Why it's wrong here
The issue is subnet-specific, not group-specific.
✗
The SD-WAN rule for 10.0.1.0/24 is misconfigured and drops authentication traffic
The domain controller is not reachable from the FortiGate
Why it's wrong here
The administrator confirmed login events are received for other subnets.
✓
The FSSO collector agent is listening on an IP that is not reachable from the 10.0.1.0/24 subnet due to SD-WAN routing
Why this is correct
The collector agent's IP is on ISP1 interface, but traffic from 10.0.1.0/24 goes via ISP2, so the domain controller may not be able to send login events to that IP.
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 FSSO authentication failures are always due to misconfigured groups or domain connectivity, overlooking how SD-WAN routing can cause asymmetric traffic flows that break the FSSO communication path.
Detailed technical explanation
How to think about this question
FSSO relies on the collector agent receiving authentication events from domain controllers and the FortiGate querying the collector agent for user-to-IP mappings. If the FortiGate's source IP for FSSO queries (e.g., from the 10.0.1.0/24 subnet) is routed via ISP2, but the collector agent listens only on the ISP1 interface (192.168.1.1), the responses may not return correctly or the collector agent may not see the queries. In real-world deployments, you must ensure the FSSO collector agent is reachable from all subnets that require authentication, often by using a loopback interface or adjusting SD-WAN rules to pin FSSO traffic to a specific path.
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: The FSSO collector agent is listening on an IP that is not reachable from the 10.0.1.0/24 subnet due to SD-WAN routing — The FSSO collector agent listens on 192.168.1.1, which is the IP of the interface connected to ISP1. SD-WAN rules send traffic from 10.0.1.0/24 via ISP2, so authentication packets from these users (e.g., to the collector agent) may be routed through ISP2 and never reach the collector agent at 192.168.1.1, causing repeated authentication prompts. This is a classic SD-WAN routing asymmetry issue where FSSO traffic does not follow the expected path.
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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