The answer is the ECMP load balancing method set to source-destination IP hash, which causes asymmetric routing. This occurs because the hash is calculated on both the source and destination IP addresses; if either address changes mid-session—such as when NAT rewrites the source IP or a reply comes from a different destination—the hash result changes, directing subsequent packets to a different WAN interface. The FortiGate’s stateful firewall expects all packets of a session to traverse the same interface for proper state tracking, so packets arriving on a different interface are dropped or cause out-of-order delivery, leading to intermittent traffic failure. On the Fortinet NSE 4 exam, this question tests your understanding of how ECMP load-balancing methods interact with stateful inspection; a common trap is confusing source-destination IP hash with session-based methods like round-robin. Memory tip: “Hash the pair, but if one changes, the path rearranges.”
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.
A FortiGate is configured with multiple WAN interfaces and ECMP routing. The administrator notices that traffic to a particular destination is intermittently failing. 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 ECMP load balancing method is set to source-destination IP hash, causing asymmetric routing.
When ECMP is configured with the source-destination IP hash load balancing method, traffic for the same session may be sent over different WAN interfaces if the source or destination IP changes mid-session (e.g., due to NAT or asymmetric routing). This causes packets to arrive out of order or be dropped by stateful inspection, leading to intermittent failures. The FortiGate expects all packets in a session to traverse the same interface for proper state tracking.
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 ECMP routes have different distances.
Why it's wrong here
ECMP requires equal distances; unequal distances would not be ECMP.
✓
The ECMP load balancing method is set to source-destination IP hash, causing asymmetric routing.
Why this is correct
Asymmetric routing due to hash changes can cause session timeouts if return traffic takes a different path.
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 session table is full.
Why it's wrong here
A full session table would affect all traffic, not just specific destinations.
✗
The firewall policies are not configured for ECMP.
Why it's wrong here
Firewall policies are independent of routing; ECMP is a routing decision.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume ECMP always works seamlessly with any load balancing method, overlooking that source-destination IP hash can cause asymmetric routing and session disruption when combined with stateful firewalling.
Detailed technical explanation
How to think about this question
The source-destination IP hash method uses a hash of source and destination IPs to select an egress interface, but if NAT changes the source IP mid-session (e.g., due to a different egress interface), the hash changes, potentially sending subsequent packets out a different interface. This breaks stateful inspection because the FortiGate expects all packets in a session to arrive on the same interface. In contrast, methods like source IP hash or per-session load balancing avoid this issue by ensuring session persistence.
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.
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 ECMP load balancing method is set to source-destination IP hash, causing asymmetric routing. — When ECMP is configured with the source-destination IP hash load balancing method, traffic for the same session may be sent over different WAN interfaces if the source or destination IP changes mid-session (e.g., due to NAT or asymmetric routing). This causes packets to arrive out of order or be dropped by stateful inspection, leading to intermittent failures. The FortiGate expects all packets in a session to traverse the same interface for proper state tracking.
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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