The answer is that the aggregate interface is using the default load-balancing algorithm. On a FortiGate, the default algorithm for an aggregate interface (LAG) hashes traffic based on source and destination MAC addresses, which often leads to uneven load balancing because most traffic flows share the same MAC pair, forcing all packets onto a single physical link. This concept is a common trap on the Fortinet NSE 4 Network Security Professional exam, where candidates must recognize that the default hash algorithm does not consider higher-layer information like IP addresses or ports, resulting in poor distribution even when multiple ports are active. To fix this, the algorithm should be changed to src-dst-ip or src-dst-ip-port for finer granularity. Remember: MAC hash is a traffic smash—switch to IP for even distribution.
NSE4 System and Network Administration Practice Question
This NSE4 practice question tests your understanding of system and network administration. 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.
An admin configures an aggregate interface on a FortiGate using two physical ports. After configuration, the admin notices that traffic is not load-balancing evenly. 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 aggregate interface is using the default load-balancing algorithm
The default load-balancing algorithm for an aggregate interface (LAG) on FortiGate is based on the source and destination MAC addresses. This algorithm often results in uneven traffic distribution, especially when traffic flows are limited to a small number of MAC pairs. To achieve more even load balancing, the algorithm should be changed to one that considers IP addresses or Layer 4 ports, such as 'src-dst-ip' or 'src-dst-ip-port'.
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 aggregate interface is set to active-passive mode
Why it's wrong here
Active-passive would only use one link at a time, not load balance.
✓
The aggregate interface is using the default load-balancing algorithm
Why this is correct
The default hash algorithm (e.g., source-dest-ip) can cause uneven distribution if many sessions share the same source-dest pair.
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 physical ports are connected to different switches
MTU mismatch would cause fragmentation or drops, not uneven load balancing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume uneven load balancing is caused by a hardware issue or physical misconfiguration, when in fact the default MAC-based hash algorithm is the root cause in most scenarios.
Detailed technical explanation
How to think about this question
FortiGate's default LAG algorithm uses a hash of source and destination MAC addresses to select the egress link. In environments with few MAC pairs (e.g., a single server communicating with a gateway), all traffic maps to the same hash bucket, resulting in one link being saturated while others remain idle. Changing the algorithm to 'src-dst-ip' or 'src-dst-ip-port' (via config system interface, edit <aggregate>, set algorithm) distributes flows based on IP addresses or port numbers, providing finer granularity and more even utilization across multiple flows.
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 aggregate interface is using the default load-balancing algorithm — The default load-balancing algorithm for an aggregate interface (LAG) on FortiGate is based on the source and destination MAC addresses. This algorithm often results in uneven traffic distribution, especially when traffic flows are limited to a small number of MAC pairs. To achieve more even load balancing, the algorithm should be changed to one that considers IP addresses or Layer 4 ports, such as 'src-dst-ip' or 'src-dst-ip-port'.
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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