The answer is a routing loop causing continuous packet processing. When a FortiGate experiences a routing loop, it repeatedly forwards the same packets between interfaces or to other routers, creating an endless cycle that saturates the CPU with forwarding decisions and interrupts, which explains the 95% CPU usage and high context switch rate despite no traffic actually leaving the unit. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this scenario tests your ability to differentiate between a routing loop and other CPU spikes caused by denial-of-service attacks or antivirus scanning—a common trap is assuming high CPU always means high traffic. Remember that a loop consumes CPU cycles internally without any throughput, so the key clue is the combination of high context switches and zero traffic passing. Memory tip: “No traffic, high CPU? Think loop, not attack.”
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.
You run 'get system performance status' and see CPU usage at 95% with high context switch rate. The FortiGate is not passing any traffic. 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
✓
A routing loop is causing continuous packet processing
A routing loop causes the FortiGate to continuously process and re-process packets as they are forwarded in a cycle between routers, leading to high CPU usage and context switch rates. The loop prevents traffic from being successfully delivered, resulting in zero traffic passing through the FortiGate. This matches the observed symptoms of 95% CPU usage and high context switching.
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.
✓
A routing loop is causing continuous packet processing
Why this is correct
Even with no traffic, a misconfigured route can cause kernel loops that consume CPU.
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 is under a DDoS attack
Why it's wrong here
Attack would show traffic, but no traffic is passing.
✗
The antivirus engine is updating signatures
Why it's wrong here
Updates cause brief CPU spikes, not sustained 95%.
✗
The FortiGate is in transparent mode
Why it's wrong here
Transparent mode does not inherently cause high CPU.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often associate high CPU usage with a DDoS attack, but the key clue is the high context switch rate combined with zero traffic passing, which points to a routing loop rather than a flood of traffic.
Trap categories for this question
Command / output trap
Attack would show traffic, but no traffic is passing.
Detailed technical explanation
How to think about this question
In a routing loop, packets are forwarded repeatedly between routers, incrementing the TTL field each hop until it reaches zero, causing the packet to be dropped. The FortiGate's CPU is consumed by processing these looping packets, including route lookups, MAC rewrites, and ARP requests, leading to high context switching as the kernel rapidly switches between interrupt handlers and packet processing tasks. This scenario is often diagnosed by observing high 'soft' interrupt counts and a large number of packets being processed per second with no corresponding throughput.
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: A routing loop is causing continuous packet processing — A routing loop causes the FortiGate to continuously process and re-process packets as they are forwarded in a cycle between routers, leading to high CPU usage and context switch rates. The loop prevents traffic from being successfully delivered, resulting in zero traffic passing through the FortiGate. This matches the observed symptoms of 95% CPU usage and high context switching.
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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This NSE4 practice question is part of Courseiva's free Fortinet certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the NSE4 exam.
Question Discussion
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