Quick Answer
The correct command is **diagnose debug flow filter src**. This is the precise syntax for enabling flow tracing with a specific source IP filter on a FortiGate, because the `filter src` subcommand narrows the real-time debug output to only traffic originating from the designated IP address, preventing the console from being flooded with irrelevant packets. On the Fortinet NSE 4 Network Security Professional exam, this question tests your ability to recall the exact `diagnose debug flow` workflow, which is a critical troubleshooting skill for verifying firewall policy behavior and NAT translations. A common trap is confusing `filter src` with `filter dst` or forgetting to first run `diagnose debug flow show function-name` to enable the flow trace itself. Remember the mnemonic: **S**ource **R**eally **C**ounts—always filter the source first to cut through the noise.
This NSE4 practice question tests your understanding of high availability and diagnostics. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 administrator wants to view real-time debug output for traffic flowing through a FortiGate. Which command should they use to enable flow tracing with a specific source IP filter?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "which command"
Why it matters: Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
diagnose debug enable
Why wrong: This enables debug output but does not set any specific filter.
diagnose debug flow filter src
This sets a source IP filter for debug flow tracing.
diagnose sys session filter src
Why wrong: This filters session list output, not real-time flow.
diagnose sniffer packet filter src
Why wrong: This is for packet capture, not flow tracing.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
diagnose debug flow filter src
Option D is correct. 'diagnose debug flow' with 'filter src' sets a source IP filter for flow debugging.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
diagnose debug enable
Why it's wrong here
This enables debug output but does not set any specific filter.
diagnose debug flow filter src
Why this is correct
This sets a source IP filter for debug flow tracing.
Clue confirmation
The clue word "which command" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
diagnose sys session filter src
Why it's wrong here
This filters session list output, not real-time flow.
diagnose sniffer packet filter src
Why it's wrong here
This is for packet capture, not flow tracing.
Common exam traps
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Trap categories for this question
Command / output trap
This enables debug output but does not set any specific filter.
Detailed technical explanation
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the NSE4 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Identify which NSE4 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
High Availability and Diagnostics — study guide chapter
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FAQ
High Availability and Diagnostics — This question tests High Availability and Diagnostics — Read the scenario before looking for a memorised answer..
The correct answer is: diagnose debug flow filter src — Option D is correct. 'diagnose debug flow' with 'filter src' sets a source IP filter for flow debugging.
Identify which NSE4 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Yes — watch for: "which command". Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
Read the scenario before looking for a memorised answer.
About these practice questions
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Same concept, more angles
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A FortiGate administrator wants to see real-time debugging output for traffic matching a specific source IP address. Which command sequence would achieve this?
easyWhy B: Option B is correct because the 'diagnose debug flow' command sequence is specifically designed for real-time debugging of traffic flows, allowing filtering by source IP with the 'filter src' option. Enabling debug output with 'diagnose debug enable' then shows flow trace information for packets matching the filter, which is the standard method for live traffic debugging on FortiGate.
Last reviewed: Jun 21, 2026
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.
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