The answer is a firewall rule tag mismatch between the rule’s target tag and the instance’s network tag. When a VPC firewall rule is configured with a target tag, it only applies to instances that carry that exact tag; if the Compute Engine instance lacks the matching tag, the rule is not enforced, allowing traffic that should have been blocked to appear in VPC Flow Logs as unexpected connections. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of how firewall rules are scoped—a common trap is assuming all rules apply globally, when in fact tag-based targeting is selective. A reliable memory tip is “tags tie the rule to the instance”: if the tag doesn’t match, the rule doesn’t bite.
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing network security. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 network engineer notices that VPC Flow Logs show connections from a Compute Engine instance to an IP address that should have been blocked by firewall rules. 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 firewall rule is not applied to the instance's network tag.
Option D is correct because if a firewall rule is not applied to the instance's network tag, the rule will not affect that instance. In Google Cloud, firewall rules can target instances by specifying target tags; if the rule's target tag does not match the instance's tag, the rule is not enforced on that instance, allowing traffic that should have been blocked.
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 firewall rule has a lower priority than a deny rule.
Why it's wrong here
A lower priority rule would not override a higher priority rule; it would be ignored.
The firewall rule is not applied to the instance's network tag.
Why this is correct
If the instance doesn't have the required tag, the firewall rule doesn't apply.
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
Google Cloud often tests the misconception that firewall rules are automatically applied to all instances in a VPC, when in reality they must be explicitly associated via target tags or service accounts, and candidates may overlook the tag mismatch as the root cause.
Detailed technical explanation
How to think about this question
Google Cloud firewall rules are stateful and evaluated based on priority, direction, and target tags. When a rule is created with a target tag, only instances that have that tag applied will have the rule enforced; if the tag is missing, the rule is effectively ignored for that instance. This is a common misconfiguration where a rule is created but not applied to the intended instances, leading to unexpected traffic being allowed or denied.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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.
Implementing network security — This question tests Implementing network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The firewall rule is not applied to the instance's network tag. — Option D is correct because if a firewall rule is not applied to the instance's network tag, the rule will not affect that instance. In Google Cloud, firewall rules can target instances by specifying target tags; if the rule's target tag does not match the instance's tag, the rule is not enforced on that instance, allowing traffic that should have been blocked.
What should I do if I get this PCNE 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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