PCDOE Optimizing service performance Practice Question
This PCDOE practice question tests your understanding of optimizing service performance. 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.
Refer to the exhibit. After applying the shown firewall rule, users report increased latency to a web application. 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.
The rule priority is set to 1000, which is too low.
Why wrong: Priority 1000 is a mid-range value, not excessively low. Lower numbers indicate higher priority, and this setting alone does not cause latency.
B
The rule contains both allow and deny for the same traffic, creating a conflict.
Correct. A firewall rule cannot contain both allow and deny actions for the same traffic; this invalid combination creates a conflict, leading to unexpected behavior and potential connectivity issues.
C
The source range covers all IPs, causing excessive traffic.
Why wrong: While a source range of 0.0.0.0/0 is broad, it does not cause increased latency directly. It allows traffic from all IPs, but network performance is not degraded solely by the range.
D
The firewall rule has logging enabled, which adds overhead.
Why wrong: Logging can add slight overhead, but it is typically minimal and not the primary cause of significant latency. The main issue is the conflicting rule actions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The rule contains both allow and deny for the same traffic, creating a conflict.
In Google Cloud VPC firewall rules, a single rule cannot specify both allow and deny actions. The rule shown is invalid because it combines both actions for the same traffic, which can lead to undefined behavior. This misconfiguration can cause the firewall to improperly deny traffic that should be allowed, resulting in connectivity issues and increased latency due to retries or timeouts. Option B correctly identifies this conflict. Option A is wrong because priority 1000 is not low (lower number higher priority); logging alone does not cause significant latency. Option C is wrong because a broad source range does not inherently cause latency. Option D is wrong because the rule's priority is not the issue here.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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 rule priority is set to 1000, which is too low.
Why it's wrong here
Priority 1000 is a mid-range value, not excessively low. Lower numbers indicate higher priority, and this setting alone does not cause latency.
✓
The rule contains both allow and deny for the same traffic, creating a conflict.
Why this is correct
Correct. A firewall rule cannot contain both allow and deny actions for the same traffic; this invalid combination creates a conflict, leading to unexpected behavior and potential connectivity issues.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The source range covers all IPs, causing excessive traffic.
Why it's wrong here
While a source range of 0.0.0.0/0 is broad, it does not cause increased latency directly. It allows traffic from all IPs, but network performance is not degraded solely by the range.
✗
The firewall rule has logging enabled, which adds overhead.
Why it's wrong here
Logging can add slight overhead, but it is typically minimal and not the primary cause of significant latency. The main issue is the conflicting rule actions.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related PCDOE ACL questions on filtering logic and placement.
Optimizing service performance — This question tests Optimizing service performance — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The rule contains both allow and deny for the same traffic, creating a conflict. — In Google Cloud VPC firewall rules, a single rule cannot specify both allow and deny actions. The rule shown is invalid because it combines both actions for the same traffic, which can lead to undefined behavior. This misconfiguration can cause the firewall to improperly deny traffic that should be allowed, resulting in connectivity issues and increased latency due to retries or timeouts. Option B correctly identifies this conflict. Option A is wrong because priority 1000 is not low (lower number higher priority); logging alone does not cause significant latency. Option C is wrong because a broad source range does not inherently cause latency. Option D is wrong because the rule's priority is not the issue here.
What should I do if I get this PCDOE question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related PCDOE ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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Question Discussion
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