The answer is that a higher priority allow rule exists that matches the traffic. In Google Cloud VPC firewall rules, priority is evaluated with lower numbers taking precedence, meaning a rule with priority 1000 is only evaluated after any rule with a lower number, such as the default priority 65535 allow rules. Since the default VPC includes an implicit ingress allow rule for traffic within the same VPC, that rule’s higher priority (lower number) overrides the custom deny rule, permitting the traffic between the subnets. On the Google Professional Cloud Network Engineer exam, this concept tests your understanding of the VPC firewall rule priority order and the common trap of assuming a deny rule always blocks traffic, when in fact a more specific or higher-priority allow rule can supersede it. A useful memory tip: think of priority numbers like a race—lower numbers cross the finish line first, so a deny at 1000 loses to an allow at 65535.
PCNE Configuring network services Practice Question
This PCNE practice question tests your understanding of configuring network services. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 company has a VPC with multiple subnets. They want to restrict traffic between two specific subnets (10.0.1.0/24 and 10.0.2.0/24) while allowing all other traffic. They create a firewall rule with priority 1000 denying ingress from 10.0.1.0/24 to 10.0.2.0/24. However, traffic is still allowed. What is the most likely reason?
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
✓
There is a higher priority allow rule that matches the traffic
Option C is correct because in Google Cloud VPC firewall rules, lower priority numbers indicate higher precedence. A rule with priority 1000 is evaluated after any rule with a priority lower than 1000 (e.g., priority 65535 is the default allow rule). If a higher priority (lower number) allow rule exists that matches the same traffic, it will override the deny rule. The default VPC firewall rules include an implicit allow rule for egress and an ingress allow rule for traffic within the same VPC, which may have a higher priority than 1000, thus permitting the traffic despite the deny rule.
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 rule is incorrectly applied to the wrong network tag
Why it's wrong here
Rule applies to subnet ranges, not tags.
✗
The traffic is going through the metadata server
Why it's wrong here
Metadata server is a different IP.
✓
There is a higher priority allow rule that matches the traffic
Why this is correct
Higher priority allow rule can override deny.
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.
✗
Firewall rules are stateless, so return traffic is blocked
Why it's wrong here
Firewall rules are stateful; return traffic is allowed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that a deny rule with a higher priority number (e.g., 1000) will override allow rules with lower priority numbers, when in fact lower numbers have higher precedence.
Detailed technical explanation
How to think about this question
Google Cloud VPC firewall rules are evaluated in order of priority, with the lowest number having the highest precedence. The default VPC includes an implied allow rule for egress and an ingress rule allowing traffic from any source within the VPC (priority 65535). If a custom rule with priority 1000 denies traffic, but a higher priority (e.g., 100) allow rule exists, the allow rule takes effect. This behavior is defined in the GCP firewall rule evaluation logic, where the first matching rule (by priority) is applied. In practice, administrators often create deny rules with very low priority numbers (e.g., 100) to ensure they override default allow rules.
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 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Configuring network services — This question tests Configuring network services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: There is a higher priority allow rule that matches the traffic — Option C is correct because in Google Cloud VPC firewall rules, lower priority numbers indicate higher precedence. A rule with priority 1000 is evaluated after any rule with a priority lower than 1000 (e.g., priority 65535 is the default allow rule). If a higher priority (lower number) allow rule exists that matches the same traffic, it will override the deny rule. The default VPC firewall rules include an implicit allow rule for egress and an ingress allow rule for traffic within the same VPC, which may have a higher priority than 1000, thus permitting the traffic despite the deny rule.
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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