The answer is a firewall egress rule with priority 1000 that denies traffic from instances tagged 'app-tier' to the on-premises network 10.1.0.0/16. This is correct because Google Cloud firewall rules are evaluated by priority, where lower numbers take precedence, so this explicit deny overrides any higher-numbered allow rules, such as the rule permitting all traffic from 10.0.0.0/16 to 10.1.0.0/16. Since the web tier works, routing and VPN are functional, isolating the issue to the app tier’s tag-based egress deny. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of hierarchical firewall evaluation and how tag-based rules can silently block VPN traffic from specific subnets. A common trap is assuming that a broad allow rule covers all subnets, forgetting that a more specific deny with a lower priority number wins. Memory tip: think of firewall priority like a bouncer—the lowest number gets the final say, so a deny at priority 1000 will always trump an allow at priority 65535.
PCNE Implementing a Virtual Private Cloud Practice Question
This PCNE practice question tests your understanding of implementing a virtual private cloud. 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.
A financial services company is deploying a multi-tier application in a custom VPC with three subnets: web (10.0.1.0/24), app (10.0.2.0/24), and db (10.0.3.0/24). They use a Cloud VPN with dynamic routing (BGP) to connect to their on-premises data center (10.1.0.0/16). The on-premises network administrator reports that traffic from the web tier (10.0.1.0/24) to on-premises is working, but traffic from the app tier (10.0.2.0/24) to on-premises is failing. The company uses an Identity-Aware Proxy (IAP) for SSH access. The following configurations are in place:
- Cloud Router advertises all VPC subnets via BGP.
- On-premises router advertises 10.1.0.0/16.
- Firewall rules allow all traffic from 10.0.0.0/16 to 10.1.0.0/16.
- The app tier instances have a network tag 'app-tier' and a service account 'app-sa@project.iam.gserviceaccount.com'.
- There is a firewall rule with priority 1000 that denies egress from tags 'app-tier' to 10.1.0.0/16.
What is the most likely cause of the failure?
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 firewall egress rule with priority 1000 denies traffic from instances with tag 'app-tier' to 10.1.0.0/16.
Option C is correct because the firewall egress rule with priority 1000 explicitly denies traffic from instances tagged 'app-tier' to the on-premises network (10.1.0.0/16). Since firewall rules are evaluated in order of priority (lower numbers are higher priority), and this rule has a relatively low priority number, it will override any higher-numbered (lower priority) allow rules. The fact that web tier traffic works confirms that routing and VPN are functional, isolating the issue to the egress deny rule targeting the app tier.
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 service account 'app-sa' does not have permissions to send traffic through the VPN.
Why it's wrong here
Service accounts do not control network traffic; firewall rules do.
✗
IAP is blocking traffic from the app tier because it is not configured for that subnet.
Why it's wrong here
IAP is used for SSH access, not for general traffic to on-premises.
✓
A firewall egress rule with priority 1000 denies traffic from instances with tag 'app-tier' to 10.1.0.0/16.
Why this is correct
This deny rule explicitly blocks the traffic, overriding any lower-priority allow rules.
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 Cloud Router is not advertising the 10.0.2.0/24 subnet to on-premises.
Why it's wrong here
The Cloud Router is configured to advertise all subnets, so 10.0.2.0/24 should be advertised.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that service accounts or IAP control network-level traffic, when in reality firewall rules and routing are the only mechanisms that govern packet flow between VPC subnets and on-premises networks.
Detailed technical explanation
How to think about this question
In Google Cloud VPC, firewall rules are stateful and evaluated based on priority (0-65535), with lower numbers taking precedence. The deny rule at priority 1000 will match egress traffic from any instance with the 'app-tier' tag destined to 10.1.0.0/16, and because it is a deny rule, it will drop the packets before any allow rule with a higher priority number (e.g., 65535) can be evaluated. This is a common pattern where a specific deny rule is used to restrict certain workloads, but if the priority is too low, it can inadvertently block legitimate traffic.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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 a Virtual Private Cloud — This question tests Implementing a Virtual Private Cloud — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: A firewall egress rule with priority 1000 denies traffic from instances with tag 'app-tier' to 10.1.0.0/16. — Option C is correct because the firewall egress rule with priority 1000 explicitly denies traffic from instances tagged 'app-tier' to the on-premises network (10.1.0.0/16). Since firewall rules are evaluated in order of priority (lower numbers are higher priority), and this rule has a relatively low priority number, it will override any higher-numbered (lower priority) allow rules. The fact that web tier traffic works confirms that routing and VPN are functional, isolating the issue to the egress deny rule targeting the app tier.
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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