The answer is a static route in VPC-A to 10.0.3.0/24 that preempts the peering route. This occurs because Google Cloud routes use longest prefix match, so a manually created static route with the same destination as a dynamically learned peering route will always take precedence, even if the peering route is more specific. In this scenario, the instance in VPC-A can reach 10.0.2.10 because no conflicting static route exists for that subnet, but the static route to 10.0.3.0/24 overrides the VPC peering route, blocking traffic to that range. On the Google Professional Cloud Network Engineer exam, this is a classic troubleshooting trap: candidates often check firewall rules first, but the real issue is route priority—peering routes have a default priority of 1000, while static routes can be set lower (higher priority). A common memory tip is “static beats dynamic in the route table,” so always inspect custom routes when connectivity is partially broken across a peering.
PCNE Practice Question: Designing, planning, and prototyping a GCP network
This PCNE practice question tests your understanding of designing, planning, and prototyping a gcp network. 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 company has two VPCs in the same project, VPC-A and VPC-B. They have set up VPC peering between them. In VPC-A, there is a subnet 10.0.1.0/24. In VPC-B, there are subnets 10.0.2.0/24 and 10.0.3.0/24. A compute instance in VPC-A can ping an instance in VPC-B with IP 10.0.2.10, but fails to ping an instance in VPC-B with IP 10.0.3.10. All subnets are in the same region. Firewall rules allow all traffic between VPC-A and VPC-B. 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
✓
VPC-A has a static route to 10.0.3.0/24 that preempts the peering route.
VPC peering routes are automatically added to the route tables of both VPCs when the peering is established. However, if a more specific static route (e.g., to 10.0.3.0/24) exists in VPC-A, it will take precedence over the peering route due to longest prefix match routing. Since the instance in VPC-A can reach 10.0.2.10 but not 10.0.3.10, the most likely cause is that VPC-A has a static route that preempts the peering route for the 10.0.3.0/24 subnet.
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.
✓
VPC-A has a static route to 10.0.3.0/24 that preempts the peering route.
Why this is correct
Static routes have higher priority than peering routes, causing traffic to go elsewhere.
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 peering is not bidirectional; VPC-B is not exporting routes.
Why it's wrong here
By default, both sides export routes.
✗
The firewall rules in VPC-B block ICMP from VPC-A to 10.0.3.0/24.
The subnet 10.0.3.0/24 was added after the peering, and the peering routes were not updated. Recreate the peering.
Why it's wrong here
VPC peering automatically updates routes when subnets are added.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume VPC peering automatically works for all subnets in the peered VPC, forgetting that static routes with more specific prefixes can override peering routes, even when firewall rules are permissive.
Detailed technical explanation
How to think about this question
In Google Cloud VPC peering, routes for subnets in the peered VPC are automatically exchanged and appear as system-generated peering routes with a priority of 100. However, custom static routes can override these if they have a more specific destination prefix (e.g., /24 vs /24) or a lower priority value (lower number = higher priority). This behavior follows the longest prefix match rule, where a more specific prefix always wins regardless of route type or priority. A common real-world scenario is when an organization has overlapping IP ranges or uses static routes for VPN or on-premises connectivity, inadvertently breaking peering connectivity.
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.
Designing, planning, and prototyping a GCP network — This question tests Designing, planning, and prototyping a GCP network — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: VPC-A has a static route to 10.0.3.0/24 that preempts the peering route. — VPC peering routes are automatically added to the route tables of both VPCs when the peering is established. However, if a more specific static route (e.g., to 10.0.3.0/24) exists in VPC-A, it will take precedence over the peering route due to longest prefix match routing. Since the instance in VPC-A can reach 10.0.2.10 but not 10.0.3.10, the most likely cause is that VPC-A has a static route that preempts the peering route for the 10.0.3.0/24 subnet.
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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This PCNE practice question is part of Courseiva's free Google Cloud 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 PCNE exam.
Question Discussion
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