- A
The firewall rules are not applied to the correct tags
Why wrong: The scenario says firewall rules allow traffic, so this is unlikely.
- B
The subnet IP ranges overlap
Why wrong: If peering is active, subnets do not overlap.
- C
The instance in VPC-A does not have a default route
Why wrong: Default routes are not needed for peering.
- D
Custom routes are not being exchanged between the VPCs
By default, custom routes are not exchanged; the export/import custom routes flag must be enabled.
Question 942 of 982
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.
An engineer is troubleshooting a VPC Network Peering connection between VPC-A and VPC-B. They have verified that the peering is active. However, an instance in VPC-A cannot reach an instance in VPC-B using its internal IP. Both VPCs have firewall rules allowing the traffic. 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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Custom routes are not being exchanged between the VPCs
When VPC Network Peering is active but traffic fails, the most common cause is that custom routes are not being exchanged. By default, only subnet routes are exchanged; custom routes (e.g., static routes or routes from VPNs) are not shared unless explicitly configured using the 'export custom routes' option in the peering settings. Without this, the instance in VPC-A has no route to the internal IP of the instance in VPC-B, even if firewall rules allow the traffic.
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 rules are not applied to the correct tags
Why it's wrong here
The scenario says firewall rules allow traffic, so this is unlikely.
- ✗
The subnet IP ranges overlap
Why it's wrong here
If peering is active, subnets do not overlap.
- ✗
The instance in VPC-A does not have a default route
Why it's wrong here
Default routes are not needed for peering.
- ✓
Custom routes are not being exchanged between the VPCs
Why this is correct
By default, custom routes are not exchanged; the export/import custom routes flag must be enabled.
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
The trap here is that candidates assume an active peering status guarantees full route exchange, but Cisco tests the nuance that custom routes are not shared by default, requiring explicit configuration to enable them.
Trap categories for this question
Scenario analysis trap
The scenario says firewall rules allow traffic, so this is unlikely.
Detailed technical explanation
How to think about this question
VPC Network Peering uses the concept of route exchange: subnet routes are automatically exchanged, but custom routes (including static routes, dynamic routes from Cloud Router, and routes from VPN tunnels) require explicit export. This is controlled via the 'export custom routes' flag on each peering direction. In a real-world scenario, if VPC-A has a custom route to an on-premises network via VPN, that route will not be propagated to VPC-B unless custom route export is enabled, causing connectivity failures that mimic a peering issue.
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.
Visual reference
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.
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Designing, Planning, and Prototyping a GCP Network — study guide chapter
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FAQ
Questions learners often ask
What does this PCNE question test?
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: Custom routes are not being exchanged between the VPCs — When VPC Network Peering is active but traffic fails, the most common cause is that custom routes are not being exchanged. By default, only subnet routes are exchanged; custom routes (e.g., static routes or routes from VPNs) are not shared unless explicitly configured using the 'export custom routes' option in the peering settings. Without this, the instance in VPC-A has no route to the internal IP of the instance in VPC-B, even if firewall rules allow the traffic.
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.
About these practice questions
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Last reviewed: Jul 4, 2026
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.
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