- A
The subnet has overlapping IP with VPC A
Why wrong: Peering does not allow overlapping subnets; the peering would not be established.
- B
The subnet in VPC B is in a different region
Why wrong: Peering works across regions.
- C
Export custom routes is not enabled on the VPC A side
Why wrong: The issue is likely on the VPC B side because VPC A is sending traffic to VPC B.
- D
Export custom routes is not enabled on the VPC B side
VPC B must export custom routes to VPC A for VPC A to see the subnet route.
Question 335 of 1,000
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 two VPCs. The peering is established, but traffic from VPC A to VPC B is not reaching a specific subnet. Both VPCs have custom routes. 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
✓
Export custom routes is not enabled on the VPC B side
A common issue is that the subnet's custom routes are not exported. VPC peering allows exporting custom routes, but this must be explicitly enabled. If not, only system-generated routes (subnet routes) are exchanged.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 subnet has overlapping IP with VPC A
Why it's wrong here
Peering does not allow overlapping subnets; the peering would not be established.
- ✗
The subnet in VPC B is in a different region
Why it's wrong here
Peering works across regions.
- ✗
Export custom routes is not enabled on the VPC A side
Why it's wrong here
The issue is likely on the VPC B side because VPC A is sending traffic to VPC B.
- ✓
Export custom routes is not enabled on the VPC B side
Why this is correct
VPC B must export custom routes to VPC A for VPC A to see the subnet route.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related PCNE subnetting questions on CIDR, address ranges, and subnet selection.
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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 — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Export custom routes is not enabled on the VPC B side — A common issue is that the subnet's custom routes are not exported. VPC peering allows exporting custom routes, but this must be explicitly enabled. If not, only system-generated routes (subnet routes) are exchanged.
What should I do if I get this PCNE question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related PCNE subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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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