The answer is a custom mode VPC with regional subnets. This configuration is correct because it gives you explicit control over subnet placement, allowing you to create subnets only in the specific regions where your instances reside, which ensures that all intra-region traffic stays within that regional boundary and uses low-latency local paths without ever crossing to another region. On the Google Professional Cloud Network Engineer exam, this question tests your understanding of the fundamental difference between auto mode VPCs, which automatically create a subnet in every region and can inadvertently route traffic across regions, and custom mode VPCs, which require manual subnet creation and are the only choice for latency-sensitive, regionally isolated designs. A common trap is assuming auto mode is simpler for multi-region setups, but it forces cross-region subnets that add unnecessary latency. Memory tip: think “Custom for Control” — custom mode lets you choose exactly where subnets live, keeping traffic local and fast.
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. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 network engineer is designing a VPC in Google Cloud with multiple subnets across different regions. The application requires low-latency communication between instances in the same region but not across regions. Which VPC network configuration should be used?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Custom mode VPC with regional subnets
Custom mode VPC with regional subnets (D) is correct because it allows you to explicitly define subnets in specific regions, ensuring that instances within the same region communicate over low-latency paths without cross-region traffic. This design avoids the automatic creation of subnets in every region (as in auto mode) and prevents the use of deprecated legacy networks, giving you full control over regional placement for latency-sensitive applications.
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.
✗
Auto mode VPC with global subnets
Why it's wrong here
Auto mode creates subnets in every region, which may introduce unnecessary cross-region traffic.
✗
Legacy network
Why it's wrong here
Legacy networks are deprecated and not recommended for new designs.
✗
Auto mode VPC with regional subnets
Why it's wrong here
Auto mode does not allow you to choose which regions to include; it creates subnets in all regions.
✓
Custom mode VPC with regional subnets
Why this is correct
Custom mode allows you to create subnets only in required regions, reducing complexity and latency.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that auto mode VPCs can be configured with regional subnets, but in reality, auto mode automatically creates subnets in every region, and only custom mode gives you the granularity to define subnets per region for low-latency designs.
Detailed technical explanation
How to think about this question
Under the hood, a custom mode VPC uses a /16 or smaller CIDR block for the network, and each regional subnet is defined with its own CIDR range (e.g., /24) that must not overlap. This allows you to design a non-contiguous IP space optimized for regional traffic, leveraging Google's Andromeda SDN to keep east-west traffic within a region's data center fabric, avoiding inter-region latency that can exceed 10 ms. In a real-world scenario, a financial trading application requiring sub-millisecond latency would use custom mode VPC with regional subnets to ensure all compute instances in us-central1 communicate over the same virtual switch, without routing through a global backbone.
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.
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 mode VPC with regional subnets — Custom mode VPC with regional subnets (D) is correct because it allows you to explicitly define subnets in specific regions, ensuring that instances within the same region communicate over low-latency paths without cross-region traffic. This design avoids the automatic creation of subnets in every region (as in auto mode) and prevents the use of deprecated legacy networks, giving you full control over regional placement for latency-sensitive applications.
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.
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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