The answer is VPC peering, as it provides the simplest secure method for cross-network communication between a Compute Engine instance and a Cloud SQL database in the same project but different VPCs. VPC peering establishes a private RFC 1918 connection using internal IP addresses, allowing traffic to flow directly between the two networks without traversing the public internet or requiring a VPN gateway. On the Google Professional Cloud Security Engineer exam, this scenario tests your understanding of network isolation versus connectivity—a common trap is assuming firewall rules alone can bridge separate VPCs, but they cannot; peering is required for layer-3 routing. Another pitfall is overcomplicating the solution with Cloud SQL Proxy, which adds unnecessary overhead for internal connectivity. Remember the memory tip: "Peering pairs private networks; firewalls filter, they don't bridge." This question reinforces that VPC peering is the default, low-latency choice for same-project, cross-VPC communication when security and simplicity are paramount.
PCSE Configuring network security Practice Question
This PCSE practice question tests your understanding of configuring network security. 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 developer needs to allow a specific Compute Engine instance to communicate with a Cloud SQL database instance. Both are in the same project but different VPC networks. What is the simplest secure method?
Create a firewall rule allowing traffic between networks
Why wrong: Firewall rules cannot allow traffic between different VPC networks without a connection like peering or VPN.
B
Use VPC peering between the two networks
VPC peering provides a direct private connection between the two VPCs.
C
Use Cloud SQL Proxy on the instance
Why wrong: Cloud SQL Proxy is used for connecting to Cloud SQL via public IP or private IP, but it requires a service account and is more complex than VPC peering.
D
Grant the instance's service account access to Cloud SQL
Why wrong: IAM permissions control authentication, not network connectivity.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use VPC peering between the two networks
VPC peering is the simplest secure method to allow communication between two VPC networks in the same project. It establishes a private RFC 1918 connection using internal IPs, avoiding the public internet. Firewall rules alone cannot bridge separate VPCs, and Cloud SQL Proxy is unnecessary overhead for internal connectivity.
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.
✗
Create a firewall rule allowing traffic between networks
Why it's wrong here
Firewall rules cannot allow traffic between different VPC networks without a connection like peering or VPN.
✓
Use VPC peering between the two networks
Why this is correct
VPC peering provides a direct private connection between the two VPCs.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use Cloud SQL Proxy on the instance
Why it's wrong here
Cloud SQL Proxy is used for connecting to Cloud SQL via public IP or private IP, but it requires a service account and is more complex than VPC peering.
✗
Grant the instance's service account access to Cloud SQL
Why it's wrong here
IAM permissions control authentication, not network connectivity.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that firewall rules can span VPCs, but in Google Cloud, firewall rules are scoped to a single VPC network and cannot enable cross-network traffic without peering or VPN.
Detailed technical explanation
How to think about this question
VPC peering uses the Google Cloud internal infrastructure to exchange routes between two VPC networks, enabling private IP communication without VPNs or NAT. The peered networks remain administratively separate, so you must still configure firewall rules to allow specific traffic (e.g., TCP 3306 for MySQL). In real-world scenarios, VPC peering is preferred over VPN for low-latency, high-throughput database connections within the same region.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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.
Configuring network security — This question tests Configuring network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use VPC peering between the two networks — VPC peering is the simplest secure method to allow communication between two VPC networks in the same project. It establishes a private RFC 1918 connection using internal IPs, avoiding the public internet. Firewall rules alone cannot bridge separate VPCs, and Cloud SQL Proxy is unnecessary overhead for internal connectivity.
What should I do if I get this PCSE 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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