The answer is that the web-server's service account lacks the Cloud SQL Client IAM role. While the successful ping confirms Layer 3 connectivity to the Cloud SQL private IP (10.0.1.3), the authentication error reveals that the application reaches the database but cannot complete the IAM-based login required for private IP connections. Cloud SQL enforces IAM authentication for private IP access, meaning the connecting VM’s service account must be granted the Cloud SQL Client role to authenticate; without it, the database rejects the connection despite an open network path. On the Google Professional Cloud Security Engineer exam, this scenario tests your understanding that private IP connectivity alone is insufficient—IAM permissions govern database access, a common trap where candidates focus only on network rules. Remember the mnemonic: “Ping works, but IAM blocks the works”—network reachability does not equal database authentication.
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 small company has a single VPC with subnets in us-central1 (10.0.1.0/24) and us-west1 (10.0.2.0/24). They have a Compute Engine VM (web-server) in us-central1 that needs to connect to a Cloud SQL MySQL instance also in us-central1 using its private IP address 10.0.1.3. The Cloud SQL instance is configured with private IP only and is deployed in the same VPC. The web-server can successfully ping the Cloud SQL private IP (10.0.1.3). However, the application on the web-server fails to connect to the MySQL database with an authentication error. There are no custom firewall rules; only the default VPC firewall rules are in place. What is the most likely cause of the connection failure?
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
✓
The web-server's service account lacks the Cloud SQL Client IAM role.
The web-server can ping the Cloud SQL private IP (10.0.1.3), confirming network connectivity at Layer 3. The authentication error indicates the application is reaching the database but being denied access. Cloud SQL uses IAM for authentication when connecting via private IP; the web-server's service account must have the Cloud SQL Client IAM role to authenticate successfully. Without this role, the connection is rejected even though the network path is open.
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 default-allow-internal firewall rule does not allow TCP port 3306.
Why it's wrong here
The default rule allows all protocols and ports, including TCP 3306.
✓
The web-server's service account lacks the Cloud SQL Client IAM role.
Why this is correct
This role is necessary to authenticate to Cloud SQL; without it, the application fails with a permission error.
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 Cloud SQL instance does not have the public IP address enabled.
Why it's wrong here
Public IP is not required for private IP connectivity.
✗
The Cloud SQL instance is in a different region than the web-server.
Why it's wrong here
Both are in us-central1, as stated in the scenario.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between network connectivity (Layer 3 reachability) and application-layer authentication, leading candidates to incorrectly blame firewall rules or IP configuration when the real issue is missing IAM permissions for Cloud SQL private IP access.
Trap categories for this question
Scenario analysis trap
Both are in us-central1, as stated in the scenario.
Detailed technical explanation
How to think about this question
Cloud SQL with private IP uses IAM database authentication by default, where the client's service account must have the roles/cloudsql.client IAM role to obtain a temporary database password. This role is separate from network permissions; even with full network reachability (ping works), the database server will reject the connection if the service account lacks this role. In contrast, Cloud SQL with public IP uses traditional username/password authentication, which is why this trap is specific to private IP configurations.
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: The web-server's service account lacks the Cloud SQL Client IAM role. — The web-server can ping the Cloud SQL private IP (10.0.1.3), confirming network connectivity at Layer 3. The authentication error indicates the application is reaching the database but being denied access. Cloud SQL uses IAM for authentication when connecting via private IP; the web-server's service account must have the Cloud SQL Client IAM role to authenticate successfully. Without this role, the connection is rejected even though the network path is open.
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.
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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Question Discussion
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