The answer is to configure the VPC peering connection to export custom routes from your VPC to the service producer VPC. This is the most likely cause of the Cloud SQL private IP timeout after VPC peering because, by default, VPC peering does not advertise your subnet routes (10.0.1.0/24 and 10.0.2.0/24) to the Google-managed VPC hosting Cloud SQL. Without these routes, the Cloud SQL instance at 10.0.3.5 cannot send response traffic back to the web instances, breaking the TCP handshake and causing the connection to hang. On the Google Professional Cloud Security Engineer exam, this scenario tests your understanding of asymmetric routing in peered networks and the critical distinction between importing and exporting routes—a common trap is assuming an ACTIVE peering status guarantees bidirectional traffic. Remember the memory tip: “Peering is a two-way street; export your routes so Cloud SQL can reply.”
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.
Your company is deploying a multi-tier application in a single VPC with two subnets: web (10.0.1.0/24) and db (10.0.2.0/24). The web instances need to connect to a private Cloud SQL instance (MySQL) that is provisioned in a service project. The Cloud SQL instance has a private IP address 10.0.3.5 assigned using private services access. You have established VPC peering between your VPC and the service producer VPC (the Google-managed VPC hosting Cloud SQL). You verified that the peering connection is in 'ACTIVE' state. The web instances can reach internet sites, but connections to the Cloud SQL instance (using the MySQL client) are timing out. The db instances do not need to connect to Cloud SQL. What is the most likely cause and recommended solution?
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
✓
Configure the VPC peering connection to export custom routes from your VPC to the service producer VPC.
The web instances are timing out when connecting to the Cloud SQL private IP (10.0.3.5) because the VPC peering connection is not exporting custom routes from your VPC to the service producer VPC. By default, VPC peering does not export custom routes (including the subnet routes for 10.0.1.0/24 and 10.0.2.0/24) unless explicitly configured. Without these routes, the Cloud SQL instance cannot send response traffic back to the web instances, causing the TCP handshake to fail (SYN sent, SYN-ACK never received). Enabling 'Export custom routes' on the peering connection from your VPC to the service producer VPC resolves this by advertising your subnet routes to the Cloud SQL host VPC.
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.
✓
Configure the VPC peering connection to export custom routes from your VPC to the service producer VPC.
Why this is correct
Exporting custom routes ensures the service VPC knows how to reach your subnets for return traffic.
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.
✗
Re-create the VPC peering connection because it might be misconfigured.
Why it's wrong here
The peering is active; recreating it without changes won't fix the route export issue.
✗
Change the Cloud SQL instance to use a public IP and allowlist your web subnet.
Why it's wrong here
Using a public IP defeats the purpose of private connectivity and reduces security.
✗
Set up Cloud NAT for the web subnet to enable outbound connections.
Why it's wrong here
Cloud NAT is for outbound internet; the Cloud SQL private IP is not internet-routable.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that an 'ACTIVE' peering status guarantees full connectivity, but the trap here is that route exchange is not automatic for custom routes — candidates overlook the need to explicitly export custom routes for return traffic.
Detailed technical explanation
How to think about this question
VPC peering in Google Cloud uses a 'route exchange' mechanism where only subnet routes are automatically exchanged by default; custom routes (e.g., static routes, dynamic routes from Cloud Router) are not exported unless the 'Export custom routes' flag is enabled on the peering connection. In this scenario, the web subnet (10.0.1.0/24) is a custom route from the perspective of the service producer VPC, so without exporting it, the Cloud SQL instance's VPC has no path back to the web instances. This is a common issue when using Private Services Access for Cloud SQL, as the service producer VPC must learn the consumer's subnet routes for return traffic to work.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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: Configure the VPC peering connection to export custom routes from your VPC to the service producer VPC. — The web instances are timing out when connecting to the Cloud SQL private IP (10.0.3.5) because the VPC peering connection is not exporting custom routes from your VPC to the service producer VPC. By default, VPC peering does not export custom routes (including the subnet routes for 10.0.1.0/24 and 10.0.2.0/24) unless explicitly configured. Without these routes, the Cloud SQL instance cannot send response traffic back to the web instances, causing the TCP handshake to fail (SYN sent, SYN-ACK never received). Enabling 'Export custom routes' on the peering connection from your VPC to the service producer VPC resolves this by advertising your subnet routes to the Cloud SQL host VPC.
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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