The answer is that the Cloud Router is not in the same VPC network as the private subnet. Cloud NAT silently fails when its associated Cloud Router resides in a different VPC because the NAT gateway relies on that router to establish dynamic BGP routes and advertise the default route for the subnet’s traffic; without this alignment, the router cannot learn the private IP ranges or direct translated packets out through the NAT IP. On the Google Professional Cloud Architect exam, this scenario tests your understanding that Cloud NAT is not a standalone resource—it is tightly coupled to a Cloud Router within the exact same VPC, a detail often overlooked when copying Terraform modules across projects. A common trap is assuming any router in the region will work, but the router must match the VPC network of the subnet needing translation. Memory tip: think of the Cloud Router as the NAT’s “address book”—if it’s in the wrong VPC, it can’t read the addresses.
Google PCA Practice Question: Analyze and optimize technical and business processes
This PCA practice question tests your understanding of analyze and optimize technical and business processes. 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.
Exhibit
Refer to the exhibit.
```
resource "google_compute_router_nat" "nat" {
name = "my-nat"
router = google_compute_router.router.name
region = "us-central1"
nat_ip_allocate_option = "AUTO_ONLY"
source_subnetwork_ip_ranges_to_nat = "LIST_OF_SUBNETWORKS"
subnetworks {
name = google_compute_subnetwork.private.self_link
source_ip_ranges_to_nat = ["PRIMARY_IP_RANGE"]
}
log_config {
enable = true
filter = "ERRORS_ONLY"
}
}
```
A team deployed the Terraform configuration shown in the exhibit. They observe that Cloud NAT is not translating traffic from the private subnet as expected. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Cloud Router is not in the same VPC network as the private subnet
Option A is correct because Cloud NAT requires a Cloud Router to be in the same VPC network as the subnet whose traffic needs translation. If the Cloud Router is in a different VPC network, the NAT gateway cannot establish the necessary BGP sessions or route traffic from the private subnet, causing NAT to fail silently.
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 Cloud Router is not in the same VPC network as the private subnet
Why this is correct
Cloud NAT requires the router to be in the same VPC network as the subnet. If the router is in a different VPC, NAT will not work.
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 log filter is set to ERRORS_ONLY, which suppresses all logs
Why it's wrong here
Logging configuration does not affect NAT functionality.
✗
The NAT IP allocation is manual and no IPs were specified
Why it's wrong here
The configuration uses AUTO_ONLY, so IPs are automatically allocated.
✗
The subnet is not included in the source_subnetwork_ip_ranges_to_nat list
Why it's wrong here
The subnet is specified in the subnetworks block, which overrides the list. So it is included.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the requirement that Cloud Router must be in the same VPC as the NATgateway and the private subnet, tempting candidates to focus on subnet inclusion or log settings instead of the cross-VPC routing dependency.
Detailed technical explanation
How to think about this question
Cloud NAT uses Cloud Router to dynamically learn routes and establish BGP sessions with the NAT gateway. If the Cloud Router is in a different VPC, the NAT gateway cannot resolve the next-hop for private subnet traffic, so packets are dropped without translation. This is distinct from a missing subnet in the NAT configuration, which would produce a different failure mode.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this PCA question in full detail.
Analyze and optimize technical and business processes — This question tests Analyze and optimize technical and business processes — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Cloud Router is not in the same VPC network as the private subnet — Option A is correct because Cloud NAT requires a Cloud Router to be in the same VPC network as the subnet whose traffic needs translation. If the Cloud Router is in a different VPC network, the NAT gateway cannot establish the necessary BGP sessions or route traffic from the private subnet, causing NAT to fail silently.
What should I do if I get this PCA 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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