- A
Remove the new subnet range (10.0.10.0/24) from the custom advertised routes on the Cloud Router associated with the Cloud NAT.
The Cloud Router is advertising that subnet to on-premises, causing Cloud NAT to think traffic for that subnet should be sent through the VPN, not NATed.
- B
Add a static route for 0.0.0.0/0 with next-hop set to the Cloud NAT gateway's IP address in the VPC.
Why wrong: The default route already points to the internet gateway; Cloud NAT intercepts traffic. Adding a static route could break routing further.
- C
Update the VPC firewall rules to explicitly allow egress traffic from the new subnet's IP range to 0.0.0.0/0.
Why wrong: Firewall rules are not blocking egress; other subnets in the same project work, and the team already verified firewall rules allow egress.
- D
Reserve static NAT IPs for the new subnet in the Cloud NAT configuration to ensure that the VMs have a consistent egress IP.
Why wrong: The issue is not about NAT IP reservations; ephemeral IPs are working for other subnets.
Quick Answer
The answer is to remove the new subnet range (10.0.10.0/24) from the custom advertised routes on the Cloud Router associated with the Cloud NAT. This is because when Cloud NAT uses a Cloud Router that advertises custom IP ranges via BGP to an on-premises router, the on-premises router can respond with a more specific route for that subnet, causing the VMs to prefer the on-premises path over the default 0.0.0.0/0 route for internet traffic, which blackholes the egress. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of how Cloud NAT interacts with dynamic routing and the subtle trap that custom advertised routes can override NAT behavior even when firewall rules and NAT IPs are correct. A common memory tip is: “If a new subnet can’t reach the internet but others can, check your custom BGP advertisements—don’t let on-premises steal your default route.”
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. 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 large multinational corporation uses a Shared VPC in Google Cloud with multiple service projects. They have a central Cloud NAT configured in the host project in the us-central1 region to provide internet egress for all VMs. Recently, the IT team added a new subnet (10.0.10.0/24) in a service project and deployed VMs there. All other VMs in the same project but in different subnets (e.g., 10.0.1.0/24) can reach the internet, but the new VMs in 10.0.10.0/24 cannot. The Cloud NAT gateway is configured in us-central1 with all IP ranges allowed. The VPC firewall rules allow egress traffic to the internet. The team verified that the VMs have a default route (0.0.0.0/0) with next-hop 'default-internet-gateway' and that the Cloud NAT router's NAT IPs are properly assigned. However, the new subnet's VMs are unable to connect to any external IP. The network engineer suspects that the Cloud NAT's NAT reservations might be the issue, but all NAT IPs are ephemeral. Further investigation shows that the Cloud Router used by Cloud NAT is advertising custom IP ranges via BGP to an on-premises router for a different use case. What is the most likely cause and 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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Remove the new subnet range (10.0.10.0/24) from the custom advertised routes on the Cloud Router associated with the Cloud NAT.
The Cloud Router associated with Cloud NAT is advertising custom IP ranges via BGP to an on-premises router. If the new subnet range (10.0.10.0/24) is included in those custom advertised routes, the on-premises router may advertise a more specific route back to the VPC, causing the VMs in that subnet to prefer the on-premises route over the default route (0.0.0.0/0) for internet-bound traffic. Since the on-premises router does not have internet access, the traffic is blackholed. Removing the subnet range from the custom advertised routes on the Cloud Router resolves the issue by ensuring the default route remains the preferred path for internet egress.
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.
- ✓
Remove the new subnet range (10.0.10.0/24) from the custom advertised routes on the Cloud Router associated with the Cloud NAT.
Why this is correct
The Cloud Router is advertising that subnet to on-premises, causing Cloud NAT to think traffic for that subnet should be sent through the VPN, not NATed.
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.
- ✗
Add a static route for 0.0.0.0/0 with next-hop set to the Cloud NAT gateway's IP address in the VPC.
Why it's wrong here
The default route already points to the internet gateway; Cloud NAT intercepts traffic. Adding a static route could break routing further.
- ✗
Update the VPC firewall rules to explicitly allow egress traffic from the new subnet's IP range to 0.0.0.0/0.
Why it's wrong here
Firewall rules are not blocking egress; other subnets in the same project work, and the team already verified firewall rules allow egress.
- ✗
Reserve static NAT IPs for the new subnet in the Cloud NAT configuration to ensure that the VMs have a consistent egress IP.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often focus on Cloud NAT configuration or firewall rules, overlooking how BGP custom route advertisements from the Cloud Router can inject more specific routes that override the default route and cause asymmetric routing or blackholing.
Detailed technical explanation
How to think about this question
Cloud NAT uses a Cloud Router to manage NAT IP allocation and to support BGP-based custom route advertisements. When a Cloud Router advertises a subnet range via BGP to an on-premises router, the on-premises router may inject a more specific route (e.g., 10.0.10.0/24) into the VPC, which takes precedence over the default route (0.0.0.0/0) due to longest prefix match. This causes traffic from the new subnet to be forwarded to the on-premises router instead of the internet gateway, breaking internet egress. This behavior is consistent with RFC 4271 BGP route selection and Google Cloud's dynamic routing rules.
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.
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FAQ
Questions learners often ask
What does this PCNE question test?
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: Remove the new subnet range (10.0.10.0/24) from the custom advertised routes on the Cloud Router associated with the Cloud NAT. — The Cloud Router associated with Cloud NAT is advertising custom IP ranges via BGP to an on-premises router. If the new subnet range (10.0.10.0/24) is included in those custom advertised routes, the on-premises router may advertise a more specific route back to the VPC, causing the VMs in that subnet to prefer the on-premises route over the default route (0.0.0.0/0) for internet-bound traffic. Since the on-premises router does not have internet access, the traffic is blackholed. Removing the subnet range from the custom advertised routes on the Cloud Router resolves the issue by ensuring the default route remains the preferred path for internet egress.
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.
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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Last reviewed: Jun 25, 2026
This PCNE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCNE exam.
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