- A
The VPC firewall rules are blocking outbound HTTPS traffic from the backend servers.
Why wrong: The team has verified firewall rules allow egress traffic.
- B
The default route (0.0.0.0/0) with next hop 'default internet gateway' preempts the Cloud NAT route.
The default route sends traffic directly to the internet, bypassing Cloud NAT. Cloud NAT requires that the default route have a higher priority (lower number) than the automatically created route for Cloud NAT, or the default route must be removed.
- C
The Cloud NAT gateway is not in the same region as the backend servers.
Why wrong: Cloud NAT is regional and can be used by instances in any zone in that region; the region is likely correct.
- D
The Cloud Router's BGP ASN is not properly configured.
Why wrong: BGP ASN is used for dynamic routing with on-premises networks; not required for Cloud NAT.
Quick Answer
The answer is a routing conflict: the default route (0.0.0.0/0) pointing to the default internet gateway preempts the Cloud NAT route. Cloud NAT only works when outbound traffic is routed through the Cloud Router, which dynamically learns the NAT path; a static default route to the internet gateway has a higher priority (lower numeric value) and sends traffic directly to the internet, bypassing NAT entirely. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of how Cloud NAT relies on the next hop being the Cloud Router, not the internet gateway—a common trap where engineers assume NAT and a default internet gateway can coexist. The key insight is that a default route to the internet gateway always wins over Cloud Router routes, so private instances with only internal IPs fail to reach external services. Memory tip: think of the default internet gateway as a “direct express lane” that skips the NAT tollbooth—if you want NAT, the route must go through the Cloud Router instead.
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing 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 financial services company is deploying a new payment processing application in Google Cloud. The architecture consists of: a VPC named 'payment-vpc' with subnet 'payment-subnet' (10.1.0.0/16), a managed instance group (MIG) of backend servers in payment-subnet, an internal TCP load balancer (ILB) with IP 10.1.0.10 distributing traffic to the MIG, and a Cloud NAT for outbound internet access. The application must communicate with an external payment gateway over TLS. The security policy requires that all outbound traffic from the backend servers to the internet must egress through a single, centralized Cloud NAT instance to allow traffic inspection. To meet this requirement, the network team has configured: a Cloud Router, a Cloud NAT gateway named 'payment-nat' in payment-vpc, and a default route (0.0.0.0/0, next hop: default internet gateway) in payment-vpc. They have also configured VPC firewall rules to allow outbound HTTPS traffic. During testing, the backend servers cannot connect to the external payment gateway. The team has verified that the Cloud NAT is properly configured and that the VPC firewall rules allow egress traffic. What is the most likely cause of the connectivity 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 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
The default route (0.0.0.0/0) with next hop 'default internet gateway' preempts the Cloud NAT route.
The default route (0.0.0.0/0) with next hop 'default internet gateway' directs all outbound internet traffic directly to the internet gateway, bypassing the Cloud NAT gateway. Cloud NAT only applies when the next hop for 0.0.0.0/0 is the Cloud Router (or when no default route to the internet gateway exists), because NAT is performed on packets that are routed through the Cloud Router. Since the default route with next hop 'default internet gateway' has a higher priority (lower numeric value) than any dynamically learned route, it preempts the Cloud NAT path, causing outbound traffic to egress without NAT and thus fail to reach the external payment gateway if the backend servers have only private IPs.
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 VPC firewall rules are blocking outbound HTTPS traffic from the backend servers.
Why it's wrong here
The team has verified firewall rules allow egress traffic.
- ✓
The default route (0.0.0.0/0) with next hop 'default internet gateway' preempts the Cloud NAT route.
Why this is correct
The default route sends traffic directly to the internet, bypassing Cloud NAT. Cloud NAT requires that the default route have a higher priority (lower number) than the automatically created route for Cloud NAT, or the default route must be removed.
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 NAT gateway is not in the same region as the backend servers.
Why it's wrong here
Cloud NAT is regional and can be used by instances in any zone in that region; the region is likely correct.
- ✗
The Cloud Router's BGP ASN is not properly configured.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that Cloud NAT automatically intercepts all outbound traffic regardless of routing, when in fact the default route's next hop must point to the Cloud Router for NAT to apply.
Detailed technical explanation
How to think about this question
In Google Cloud, Cloud NAT is implemented as a regional managed service that uses the Cloud Router to hold the NAT configuration and to enable the VPC to send packets through the NAT gateway. The key behavior is that Cloud NAT only applies when the next hop for the default route (0.0.0.0/0) is the Cloud Router (i.e., a route with next hop 'default internet gateway' is absent or has a higher numeric priority). If a static default route with next hop 'default internet gateway' exists, it takes precedence because static routes have a default priority of 1000, while Cloud Router–learned routes have a priority of 2000; thus, traffic egresses directly via the internet gateway without NAT, breaking connectivity for instances with only private IPs.
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.
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FAQ
Questions learners often ask
What does this PCNE question test?
Implementing network security — This question tests Implementing network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The default route (0.0.0.0/0) with next hop 'default internet gateway' preempts the Cloud NAT route. — The default route (0.0.0.0/0) with next hop 'default internet gateway' directs all outbound internet traffic directly to the internet gateway, bypassing the Cloud NAT gateway. Cloud NAT only applies when the next hop for 0.0.0.0/0 is the Cloud Router (or when no default route to the internet gateway exists), because NAT is performed on packets that are routed through the Cloud Router. Since the default route with next hop 'default internet gateway' has a higher priority (lower numeric value) than any dynamically learned route, it preempts the Cloud NAT path, causing outbound traffic to egress without NAT and thus fail to reach the external payment gateway if the backend servers have only private IPs.
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.
About these practice questions
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Last reviewed: Jun 30, 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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