The answer is a missing or misconfigured firewall rule that does not include the required health check source IP ranges, specifically 35.191.0.0/16 and 130.211.0.0/22. When configuring an internal load balancer health check firewall, GCP probes originate from these external ranges, not from within the VPC, so the firewall must explicitly allow ingress on the health check port (8080) from those IPs. If the rule only permits traffic from internal VPC subnets, the health checks will fail even though the application is running fine. This scenario tests your understanding of the Google Professional Cloud Network Engineer exam’s focus on separating health check firewall rules from general internal traffic—a common trap where engineers assume health check probes come from the same network as the backend. A reliable memory tip is “Health checks come from Google’s cloud, not your VPC,” so always verify the source ranges in your firewall rules.
PCNE Configuring network services Practice Question
This PCNE practice question tests your understanding of configuring network services. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 organization has an internal HTTP load balancer (ILB) in us-central1. The backend service is a managed instance group with a health check on port 8080. Recently, some instances are reported as unhealthy despite the application running fine. 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 firewall rule allowing health check probes does not include the required source IP ranges (e.g., 35.191.0.0/16).
Option A is correct because the firewall rule for health check sources must allow traffic from the GCP health check IP ranges (35.191.0.0/16, 130.211.0.0/22). If only allowing traffic from the VPC, health checks fail. Option B is wrong because the load balancer does not interfere with health check source IPs. Option C is wrong because preemptible VMs are still healthy if app runs. Option D is wrong because the health check firewall rule must be separate from VPC internal traffic.
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 health check firewall rule is configured for the same source range as the VPC internal traffic.
Why it's wrong here
Health check sources are Google IPs, not internal VPC IPs.
✗
The internal load balancer is using a proxy protocol which changes the health check source IP.
The firewall rule allowing health check probes does not include the required source IP ranges (e.g., 35.191.0.0/16).
Why this is correct
Health check probes come from Google's probe IPs, not from within the VPC.
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 instances are preemptible and become unhealthy after 24 hours.
Why it's wrong here
Preemptible VMs can run up to 24h but are not marked unhealthy by the app.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
Got this wrong? Here's your next step.
Identify which PCNE exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Configuring network services — This question tests Configuring network services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The firewall rule allowing health check probes does not include the required source IP ranges (e.g., 35.191.0.0/16). — Option A is correct because the firewall rule for health check sources must allow traffic from the GCP health check IP ranges (35.191.0.0/16, 130.211.0.0/22). If only allowing traffic from the VPC, health checks fail. Option B is wrong because the load balancer does not interfere with health check source IPs. Option C is wrong because preemptible VMs are still healthy if app runs. Option D is wrong because the health check firewall rule must be separate from VPC internal traffic.
What should I do if I get this PCNE question wrong?
Identify which PCNE exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company has deployed an HTTP load balancer with a backend service configured to use an unmanaged instance group. Users report that traffic is not reaching the backend instances. The backend instances are healthy and have proper firewall rules allowing traffic from the load balancer. What step should the network engineer take to resolve the issue?
easy
✓ A.Configure a firewall rule to allow health check probes from the load balancer's health check ranges.
B.Enable HTTP health check on the backend service.
C.Assign an external IP address to each backend instance.
D.Add a route for the load balancer's IP range.
Why A: The most likely issue is that firewall rules are not allowing health check probes from the load balancer's health check ranges. Option C is correct because health check probes must be allowed by a firewall rule for the load balancer to consider instances healthy. Option A is unnecessary because the load balancer VIP is not a destination route; Option B is not needed if health checks are already configured; Option D would give instances external IPs, defeating the purpose of internal load balancing.
Last reviewed: Jun 24, 2026
Question Discussion
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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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