The answer is that firewall rules blocking the health check probe IP ranges are the most likely cause. This happens because Google Cloud load balancers send health checks from specific, documented probe IP ranges, not from the load balancer’s frontend IP. Even if your instances are healthy and serving traffic on port 80 when accessed directly, a firewall rule that denies traffic from those probe IPs will cause the load balancer to mark the instances as unhealthy, as the probes never reach the backend. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how load balancer health checks differ from direct user traffic—a common trap is assuming that if the instance responds on port 80, the health check must also succeed. To avoid this, always verify that your firewall rules explicitly allow the health check probe IP ranges. A useful memory tip: “Probes come from the cloud, not the client—check the source range, not the frontend IP.”
PCD Practice Question: Designing highly scalable, available, and reliable cloud-native applications
This PCD practice question tests your understanding of designing highly scalable, available, and reliable cloud-native applications. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
The developer runs the command above and sees both instances are unhealthy. The instances are running and serving traffic on port 80 when accessed directly. 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
✓
Firewall rules block the health check probe IP ranges
The most likely cause is that firewall rules are blocking the health check probe IP ranges. Google Cloud Platform (GCP) load balancers use specific, documented IP ranges for health check probes. If a firewall rule denies traffic from these ranges, the load balancer will mark the instances as unhealthy even though the instances are running and serving traffic on port 80 when accessed directly. This is a common misconfiguration because the health check probes originate from these special IP ranges, not from the load balancer's frontend IP.
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.
✓
Firewall rules block the health check probe IP ranges
Why this is correct
Health check probes originate from Google's health checker IP ranges; they must be allowed in firewall rules.
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 have been deleted
Why it's wrong here
The ipAddresses appear in the output, so instances exist.
✗
The instances are not running the specified health check port
Why it's wrong here
They are listening on port 80, which matches the health check port.
✗
The load balancer is misconfigured
Why it's wrong here
The load balancer configuration is correct enough to query health status.
✗
The instances are out of memory and unable to respond
Why it's wrong here
If out of memory, they likely would not respond to direct access either.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that health checks originate from the load balancer's frontend IP or that the instance's direct accessibility implies it will pass health checks, ignoring that health check probes come from specific, separate IP ranges that must be explicitly allowed in firewall rules.
Trap categories for this question
Command / output trap
The ipAddresses appear in the output, so instances exist.
Detailed technical explanation
How to think about this question
GCP health check probes use source IP addresses in the ranges 35.191.0.0/16 and 130.211.0.0/22 for HTTP/HTTPS/SSL health checks. These probes are sent from Google's infrastructure, not from the load balancer's frontend IP. If a firewall rule (e.g., a VPC firewall rule or a guest OS firewall like iptables) does not explicitly allow traffic from these ranges on the health check port, the probe packets are dropped. The instance's application may be healthy, but the load balancer never receives a response to its health check, causing it to mark the instance as unhealthy. This is a subtle but critical distinction: the health check path is separate from the user traffic path.
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.
Designing highly scalable, available, and reliable cloud-native applications — This question tests Designing highly scalable, available, and reliable cloud-native applications — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Firewall rules block the health check probe IP ranges — The most likely cause is that firewall rules are blocking the health check probe IP ranges. Google Cloud Platform (GCP) load balancers use specific, documented IP ranges for health check probes. If a firewall rule denies traffic from these ranges, the load balancer will mark the instances as unhealthy even though the instances are running and serving traffic on port 80 when accessed directly. This is a common misconfiguration because the health check probes originate from these special IP ranges, not from the load balancer's frontend IP.
What should I do if I get this PCD 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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