Quick Answer
The answer is that the firewall is blocking the uptime check IP ranges, which is the most likely cause of periodic failures on a healthy external HTTP endpoint. Cloud Monitoring uses a specific set of source IP ranges to perform its probes, and if these ranges are not explicitly whitelisted in your firewall rules, the checks will fail even when your service is running perfectly. On the Google Professional Cloud Developer exam, this tests your understanding of how monitoring infrastructure interacts with network security—a common trap is assuming the issue is with the application itself rather than the monitoring probes. The key is to remember that uptime checks originate from Google’s infrastructure, not from your VPC, so you must allow those ranges. Memory tip: think “probes need passes”—your firewall must pass the probe IPs before it can pass the test.
This PCD practice question tests your understanding of managing application performance monitoring. 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 company uses Cloud Monitoring to create an uptime check for their external HTTP endpoint. The check fails periodically even though the service is healthy. 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.
The service is down during those periods
Why wrong: The service is reported healthy.
The SSL certificate is expired
Why wrong: An expired certificate would cause persistent failure, not intermittent.
The firewall is blocking the uptime check IP ranges
Uptime checks come from specific Google IP ranges that must be allowed.
The endpoint has high latency
Why wrong: High latency may cause timeout but not always failure.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The firewall is blocking the uptime check IP ranges
The most likely cause is that the firewall is blocking the uptime check IP ranges. Cloud Monitoring uses specific source IP ranges for its uptime checks, and if these are not explicitly allowed through the firewall, the checks will fail even though the service itself is healthy. This is a common configuration issue where the firewall rules are not updated to include the monitoring system's probe IPs.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The service is down during those periods
Why it's wrong here
The service is reported healthy.
The SSL certificate is expired
Why it's wrong here
An expired certificate would cause persistent failure, not intermittent.
The firewall is blocking the uptime check IP ranges
Why this is correct
Uptime checks come from specific Google IP ranges that must be allowed.
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 endpoint has high latency
Why it's wrong here
High latency may cause timeout but not always failure.
Common exam traps
The trap here is that candidates often assume an expired SSL certificate is the cause, but the periodic nature of the failure points to a network filtering issue rather than a consistent certificate problem.
Detailed technical explanation
Cloud Monitoring uptime checks are sent from a set of global IP ranges that are documented in the Google Cloud documentation. These IPs must be allowed through any firewall or network security group that protects the endpoint. If the firewall only allows traffic from certain IPs (e.g., corporate VPN) and not the monitoring IPs, the checks will fail intermittently as the probes are blocked. This is a common misconfiguration in hybrid or on-premises environments where the firewall rules are not updated to include the monitoring source ranges.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
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
Managing application performance monitoring — This question tests Managing application performance monitoring — Read the scenario before looking for a memorised answer..
The correct answer is: The firewall is blocking the uptime check IP ranges — The most likely cause is that the firewall is blocking the uptime check IP ranges. Cloud Monitoring uses specific source IP ranges for its uptime checks, and if these are not explicitly allowed through the firewall, the checks will fail even though the service itself is healthy. This is a common configuration issue where the firewall rules are not updated to include the monitoring system's probe IPs.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
This PCD 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 PCD exam.
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