- A
The ALB's idle timeout is set too low.
Why wrong: A low idle timeout would cause connection drops, not a drop in healthy host count.
- B
The security group for the EC2 instances does not allow inbound traffic from the ALB.
Why wrong: This would cause health checks to fail with no response, but the healthy host count would drop to zero only if all instances are affected; however, the more likely first step is a health check path issue.
- C
The EC2 instances are in an Auto Scaling group with insufficient capacity.
Why wrong: Insufficient capacity would reduce the number of instances, but the healthy host count would not drop to zero if some instances are running.
- D
The health check path is misconfigured and returns HTTP 404.
A 404 response causes the ALB to mark the instance as unhealthy, reducing the healthy host count.
Quick Answer
The answer is a misconfigured health check path that returns HTTP 404. This is correct because the ALB relies on health checks to determine target availability; if the health check path points to a non-existent endpoint, each target returns a non-200 status, causing the healthy host count to drop to zero and triggering 503 errors. On the AWS Certified Solutions Architect Professional SAP-C02 exam, this scenario tests your understanding of how ALB health checks interact with target group registration—a common trap is assuming scaling or capacity issues cause the drop, but the key clue is the periodic, not sustained, nature of the zero count. Remember that a 503 error with zero healthy hosts almost always points to a health check misconfiguration, not a load or network problem. Memory tip: “404 on the path, 503 on the math”—if the health check path returns a 404, the ALB math says zero healthy hosts.
SAP-C02 Continuous Improvement for Existing Solutions Practice Question
This SAP-C02 practice question tests your understanding of continuous improvement for existing solutions. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 runs a web application on EC2 instances behind an ALB. Users report intermittent 503 errors during peak hours. The metrics show the target group's healthy host count drops to zero periodically. 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 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 health check path is misconfigured and returns HTTP 404.
The ALB sends requests to targets based on health checks. If the health check path returns a non-200 status, the target is marked unhealthy. A misconfigured health check path (e.g., pointing to a non-existent endpoint) can cause all instances to be marked unhealthy, leading to 503 errors. Option B is correct because it directly affects health check status. Options A, C, and D would not cause the healthy host count to drop to zero.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 ALB's idle timeout is set too low.
Why it's wrong here
A low idle timeout would cause connection drops, not a drop in healthy host count.
- ✗
The security group for the EC2 instances does not allow inbound traffic from the ALB.
Why it's wrong here
This would cause health checks to fail with no response, but the healthy host count would drop to zero only if all instances are affected; however, the more likely first step is a health check path issue.
- ✗
The EC2 instances are in an Auto Scaling group with insufficient capacity.
Why it's wrong here
Insufficient capacity would reduce the number of instances, but the healthy host count would not drop to zero if some instances are running.
- ✓
The health check path is misconfigured and returns HTTP 404.
Why this is correct
A 404 response causes the ALB to mark the instance as unhealthy, reducing the healthy host count.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
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
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SAP-C02 NAT questions on configuration and troubleshooting.
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Continuous Improvement for Existing Solutions — study guide chapter
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FAQ
Questions learners often ask
What does this SAP-C02 question test?
Continuous Improvement for Existing Solutions — This question tests Continuous Improvement for Existing Solutions — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The health check path is misconfigured and returns HTTP 404. — The ALB sends requests to targets based on health checks. If the health check path returns a non-200 status, the target is marked unhealthy. A misconfigured health check path (e.g., pointing to a non-existent endpoint) can cause all instances to be marked unhealthy, leading to 503 errors. Option B is correct because it directly affects health check status. Options A, C, and D would not cause the healthy host count to drop to zero.
What should I do if I get this SAP-C02 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SAP-C02 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 20, 2026
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