- A
The health check path is incorrect
Why wrong: The path is correctly configured as /health.
- B
The target group's slow start duration is too long
Why wrong: Slow start does not cause health check failures.
- C
The security group for the instances does not allow inbound traffic from the ALB's security group on the health check port
The ALB's health checks must be allowed by the instance security group.
- D
The ALB's stickiness is preventing health checks
Why wrong: Stickiness does not affect health checks.
Question 730 of 1,705
Why ALB Health Checks Fail Despite Healthy Application
This ANS-C01 practice question tests your understanding of network design. 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 has an Application Load Balancer (ALB) in front of an Auto Scaling group of EC2 instances. The ALB is configured with a target group that has a health check path of /health. Some instances are failing health checks and being marked unhealthy, but the application logs show the instances are 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.
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 security group for the instances does not allow inbound traffic from the ALB's security group on the health check port
The most likely cause is that the security group for the EC2 instances does not allow inbound traffic from the ALB's security group on the health check port. ALB health checks originate from the ALB's private IP addresses, and if the instance security group lacks an inbound rule permitting traffic from the ALB's security group (or its CIDR) on the health check port, the health check requests are dropped, causing the target to be marked unhealthy even though the application itself is running fine.
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 path is incorrect
Why it's wrong here
The path is correctly configured as /health.
- ✗
The target group's slow start duration is too long
Why it's wrong here
Slow start does not cause health check failures.
- ✓
The security group for the instances does not allow inbound traffic from the ALB's security group on the health check port
Why this is correct
The ALB's health checks must be allowed by the instance security group.
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 ALB's stickiness is preventing health checks
Why it's wrong here
Stickiness does not affect health checks.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume health check failures always indicate application problems, but The ANS-C01 exam often tests the security group misconfiguration where the ALB's health check traffic is blocked, causing a false unhealthy status despite the application being healthy.
Detailed technical explanation
How to think about this question
ALB health checks are sent from the ALB's own IP addresses (in the VPC subnet) to the target instances on the configured health check port (default 80 or 443). The instance security group must have an inbound rule allowing traffic from the ALB's security group (or the VPC CIDR) on that port; otherwise, the health check packets are silently dropped by the OS network stack, resulting in a 504 or timeout. This is a common misconfiguration because engineers often focus on application-level health but forget the network-level access control.
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 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.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The security group for the instances does not allow inbound traffic from the ALB's security group on the health check port — The most likely cause is that the security group for the EC2 instances does not allow inbound traffic from the ALB's security group on the health check port. ALB health checks originate from the ALB's private IP addresses, and if the instance security group lacks an inbound rule permitting traffic from the ALB's security group (or its CIDR) on the health check port, the health check requests are dropped, causing the target to be marked unhealthy even though the application itself is running fine.
What should I do if I get this ANS-C01 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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Last reviewed: Jul 4, 2026
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