- A
The ALB is not scaled to handle the traffic.
Why wrong: Scaling would cause 5xx but not specifically health check failures.
- B
The security group for the EC2 instances is not allowing traffic from the ALB.
Health checks fail if security group blocks ALB traffic.
- C
The DNS resolution via Route53 is misconfigured.
Why wrong: DNS issues would affect routing, not health checks.
- D
Sticky sessions are not enabled on the ALB.
Why wrong: Sticky sessions affect session persistence, not health.
Question 371 of 1,616
DVA-C02 Troubleshooting and Optimization Practice Question
This DVA-C02 practice question tests your understanding of troubleshooting and optimization. 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 runs a web application on EC2 instances behind an Application Load Balancer (ALB). Users report intermittent 503 errors. The ALB health checks are failing for a few instances, but the instances themselves are running and have healthy application processes. 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 EC2 instances is not allowing traffic from the ALB.
The ALB health checks are failing despite the instances and application processes being healthy, which indicates a network-level issue. The most likely cause is that the EC2 instances' security group is not allowing inbound traffic from the ALB's security group on the health check port (e.g., HTTP/HTTPS). Without this rule, the ALB cannot reach the health check endpoint, marking the instances as unhealthy and causing intermittent 503 errors when traffic is routed to those instances.
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 ALB is not scaled to handle the traffic.
Why it's wrong here
Scaling would cause 5xx but not specifically health check failures.
- ✓
The security group for the EC2 instances is not allowing traffic from the ALB.
Why this is correct
Health checks fail if security group blocks ALB traffic.
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 DNS resolution via Route53 is misconfigured.
Why it's wrong here
DNS issues would affect routing, not health checks.
- ✗
Sticky sessions are not enabled on the ALB.
Why it's wrong here
Sticky sessions affect session persistence, not health.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume health check failures are always due to application issues (e.g., process crashes) rather than network-layer misconfigurations like security group rules, especially when the instance appears healthy from within the OS.
Detailed technical explanation
How to think about this question
The ALB health check is an HTTP/HTTPS request sent from the ALB's private IP addresses (in the VPC) to the instance's health check path (e.g., /health). The security group on the EC2 instance must have an inbound rule allowing traffic from the ALB's security group (or the ALB's subnet CIDR) on the health check port. If the rule is missing, the health check request is silently dropped, causing the ALB to mark the instance as unhealthy and stop routing traffic to it, leading to 503 errors for users. This is a common misconfiguration when using custom security groups for the ALB and instances.
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
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 exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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Troubleshooting and Optimization — study guide chapter
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FAQ
Questions learners often ask
What does this DVA-C02 question test?
Troubleshooting and Optimization — This question tests Troubleshooting and Optimization — 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 EC2 instances is not allowing traffic from the ALB. — The ALB health checks are failing despite the instances and application processes being healthy, which indicates a network-level issue. The most likely cause is that the EC2 instances' security group is not allowing inbound traffic from the ALB's security group on the health check port (e.g., HTTP/HTTPS). Without this rule, the ALB cannot reach the health check endpoint, marking the instances as unhealthy and causing intermittent 503 errors when traffic is routed to those instances.
What should I do if I get this DVA-C02 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: Jun 24, 2026
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