The answer is a healthy threshold set too high, causing instances to be considered unhealthy for too long. With a healthy threshold of 5 and a 30-second health check interval, an instance must pass five consecutive checks—taking 2.5 minutes—to be marked healthy. However, if the application takes longer than 30 seconds to start, the instance initially fails checks, and even after the 300-second grace period expires, it may not have accumulated five passes, so the Auto Scaling group prematurely terminates it while launching a replacement, creating the flapping cycle. On the AWS Certified Developer Associate DVA-C02 exam, this scenario tests your understanding of how health check thresholds interact with grace periods to cause instability; a common trap is assuming the grace period alone prevents flapping, but a high healthy threshold can override it. Remember the memory tip: “Five checks, two and a half minutes—if your app is slow, flapping begins.”
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 production web application on EC2 instances in an Auto Scaling group behind an Application Load Balancer (ALB). The application uses a custom health check endpoint /health that returns HTTP 200 when the application is healthy. Recently, the operations team noticed that the Auto Scaling group is repeatedly launching and terminating instances (flapping) even though the application is stable. The health check grace period is set to 300 seconds. The ALB health check interval is 30 seconds, unhealthy threshold is 2, and healthy threshold is 5. The Auto Scaling group uses the ELB health check type. The application logs show no errors. What is the MOST likely cause of the flapping?
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
✓
The healthy threshold is set too high, causing instances to be considered unhealthy for too long.
Option D is correct. The healthy threshold is 5, meaning an instance must pass 5 consecutive health checks to be considered healthy. With a 30-second interval, it takes 2.5 minutes to become healthy, but the health check grace period is 300 seconds (5 minutes). However, if the instance initially fails health checks because the application takes longer than 30 seconds to start, it will be marked unhealthy. More importantly, the healthy threshold of 5 is high, so after the grace period, the instance may still be considered unhealthy if it has not passed 5 checks. The combination of high healthy threshold and low unhealthy threshold can cause flapping. Option A is wrong because the ALB is not failing. Option B is wrong because scaling policies are not mentioned. Option C is wrong because the instance type is unlikely to cause this.
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 EC2 instance type is not suitable for the application, causing slow responses.
Why it's wrong here
The application is stable, so instance type is not the issue.
✓
The healthy threshold is set too high, causing instances to be considered unhealthy for too long.
Why this is correct
A high healthy threshold (5) means instances take longer to be marked healthy, leading to cycles of termination and launch.
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.
✗
The ALB is experiencing high latency and marking instances unhealthy.
Why it's wrong here
The application logs show no errors, so latency is unlikely.
✗
The Auto Scaling group has a target tracking scaling policy that is too aggressive.
Why it's wrong here
There is no mention of scaling policies causing flapping.
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.
Trap categories for this question
Command / output trap
The application logs show no errors, so latency is unlikely.
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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
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 DVA-C02 NAT questions on configuration and troubleshooting.
Troubleshooting and Optimization — This question tests Troubleshooting and Optimization — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The healthy threshold is set too high, causing instances to be considered unhealthy for too long. — Option D is correct. The healthy threshold is 5, meaning an instance must pass 5 consecutive health checks to be considered healthy. With a 30-second interval, it takes 2.5 minutes to become healthy, but the health check grace period is 300 seconds (5 minutes). However, if the instance initially fails health checks because the application takes longer than 30 seconds to start, it will be marked unhealthy. More importantly, the healthy threshold of 5 is high, so after the grace period, the instance may still be considered unhealthy if it has not passed 5 checks. The combination of high healthy threshold and low unhealthy threshold can cause flapping. Option A is wrong because the ALB is not failing. Option B is wrong because scaling policies are not mentioned. Option C is wrong because the instance type is unlikely to cause this.
What should I do if I get this DVA-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 DVA-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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