The correct answer is a health check grace period that is too short combined with an ELB health check type where the load balancer health check is failing. This configuration causes the Auto Scaling group to repeatedly launch and terminate instances because new instances are marked unhealthy before they finish initializing, triggering immediate replacement and creating an endless cycle of instance churn. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of how health check misconfiguration directly drives erratic scaling behavior, often masked by high CPU alarms that result from the constant launch and termination overhead. A common trap is assuming the issue is a faulty AMI or launch template, but the key distinction is that instances are successfully launching—they are just being killed prematurely by the health check system. Memory tip: think of the grace period as a “buffer zone”—if it’s too short, healthy instances get the boot before they can prove themselves.
DOP-C02 Incident and Event Response Practice Question
This DOP-C02 practice question tests your understanding of incident and event response. 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 DevOps engineer notices that an Auto Scaling group is repeatedly launching and terminating instances. CloudWatch alarms show high CPU but the group's metrics are erratic. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The health check grace period is too short and the health check type is ELB, but the load balancer health check is failing.
Option C is correct because incorrect health check configuration can cause the Auto Scaling group to continuously replace instances that are actually healthy, leading to the described behavior. Option A is wrong because insufficient AMI configuration would cause launch failures, not repeated cycling. Option B is wrong because load balancer health checks determine instance health; if they are not configured, instances are considered healthy. Option D is wrong because changing launch template user data would not cause cycling.
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 Auto Scaling group is not associated with a load balancer.
Why it's wrong here
Without a load balancer, Auto Scaling uses EC2 status checks, which typically pass.
✗
The launch template user data is causing instances to fail during boot.
Why it's wrong here
User data failure would cause immediate launch failure, not cycling.
✓
The health check grace period is too short and the health check type is ELB, but the load balancer health check is failing.
Why this is correct
Short grace period and failing ELB health checks cause instances to be terminated and replaced.
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 AMI used in the launch template is not properly configured.
Why it's wrong here
Insufficient AMI configuration would cause launch failures, not repeated cycling.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 DOP-C02 NAT questions on configuration and troubleshooting.
Incident and Event Response — This question tests Incident and Event Response — 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 grace period is too short and the health check type is ELB, but the load balancer health check is failing. — Option C is correct because incorrect health check configuration can cause the Auto Scaling group to continuously replace instances that are actually healthy, leading to the described behavior. Option A is wrong because insufficient AMI configuration would cause launch failures, not repeated cycling. Option B is wrong because load balancer health checks determine instance health; if they are not configured, instances are considered healthy. Option D is wrong because changing launch template user data would not cause cycling.
What should I do if I get this DOP-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 DOP-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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