The answer is to use an Elastic Load Balancer to distribute traffic across instances in multiple Availability Zones, configure the Auto Scaling group across at least three AZs, and enable health checks on the ALB. This combination ensures AZ failure resilience because the load balancer automatically reroutes traffic away from a failed AZ, while the Auto Scaling group replaces unhealthy instances in the remaining zones. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of high-availability architecture patterns, often appearing as a multi-select question where a single instance per AZ is a common trap—it fails if that one AZ goes down. Remember, resilience requires redundancy across zones, not just one instance per zone. A useful memory tip: think "three zones, one balancer, health checks on" to lock in the three actions for AZ failure resilience.
DOP-C02 Resilient Cloud Solutions Practice Question
This DOP-C02 practice question tests your understanding of resilient cloud solutions. 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 critical application on AWS that uses an Auto Scaling group of EC2 instances. The application must remain available even if an entire Availability Zone fails. Which THREE actions should the company take?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure an ALB health check to automatically replace unhealthy instances.
Options A, B, and D are correct. Using multiple AZs ensures AZ failure tolerance; an ELB distributes traffic across AZs; an ALB health check ensures unhealthy instances are replaced. Option C is wrong because a single instance in each AZ is not resilient to AZ failure of one AZ. Option E is wrong because a single subnet per AZ is sufficient; multiple subnets per AZ are not required for resilience.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Configure an ALB health check to automatically replace unhealthy instances.
Why this is correct
Health checks ensure replacement of failed instances.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Resilient Cloud Solutions — This question tests Resilient Cloud Solutions — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Configure an ALB health check to automatically replace unhealthy instances. — Options A, B, and D are correct. Using multiple AZs ensures AZ failure tolerance; an ELB distributes traffic across AZs; an ALB health check ensures unhealthy instances are replaced. Option C is wrong because a single instance in each AZ is not resilient to AZ failure of one AZ. Option E is wrong because a single subnet per AZ is sufficient; multiple subnets per AZ are not required for resilience.
What should I do if I get this DOP-C02 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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