The correct design is to launch EC2 instances in at least two Availability Zones behind an Application Load Balancer with cross-zone load balancing enabled. This works because cross-zone load balancing allows the ALB to distribute traffic evenly across all healthy instances in every enabled AZ, so when an entire AZ fails, the ALB automatically routes requests solely to the surviving AZs, eliminating a single point of failure at the AZ level. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this concept tests your understanding of high-availability architecture patterns and how ALB’s cross-zone setting directly supports AZ resilience—a common trap is assuming that simply having instances in multiple AZs is enough without enabling cross-zone load balancing, which can cause uneven traffic distribution and resource waste. Remember the mnemonic: “Cross-zone, no alone zone”—always enable cross-zone load balancing to ensure your ALB can survive an AZ outage.
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 is deploying a critical application on Amazon EC2 instances behind an Application Load Balancer (ALB) across multiple Availability Zones. The application must be resilient to the failure of an entire Availability Zone. Which design should the company implement?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Launch EC2 instances in at least two Availability Zones and place them behind an Application Load Balancer with cross-zone load balancing enabled.
Option A is correct because deploying EC2 instances across at least two Availability Zones (AZs) behind an Application Load Balancer (ALB) with cross-zone load balancing enabled ensures that if an entire AZ fails, the ALB can route traffic to healthy instances in the remaining AZs. Cross-zone load balancing allows the ALB to distribute incoming requests evenly across all registered instances in all enabled AZs, which improves fault tolerance and resource utilization. This design meets the requirement for resilience to an AZ failure by eliminating a single point of failure at the AZ level.
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.
✓
Launch EC2 instances in at least two Availability Zones and place them behind an Application Load Balancer with cross-zone load balancing enabled.
Why this is correct
Multiple AZs provide resilience; ALB distributes traffic and performs health checks.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use one EC2 instance in a single Availability Zone behind a Network Load Balancer.
Why it's wrong here
A single instance in one AZ is not resilient to AZ failure.
✗
Launch EC2 instances in one Availability Zone and use an Application Load Balancer to distribute traffic.
Why it's wrong here
One AZ is a single point of failure.
✗
Deploy EC2 instances in two Availability Zones but use a single Application Load Balancer in one AZ.
Why it's wrong here
The ALB is a single point of failure if it is in only one AZ.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume that simply placing instances in multiple AZs behind a load balancer is sufficient, but they overlook the critical requirement that the load balancer itself must be deployed across multiple AZs to avoid being a single point of failure.
Detailed technical explanation
How to think about this question
Under the hood, an ALB with cross-zone load balancing enabled distributes traffic across all healthy targets in all registered AZs, regardless of which AZ the client request originates from. This is achieved by the ALB nodes in each AZ maintaining connections to targets in other AZs, which requires the ALB to have a public or private IP in each AZ. In a real-world scenario, if cross-zone load balancing is disabled, the ALB only distributes traffic to targets in the same AZ as the requesting client, which can lead to uneven load and reduced resilience during an AZ failure.
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.
Resilient Cloud Solutions — This question tests Resilient Cloud Solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Launch EC2 instances in at least two Availability Zones and place them behind an Application Load Balancer with cross-zone load balancing enabled. — Option A is correct because deploying EC2 instances across at least two Availability Zones (AZs) behind an Application Load Balancer (ALB) with cross-zone load balancing enabled ensures that if an entire AZ fails, the ALB can route traffic to healthy instances in the remaining AZs. Cross-zone load balancing allows the ALB to distribute incoming requests evenly across all registered instances in all enabled AZs, which improves fault tolerance and resource utilization. This design meets the requirement for resilience to an AZ failure by eliminating a single point of failure at the AZ level.
What should I do if I get this DOP-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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