Quick Answer
The answer is to enable cross-zone load balancing on the ALB. This configuration allows the load balancer to distribute incoming traffic evenly across all registered instances in every enabled Availability Zone, regardless of which AZ the traffic originally arrives in. Without it, each AZ’s nodes only route traffic to instances within that same AZ, meaning a full AZ failure would strand traffic destined for that subnet, even if healthy instances exist in another AZ. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this question tests your understanding of high-availability architecture and the subtle but critical distinction between AZ-level and instance-level routing. A common trap is assuming that simply adding more AZs or enabling health checks guarantees failover; in reality, cross-zone balancing is the key to distributing load across surviving AZs. Memory tip: think of cross-zone as “crossing the zone boundary” to reach any healthy target—without it, traffic stays stuck in its own AZ silo.
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 uses an Application Load Balancer (ALB) to distribute traffic to EC2 instances. The ALB is in us-east-1a and us-east-1b. They want to ensure that if one AZ fails, traffic is routed only to healthy instances in the other AZ. What configuration is necessary?
Enable sticky sessions (session affinity)
Why wrong: Does not help with failover.
Configure health checks on the target group
Why wrong: Necessary but not sufficient for AZ failover.
Add more subnets in additional AZs
Why wrong: Does not solve failover without cross-zone balancing.
Enable cross-zone load balancing on the ALB
Allows traffic to be routed to healthy instances in any AZ.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Enable cross-zone load balancing on the ALB
Cross-zone load balancing must be enabled on the ALB so that traffic can be distributed across instances in all AZs. If disabled, each AZ receives traffic only from its own subnet. Option B is wrong because health checks are already required. Option C is wrong because stickiness doesn't affect failover. Option D is wrong because enabling more AZs helps but without cross-zone balancing, traffic may not be evenly distributed.
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
For each option: why learners choose it and why it is or isn't the right answer here.
Enable sticky sessions (session affinity)
Why it's wrong here
Does not help with failover.
Configure health checks on the target group
Why it's wrong here
Necessary but not sufficient for AZ failover.
Add more subnets in additional AZs
Why it's wrong here
Does not solve failover without cross-zone balancing.
Enable cross-zone load balancing on the ALB
Why this is correct
Allows traffic to be routed to healthy instances in any AZ.
Related concept
CIDR notation defines the prefix length.
Common exam traps
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
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.
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
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
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.
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FAQ
Resilient Cloud Solutions — This question tests Resilient Cloud Solutions — CIDR notation defines the prefix length..
The correct answer is: Enable cross-zone load balancing on the ALB — Cross-zone load balancing must be enabled on the ALB so that traffic can be distributed across instances in all AZs. If disabled, each AZ receives traffic only from its own subnet. Option B is wrong because health checks are already required. Option C is wrong because stickiness doesn't affect failover. Option D is wrong because enabling more AZs helps but without cross-zone balancing, traffic may not be evenly distributed.
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.
CIDR notation defines the prefix length.
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Last reviewed: Jun 20, 2026
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