The answer is that the instances in the other availability zones are marked as unhealthy due to failing health checks. When an Application Load Balancer’s health checks fail for targets in a given zone, the ALB automatically deregisters those instances and routes all traffic exclusively to healthy targets in the remaining zone, even if cross-zone load balancing is enabled. On the CompTIA Cloud+ CV0-004 exam, this scenario tests your understanding of how health check status directly overrides load balancing algorithms—a common trap is assuming cross-zone settings alone guarantee even distribution, when in reality unhealthy targets are simply removed from the pool. Remember the memory tip: “Health check failure equals zone isolation,” meaning a failing health check in one zone can collapse all traffic into a single zone, regardless of configuration.
CV0-004 Troubleshooting Practice Question
This CV0-004 practice question tests your understanding of troubleshooting. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 web application is deployed across multiple availability zones behind an application load balancer (ALB). The administrator notices that all traffic is being routed to instances in only one availability zone, causing performance issues. The ALB cross-zone load balancing is enabled. 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 instances in the other zones are marked as unhealthy due to failing health checks.
Option A is correct because if health checks fail, instances are deregistered, and ALB only sends traffic to healthy targets. Option B is wrong because a single target group can have instances from multiple zones. Option C is wrong because security groups are not zone-specific. Option D is wrong because route tables affect instance outbound, not ALB traffic distribution.
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.
✗
The security group for the ALB only allows traffic from one zone.
Why it's wrong here
Security groups are not zone-specific; they filter traffic based on IP addresses, not zones.
✓
The instances in the other zones are marked as unhealthy due to failing health checks.
Why this is correct
Health check failures cause instances to be deregistered, so traffic is sent only to healthy instances in the remaining zone.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The route table for the subnets in the other zones is missing a default route.
Why it's wrong here
Route tables affect instance outbound traffic, not ALB traffic distribution.
✗
The listener rules are configured to forward traffic to a single target group.
Why it's wrong here
A single target group can contain instances from multiple zones; this does not limit to one zone.
Common exam traps
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
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 CV0-004 subnetting questions on CIDR, address ranges, and subnet selection.
Troubleshooting — This question tests Troubleshooting — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The instances in the other zones are marked as unhealthy due to failing health checks. — Option A is correct because if health checks fail, instances are deregistered, and ALB only sends traffic to healthy targets. Option B is wrong because a single target group can have instances from multiple zones. Option C is wrong because security groups are not zone-specific. Option D is wrong because route tables affect instance outbound, not ALB traffic distribution.
What should I do if I get this CV0-004 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 CV0-004 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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