The answer is that the instances are in a private subnet without a route to the NLB's subnet. This is the most likely cause when NLB is not passing traffic but health checks are passing, because Network Load Balancers preserve the source IP of clients, meaning the targets must have a return path to the NLB’s subnet to complete the TCP three-way handshake. Health checks originate from the NLB’s own subnet and succeed because they do not require a return route from the target—they are sent and received within the NLB’s network space. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of asymmetric routing and the critical difference between health check connectivity and actual traffic flow. A common trap is assuming that passing health checks guarantee full traffic delivery, but the key is that targets must be able to route back to the NLB’s IPs. Memory tip: “Health checks are a one-way street; traffic is a round trip.”
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. 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 sysadmin receives an alert that a Network Load Balancer (NLB) is not passing traffic to targets. The target group health checks are passing. 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 are in a private subnet without a route to the NLB's subnet.
When health checks are passing but traffic is not reaching targets, the issue is typically network connectivity. In this scenario, the instances are in a private subnet without a route to the NLB's subnet, meaning the NLB's IP addresses are unreachable from the targets. Since NLB preserves the source IP of clients, targets must have a route back to the NLB's subnet (or the client) to return traffic; without this, the three-way TCP handshake fails even though health checks (which originate from the NLB's subnet) succeed.
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.
✗
Cross-zone load balancing is disabled.
Why it's wrong here
Disabling cross-zone may cause uneven distribution but not total failure.
✓
The instances are in a private subnet without a route to the NLB's subnet.
Why this is correct
NLB sends traffic directly to instance IPs; if no route back, traffic fails.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The listener is not configured for the correct protocol.
Why it's wrong here
Listener configuration would affect health checks if incorrect.
✗
The target group health check interval is too long.
Why it's wrong here
Health checks passing means interval is not the issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume passing health checks guarantee traffic flow, but NLB's asymmetric routing requirement means targets must have a return path to the client, not just the NLB's health check source IPs.
Detailed technical explanation
How to think about this question
NLB operates at Layer 4 and does not perform NAT or proxy; it forwards packets with the source IP preserved, so targets must have a route back to the client IP (or the NLB's subnet for health checks). Health checks are sent from the NLB's private IPs in its subnet, so they succeed if the target can reach those IPs, but actual client traffic may come from different IPs that the target cannot route to. In a real-world scenario, this often occurs when targets are in a VPC without an Internet Gateway or NAT Gateway, and the NLB is in a public subnet; the targets can respond to health checks (from the NLB's subnet) but cannot return traffic to external clients.
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
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
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.
Networking and Content Delivery — This question tests Networking and Content Delivery — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The instances are in a private subnet without a route to the NLB's subnet. — When health checks are passing but traffic is not reaching targets, the issue is typically network connectivity. In this scenario, the instances are in a private subnet without a route to the NLB's subnet, meaning the NLB's IP addresses are unreachable from the targets. Since NLB preserves the source IP of clients, targets must have a route back to the NLB's subnet (or the client) to return traffic; without this, the three-way TCP handshake fails even though health checks (which originate from the NLB's subnet) succeed.
What should I do if I get this SOA-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Question Discussion
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