- A
Cross-zone load balancing is disabled.
Why wrong: Cross-zone load balancing affects distribution across AZs, but if health checks pass, targets are reachable in at least one AZ. Disabling cross-zone would not cause complete traffic failure unless no targets are in the NLB's AZ, which would also cause health checks to fail.
- B
The instances are in a private subnet without a route to the NLB's subnet.
Why wrong: Health checks passing indicate that the instances have a route back to the NLB's subnet (since health check traffic originates from there). A missing route to the NLB subnet would cause health checks to fail, so this is not the cause.
- C
The listener is not configured for the correct protocol.
Correct. If the listener protocol (e.g., TCP) is different from the protocol expected by the targets (e.g., UDP), the NLB will establish TCP connections that the targets cannot process, causing traffic failure while health checks (configured with a different protocol) may still pass.
- D
The target group health check interval is too long.
Why wrong: A long health check interval does not block traffic; it only delays detection of unhealthy targets. Traffic would still flow if targets are healthy.
SOA-C02 Network Load Balancer (NLB) protocol handling 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. A key principle to apply: network Load Balancer (NLB) protocol handling. 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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The listener is not configured for the correct protocol.
When health checks are passing but traffic is not reaching targets, a common cause is a mismatch between the listener protocol and the application protocol expected by the targets. For example, a TCP listener sending traffic to a UDP-based service will cause the NLB to establish TCP connections that the targets cannot handle, resulting in traffic failure while health checks (which may use a different protocol like HTTP) still succeed. Other options like cross-zone load balancing or routing issues would typically affect health checks as well.
Key principle: Network Load Balancer (NLB) protocol handling
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
Cross-zone load balancing affects distribution across AZs, but if health checks pass, targets are reachable in at least one AZ. Disabling cross-zone would not cause complete traffic failure unless no targets are in the NLB's AZ, which would also cause health checks to fail.
- ✗
The instances are in a private subnet without a route to the NLB's subnet.
Why it's wrong here
Health checks passing indicate that the instances have a route back to the NLB's subnet (since health check traffic originates from there). A missing route to the NLB subnet would cause health checks to fail, so this is not the cause.
- ✓
The listener is not configured for the correct protocol.
Why this is correct
Correct. If the listener protocol (e.g., TCP) is different from the protocol expected by the targets (e.g., UDP), the NLB will establish TCP connections that the targets cannot process, causing traffic failure while health checks (configured with a different protocol) may still pass.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Network Load Balancer (NLB) protocol handling
- ✗
The target group health check interval is too long.
Why it's wrong here
A long health check interval does not block traffic; it only delays detection of unhealthy targets. Traffic would still flow if targets are healthy.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is assuming that passing health checks guarantee traffic flow. Health checks can use a different protocol from the listener, so a protocol mismatch can cause client traffic to fail even though health checks succeed.
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
- Network Load Balancer (NLB) protocol handling
- Health check independence
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
Network Load Balancer (NLB) protocol handling
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review network Load Balancer (NLB) protocol handling, then practise related SOA-C02 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this SOA-C02 question test?
Networking and Content Delivery — This question tests Networking and Content Delivery — Network Load Balancer (NLB) protocol handling.
What is the correct answer to this question?
The correct answer is: The listener is not configured for the correct protocol. — When health checks are passing but traffic is not reaching targets, a common cause is a mismatch between the listener protocol and the application protocol expected by the targets. For example, a TCP listener sending traffic to a UDP-based service will cause the NLB to establish TCP connections that the targets cannot handle, resulting in traffic failure while health checks (which may use a different protocol like HTTP) still succeed. Other options like cross-zone load balancing or routing issues would typically affect health checks as well.
What should I do if I get this SOA-C02 question wrong?
Review network Load Balancer (NLB) protocol handling, then practise related SOA-C02 questions on the same topic to reinforce the concept.
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?
Network Load Balancer (NLB) protocol handling
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Last reviewed: Jun 24, 2026
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