The answer is that security groups or network ACLs blocking traffic from the ALB to the instances are the most likely cause of 503 errors with healthy targets. This occurs because the ALB’s health checks originate from a different set of IP addresses than the actual client traffic; if the instance’s security group or subnet NACL permits health check pings on port 80 but denies the broader range of client traffic, the targets remain healthy in the group while real requests fail, producing the 503. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of the separation between health check traffic and data plane traffic—a common trap is assuming healthy targets guarantee successful client connections. Remember that ALB health checks use specific IP ranges listed in the AWS documentation, so always verify that your security group rules explicitly allow inbound traffic from the ALB’s subnet CIDR, not just the health check source. A useful memory tip: “Healthy targets, 503 errors? Check the security group—health checks pass, but client traffic gets blocked.”
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
An application hosted on EC2 instances behind an ALB is experiencing intermittent connectivity errors. The ALB target group is configured with health checks on port 80. The SysOps team notices that the EC2 instances pass health checks but clients still receive 503 errors. 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
✓
Security groups or network ACLs are blocking traffic from the ALB to the instances
Option D is correct because if security groups or network ACLs block traffic from the ALB to the instances, the ALB may still mark them as healthy if the health check is coming from a different source IP, but actual client traffic is blocked. Option A is wrong because if health checks pass, the target group is not empty. Option B is wrong because deregistration delay does not cause 503s. Option C is wrong because if the ALB is not in multiple AZs, it would not cause 503 errors specifically.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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 deregistration delay is too high
Why it's wrong here
Deregistration delay affects draining, not healthy traffic.
✗
The ALB is only configured in one Availability Zone
Why it's wrong here
This would cause reduced availability but not specifically 503 errors.
✗
The target group is empty
Why it's wrong here
If health checks pass, instances are registered in the target group.
✓
Security groups or network ACLs are blocking traffic from the ALB to the instances
Why this is correct
If health checks use a different source (e.g., from the ALB's private IP) but client traffic is blocked, the ALB returns 503.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related SOA-C02 ACL questions on filtering logic and placement.
Networking and Content Delivery — This question tests Networking and Content Delivery — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Security groups or network ACLs are blocking traffic from the ALB to the instances — Option D is correct because if security groups or network ACLs block traffic from the ALB to the instances, the ALB may still mark them as healthy if the health check is coming from a different source IP, but actual client traffic is blocked. Option A is wrong because if health checks pass, the target group is not empty. Option B is wrong because deregistration delay does not cause 503s. Option C is wrong because if the ALB is not in multiple AZs, it would not cause 503 errors specifically.
What should I do if I get this SOA-C02 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related SOA-C02 ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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