The answer is the security group for the ALB not allowing inbound traffic on port 443 from the internet. When an Application Load Balancer receives a request but its associated security group blocks the incoming traffic on the listener port, it cannot forward the request to the target group, resulting in a 503 error. This scenario tests your understanding of the ALB’s network path: even with a correct Route 53 alias record and health checks passing, the load balancer itself must be reachable. On the AWS Certified SAP on AWS Specialty PAS-C01 exam, this is a common trap where candidates focus on DNS or health check misconfigurations while overlooking the security group’s inbound rules. Remember the “first hop” rule: for any ALB 503 error, always verify that the security group’s inbound rules match the listener port and source—if the traffic can’t reach the balancer, nothing else matters. A useful memory tip is “SG first, then health,” meaning check the security group before diving into target health or DNS settings.
PAS-C01 Operations and Maintenance Practice Question
This PAS-C01 practice question tests your understanding of operations and maintenance. 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.
Refer to the exhibit. An SAP administrator is configuring Route 53 to point a custom domain name to an Application Load Balancer (ALB) that fronts SAP Web Dispatchers. The record set is configured as shown. However, when users access sap.example.com, they receive a 503 error. 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 security group for the ALB is not allowing inbound traffic on port 443 from the internet.
Option D is correct because the ALB's security group must allow inbound traffic on port 443 from the internet. Option A is wrong because the Alias record is correctly configured for an ALB. Option B is wrong because EvaluateTargetHealth is set to true, which is fine. Option C is wrong because the ALB health checks are separate from Route 53 health checks.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 EvaluateTargetHealth is set to true, which is causing the record to be marked as unhealthy.
Why it's wrong here
EvaluateTargetHealth checks the health of the ALB; if the ALB is healthy, this should not cause issues.
✓
The security group for the ALB is not allowing inbound traffic on port 443 from the internet.
Why this is correct
If the security group blocks traffic, the ALB will return 503 errors because it cannot forward requests to targets.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The Alias Target is incorrect; it should be the ALB's DNS name without 'dualstack'.
Why it's wrong here
The 'dualstack' prefix is correct for IPv4 and IPv6 support.
✗
The health check for the ALB target group is failing, causing Route 53 to return 503.
Why it's wrong here
Route 53 does not directly return 503; 503 errors come from the ALB itself if no healthy targets.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related PAS-C01 NAT questions on configuration and troubleshooting.
Operations and Maintenance — This question tests Operations and Maintenance — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The security group for the ALB is not allowing inbound traffic on port 443 from the internet. — Option D is correct because the ALB's security group must allow inbound traffic on port 443 from the internet. Option A is wrong because the Alias record is correctly configured for an ALB. Option B is wrong because EvaluateTargetHealth is set to true, which is fine. Option C is wrong because the ALB health checks are separate from Route 53 health checks.
What should I do if I get this PAS-C01 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related PAS-C01 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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