ANS-C01 Network Design Practice Question
A company is deploying a new application in a VPC that uses a single Availability Zone. The application consists of an Application Load Balancer (ALB) in a public subnet and EC2 instances in a private subnet. The EC2 instances need to send logs to an Amazon S3 bucket. The company has created a VPC gateway endpoint for S3 and associated it with the route table for the private subnet. The EC2 instances have an instance profile that grants access to the S3 bucket. However, the log delivery fails. The network team has verified that the route table for the private subnet includes a route to the S3 prefix list via the gateway endpoint. What is the MOST likely cause of the failure?
⚠ Common exam trap
Watch out — candidates often assume that if routing is correct and the instance has IAM permissions, the request will succeed, overlooking that the VPC endpoint policy is a separate authorization layer that must explicitly allow the action.
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 VPC endpoint policy does not allow the PutObject action on the S3 bucket.
The most likely cause is that the VPC endpoint policy does not allow the PutObject action on the S3 bucket. Even though the route table has a route to the S3 prefix list via the gateway endpoint and the EC2 instance profile grants access, the endpoint policy acts as an additional layer of authorization. If the endpoint policy does not explicitly allow the s3:PutObject action for the target bucket, requests will be denied, causing the log delivery to fail.
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 route table does not have a route to the S3 prefix list.
Why it's wrong here
Route is present.
- ✗
The VPC endpoint is not configured with private DNS.
Why it's wrong here
Private DNS is optional for gateway endpoints.
- ✗
The security group on the EC2 instances does not allow outbound traffic to the S3 prefix list.
Why it's wrong here
Gateway endpoints do not use security groups.
- ✓
The VPC endpoint policy does not allow the PutObject action on the S3 bucket.
Why this is correct
Correct: Endpoint policy must allow the action.
Visual reference
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
Go deeper
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.