The answer is the VPC endpoint policy does not grant access to the S3 bucket. Even when a Gateway VPC Endpoint for S3 is correctly created and associated with the private subnet route table via a route for the S3 prefix list, the endpoint’s policy acts as the final gatekeeper, explicitly controlling which buckets and actions are permitted. If this policy is too restrictive—such as a default deny or lacking an explicit allow for the specific bucket—traffic is silently dropped at the endpoint, blocking the EC2 instance from downloading updates despite permissive security groups and network ACLs. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that Gateway Endpoints rely on route table entries for traffic direction but on endpoint policies for authorization; a common trap is assuming that creating the endpoint and adding the route is sufficient. Remember the mnemonic: “Route directs, but policy protects”—the route gets traffic to the endpoint, but the policy decides if it gets through.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 company has an existing VPC with a public subnet and a private subnet. The company launches an EC2 instance in the private subnet. The instance needs to access an S3 bucket to download software updates. The company does not want the instance to have a public IP address. The company creates a VPC endpoint for S3 (Gateway type) in the VPC, and associates it with the private subnet route table by adding a route for the S3 prefix list. However, the instance still cannot access the S3 bucket. The security group for the instance allows all outbound traffic. The network ACL for the private subnet allows all inbound and outbound traffic. What is the MOST likely reason for the connectivity failure?
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 VPC endpoint policy does not grant access to the S3 bucket
The most likely reason is that the VPC endpoint policy does not grant access to the S3 bucket. Even though the Gateway VPC Endpoint is created and associated with the private subnet route table, the endpoint's policy controls which S3 buckets and actions are allowed. If the policy is too restrictive (e.g., default deny or does not explicitly allow access to the specific bucket), traffic will be dropped at the endpoint, preventing the instance from downloading updates. The security group and network ACL are already permissive, and internet access is not required because the Gateway endpoint uses AWS's private network.
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.
✗
The network ACL is blocking the traffic
Why it's wrong here
Incorrect; the NACL allows all traffic.
✗
The security group does not allow outbound HTTPS traffic
Why it's wrong here
Incorrect; the security group allows all outbound traffic.
✓
The VPC endpoint policy does not grant access to the S3 bucket
Why this is correct
Correct; the endpoint policy may be restrictive.
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 instance does not have a route to the internet
Why it's wrong here
Incorrect; a gateway endpoint does not require internet access.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that a Gateway VPC Endpoint automatically grants access to all S3 buckets once the route is added, ignoring that the endpoint policy must explicitly allow the desired S3 actions and resources.
Detailed technical explanation
How to think about this question
A Gateway VPC Endpoint uses prefix lists in the route table to direct S3 traffic through AWS's internal network, bypassing the internet. The endpoint policy is an IAM resource policy evaluated in addition to any bucket policies or IAM user policies; if it denies access to the specific S3 bucket or action (e.g., s3:GetObject), the request fails with an AccessDenied error. In practice, a common misconfiguration is leaving the endpoint policy as the default full-access policy but then accidentally modifying it to a restrictive policy that does not include the target bucket's ARN.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The VPC endpoint policy does not grant access to the S3 bucket — The most likely reason is that the VPC endpoint policy does not grant access to the S3 bucket. Even though the Gateway VPC Endpoint is created and associated with the private subnet route table, the endpoint's policy controls which S3 buckets and actions are allowed. If the policy is too restrictive (e.g., default deny or does not explicitly allow access to the specific bucket), traffic will be dropped at the endpoint, preventing the instance from downloading updates. The security group and network ACL are already permissive, and internet access is not required because the Gateway endpoint uses AWS's private network.
What should I do if I get this ANS-C01 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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