The answer is that the on-premises router is not advertising the S3 prefix list via BGP over the public VIF. This is because a Direct Connect public VIF relies entirely on BGP route advertisements from the on-premises side to establish bidirectional routing for public AWS services like S3; if the on-premises router fails to advertise the S3 prefix, the Direct Connect link cannot be used for return traffic, forcing the path over the internet instead. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding that public VIFs require BGP peering for route exchange, and a common trap is assuming the VPC route table alone controls traffic direction—it does not, because the public VIF operates outside the VPC and depends on BGP advertisements from the customer edge. A helpful memory tip: for public VIFs, think “advertise to use”—your on-premises router must advertise the S3 prefix to Direct Connect for the path to be preferred.
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 uses AWS Direct Connect with a public VIF to access Amazon S3. They notice that traffic to S3 is taking a suboptimal path (going through the internet) instead of the Direct Connect connection. The VPC has a route table with a route for S3 prefix list via the virtual private gateway. 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 on-premises router is not advertising the S3 prefix list via BGP over the public VIF
Option A is correct because Direct Connect public VIF routes are learned via BGP, and if the on-premises router does not advertise the S3 prefix, traffic may go over the internet. Option B is wrong because a private VIF is for VPC access, not public services. Option C is wrong because VPC endpoints are for private connectivity, but issue is with Direct Connect public VIF. Option D is wrong because the route table has a route for the prefix list.
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 VPC does not have a VPC endpoint for S3
Why it's wrong here
VPC endpoint is optional; public VIF can access S3 directly.
✗
The public VIF is not associated with the correct Direct Connect gateway
Why it's wrong here
Association is likely correct if other routes work.
✓
The on-premises router is not advertising the S3 prefix list via BGP over the public VIF
Why this is correct
S3 prefix must be advertised to route traffic over Direct Connect.
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 route table in the VPC does not have a route to the S3 prefix list via the virtual private gateway
Why it's wrong here
Route exists as stated.
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 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.
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 ANS-C01 NAT questions on configuration and troubleshooting.
Network Implementation — This question tests Network Implementation — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The on-premises router is not advertising the S3 prefix list via BGP over the public VIF — Option A is correct because Direct Connect public VIF routes are learned via BGP, and if the on-premises router does not advertise the S3 prefix, traffic may go over the internet. Option B is wrong because a private VIF is for VPC access, not public services. Option C is wrong because VPC endpoints are for private connectivity, but issue is with Direct Connect public VIF. Option D is wrong because the route table has a route for the prefix list.
What should I do if I get this ANS-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 ANS-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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