The answer is the on-premises router advertising a route more specific than the allowed prefix list on the Direct Connect gateway. This is correct because the Direct Connect gateway (DXGW) enforces allowed prefix lists to filter BGP advertisements from the on-premises side; when a prefix like a /28 is sent but only a /24 is permitted, the DXGW silently rejects the more specific route, leaving the VPC route table empty for that prefix even though the BGP session remains up. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of BGP route propagation and prefix list filtering on DXGW, often appearing as a trick where candidates assume a BGP-up state guarantees route installation. A common trap is overlooking that BGP session status and route acceptance are independent—the session stays established while the route is dropped. Memory tip: “BGP up doesn’t mean routes are in the cup”—always check the allowed prefix list for specificity mismatches.
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 a Direct Connect connection with a private VIF connected to a VPC. The network engineer notices that traffic from on-premises to the VPC is being dropped intermittently. The on-premises router shows BGP session is up, but the VPC route table does not have the on-premises prefix. 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 advertising a route that is more specific than the allowed prefix list on the Direct Connect gateway.
Option C is correct because the Direct Connect gateway (DXGW) uses allowed prefix lists to control which routes are accepted from the on-premises router via BGP. If the on-premises router advertises a prefix that is more specific (e.g., /28) than the allowed prefix list (e.g., /24), the DXGW will reject the route, causing the VPC route table to lack the on-premises prefix. The BGP session remains up because the session itself is not affected, but the specific route is not installed.
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 MTU on the on-premises router is set too high.
Why it's wrong here
MTU mismatch typically causes packet drops, not route table absence.
The on-premises router is advertising a route that is more specific than the allowed prefix list on the Direct Connect gateway.
Why this is correct
The allowed prefix list on the Direct Connect gateway may not include the specific prefix, causing the route to be rejected.
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 on-premises router is using AS_PATH prepending.
Why it's wrong here
AS_PATH prepending affects route preference, not acceptance.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the distinction between BGP session state and route acceptance, trapping candidates who assume a stable BGP session guarantees route installation, when in fact prefix filtering on the DXGW can silently drop specific routes.
Detailed technical explanation
How to think about this question
Under the hood, the DXGW implements a prefix-based route filtering mechanism similar to a route map with a match ip address prefix-list. When a BGP update carries a prefix that does not match the allowed prefix list (e.g., a /28 instead of a /24), the DXGW silently discards the update without tearing down the BGP session. This behavior is defined in RFC 4271 section 9.1.2, where a BGP speaker can reject a route based on policy without affecting the session state.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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 Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The on-premises router is advertising a route that is more specific than the allowed prefix list on the Direct Connect gateway. — Option C is correct because the Direct Connect gateway (DXGW) uses allowed prefix lists to control which routes are accepted from the on-premises router via BGP. If the on-premises router advertises a prefix that is more specific (e.g., /28) than the allowed prefix list (e.g., /24), the DXGW will reject the route, causing the VPC route table to lack the on-premises prefix. The BGP session remains up because the session itself is not affected, but the specific route is not installed.
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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Question Discussion
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