ANS-C01 Network Implementation Practice Question
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?
⚠ Common exam trap
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.
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 on-premises router is advertising a route that is more specific than the allowed prefix list on the Direct Connect gateway.
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.
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 BGP session is flapping due to high latency.
Why it's wrong here
BGP session is up, so flapping is not the issue.
- ✓
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.
- ✗
The on-premises router is using AS_PATH prepending.
Why it's wrong here
AS_PATH prepending affects route preference, not acceptance.
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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.