ANS-C01 Network Design Practice Question
A company uses AWS Direct Connect with a private VIF to access a VPC. They also have a site-to-site VPN as backup. They notice that during Direct Connect maintenance, the VPN does not take over traffic as expected. All routes are advertised over BGP. What is the most likely reason?
⚠ Common exam trap
Many candidates assume BGP failover is automatic based on reachability, but BGP path selection is deterministic and based on attribute comparison; without explicit tuning (e.g., local preference or AS_PATH prepending), the backup path with a longer AS_PATH will never be used as long as the primary path's routes are still in the routing table, even if the primary path is down.
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 VPN BGP advertisements have a longer AS PATH than the Direct Connect advertisements, making them less preferred.
BGP path selection prefers routes with the shortest AS_PATH. During normal operation, the Direct Connect private VIF advertises routes with a shorter AS_PATH than the VPN backup. When the Direct Connect physical link goes down, the BGP session over the private VIF also goes down, and the VPN routes should take over. However, if the VPN BGP advertisements have a longer AS_PATH, they remain less preferred in the BGP routing table, and the router may still try to use the now-unreachable Direct Connect routes until they are withdrawn, causing traffic to drop instead of failing over to the VPN.
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 Direct Connect virtual interface is still in the 'available' state even though the physical link is down.
Why it's wrong here
If the physical link is down, the virtual interface should also be down.
- ✗
The VPN tunnel is using a different BGP ASN than the Direct Connect.
Why it's wrong here
BGP ASN mismatch would prevent the VPN from establishing, but the question states the VPN is a backup and is expected to work.
- ✓
The VPN BGP advertisements have a longer AS PATH than the Direct Connect advertisements, making them less preferred.
Why this is correct
BGP prefers the shortest AS PATH, so VPN routes with longer AS PATH are not used when Direct Connect is available. During maintenance, they should become active but may be delayed.
- ✗
The VPN is configured with a higher local preference than Direct Connect.
Why it's wrong here
Typically Direct Connect has a higher local preference to prefer it over VPN.
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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.