ANS-C01 Network Implementation Practice Question
A network engineer is setting up a Direct Connect connection from an on-premises data center to AWS. The connection uses a private VIF to connect to a VPC via a Direct Connect gateway. The on-premises network is advertising a BGP prefix 10.0.0.0/16, which overlaps with the VPC CIDR 10.0.0.0/16. What is the expected behavior?
⚠ Common exam trap
Many candidates assume BGP session failure due to prefix overlap, but BGP itself has no mechanism to reject a session based on prefix overlap; the overlap only affects route installation, not session state.
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 BGP session will be established, but the overlapping prefix will be ignored and not programmed into the VPC route tables.
When a BGP prefix advertised over a Direct Connect private VIF overlaps with the VPC CIDR, the BGP session establishes successfully, but AWS ignores the overlapping prefix and does not install it into the VPC route tables. This is because AWS always prefers the local VPC route over any propagated BGP route for the same prefix, preventing traffic destined for the VPC from being blackholed or misrouted to on-premises.
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 will automatically reassign a new CIDR to avoid the conflict.
Why it's wrong here
AWS does not automatically modify VPC CIDRs.
- ✗
The BGP session will fail to establish due to the prefix conflict.
Why it's wrong here
BGP session establishment is not affected by prefix overlap; only route advertisement is rejected.
- ✓
The BGP session will be established, but the overlapping prefix will be ignored and not programmed into the VPC route tables.
Why this is correct
AWS does not allow overlapping prefixes to be injected into VPC route tables to prevent routing conflicts.
- ✗
The on-premises prefix will take precedence and override the VPC route.
Why it's wrong here
AWS prioritizes local VPC routes over learned routes, so the VPC CIDR remains.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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