ANS-C01 Network Implementation Practice Question
A company uses AWS Direct Connect with a private VIF to connect to a VPC. The VPC has a virtual private gateway (VGW). The on-premises network uses BGP to exchange routes with the VGW. The company wants to route traffic from the VPC to an on-premises subnet 192.168.1.0/24. The on-premises router advertises 192.168.1.0/24 over BGP. However, instances in the VPC cannot reach that subnet. The VPC route table has the local route and a route to the VGW for 0.0.0.0/0. What is the most likely cause?
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 192.168.1.0/24 subnet overlaps with the VPC CIDR, causing the local route to take precedence
If the on-premises subnet 192.168.1.0/24 overlaps with the VPC CIDR (e.g., VPC CIDR 192.168.0.0/16), the VPC's local route takes precedence over any propagated BGP routes. Traffic destined for 192.168.1.0/24 stays within the VPC and never reaches the virtual private gateway, even though the on-premises router is advertising the route. Option A is incorrect because BGP route propagation is enabled by default for Direct Connect private VIFs, and even if disabled, a static route to the VGW would be needed. Option C is incorrect because if the subnet overlaps, a specific route to the VGW would be ignored due to the local route's higher priority. Option D is incorrect because allowed prefixes are configured on the Direct Connect gateway, not the VIF, and the route is being advertised successfully.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
BGP route propagation is disabled on the VPC route table
Why it's wrong here
If disabled, routes wouldn't be in route table; but route propagation is enabled by default for VGW.
- ✓
The 192.168.1.0/24 subnet overlaps with the VPC CIDR, causing the local route to take precedence
Why this is correct
Local routes have higher priority than propagated routes.
- ✗
The VPC route table does not have a specific route for 192.168.1.0/24 pointing to the virtual private gateway
Why it's wrong here
Even if it did, the local route might override.
- ✗
The Direct Connect virtual interface does not have the allowed prefixes configured
Why it's wrong here
Allowed prefixes are for Direct Connect Gateway, but the route is advertised.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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