ANS-C01 Network Design Practice Question
A company has a VPC with CIDR 10.0.0.0/16. They have an on-premises network with CIDR 172.16.0.0/12 connected via AWS Site-to-Site VPN. The company also has a second VPC (VPC B) with CIDR 10.1.0.0/16 peered with the first VPC. They notice that instances in VPC B cannot reach the on-premises network. What is the most likely cause?
⚠ Common exam trap
Many exam-takers assume VPC peering works like a router or a hub-and-spoke model, not realizing that AWS explicitly disables transitive routing across VPC peering connections to prevent unintended network loops and complexity.
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
✓
VPC peering does not support transitive routing.
VPC peering does not support transitive routing. This means that if VPC A is peered with VPC B and also connected to an on-premises network via VPN, traffic from VPC B cannot use VPC A as a transit point to reach the on-premises network. Each VPC must have its own direct connection to the on-premises network, or a transit gateway must be used to enable transitive routing.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Route propagation is disabled in VPC B.
Why it's wrong here
Route propagation in VPC B would not help because routes from VPN are not propagated to peered VPCs.
- ✗
The VPN connection is not compatible with VPC peering.
Why it's wrong here
VPN and VPC peering can coexist.
- ✗
The VPN tunnel is down.
Why it's wrong here
If first VPC can reach on-premises, tunnel is up.
- ✓
VPC peering does not support transitive routing.
Why this is correct
Transitive routing is not supported; on-premises cannot reach VPC B through VPC peering.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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