- A
Set up an AWS Transit Gateway in each region and connect them via a VPN attachment.
This is correct. Although not the most optimal solution (Transit Gateway peering would be ideal), a VPN attachment between Transit Gateways can provide private connectivity between regions, even with overlapping CIDRs, by using separate route tables per VPC attachment.
- B
Use a Direct Connect connection between the regions.
Why wrong: Incorrect. Direct Connect is used for on-premises connections, not for inter-region connectivity between VPCs.
- C
Use VPC peering between the two VPCs.
Why wrong: Incorrect. VPC peering does not support overlapping CIDR blocks, so it cannot be used here.
- D
Place all resources in a single VPC with multiple Availability Zones.
Why wrong: Incorrect. Placing all resources in a single VPC with multiple Availability Zones does not meet the multi-region requirement.
Connecting Multi-Region VPCs with Overlapping CIDRs Using Transit Gateway
This ANS-C01 practice question tests your understanding of network design. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: transit Gateway. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company is designing a multi-region architecture with VPCs in us-east-1 and eu-west-1. The company needs low-latency connectivity between the VPCs and wants to avoid traffic over the public internet. The VPCs have overlapping CIDR blocks (10.0.0.0/16). Which solution should the network engineer recommend?
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
Set up an AWS Transit Gateway in each region and connect them via a VPN attachment.
The correct answer is A. AWS Transit Gateway supports inter-region peering, but the option specifies a VPN attachment between Transit Gateways. While Transit Gateway peering is the ideal low-latency solution using the AWS global backbone, a VPN attachment (e.g., using a third-party VPN appliance or AWS Site-to-Site VPN) can also provide private connectivity between Transit Gateways in different regions, especially when the VPCs have overlapping CIDRs. Transit Gateways allow you to manage overlapping CIDRs by using separate route tables per VPC attachment. Option B is incorrect because Direct Connect is for on-premises connections, not inter-region. Option C fails because VPC peering does not support overlapping CIDRs. Option D does not meet the multi-region requirement. Therefore, A is the best available choice.
Key principle: Transit Gateway
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Set up an AWS Transit Gateway in each region and connect them via a VPN attachment.
Why this is correct
This is correct. Although not the most optimal solution (Transit Gateway peering would be ideal), a VPN attachment between Transit Gateways can provide private connectivity between regions, even with overlapping CIDRs, by using separate route tables per VPC attachment.
Related concept
Transit Gateway
- ✗
Use a Direct Connect connection between the regions.
Why it's wrong here
Incorrect. Direct Connect is used for on-premises connections, not for inter-region connectivity between VPCs.
- ✗
Use VPC peering between the two VPCs.
Why it's wrong here
Incorrect. VPC peering does not support overlapping CIDR blocks, so it cannot be used here.
- ✗
Place all resources in a single VPC with multiple Availability Zones.
Why it's wrong here
Incorrect. Placing all resources in a single VPC with multiple Availability Zones does not meet the multi-region requirement.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that candidates may dismiss option A because VPN attachments are typically used for site-to-site VPNs to on-premises, but in this scenario, a VPN attachment between Transit Gateways can still provide the required connectivity, especially if combined with Direct Connect for a private path.
Detailed technical explanation
How to think about this question
Transit Gateway inter-region peering uses the AWS global network to route traffic, with each peering attachment supporting up to 20 Gbps of bandwidth and less than 10 ms latency between US and EU regions. The VPN attachment between Transit Gateways can use IPsec with AES-256 encryption, and when combined with BGP dynamic routing, it allows for failover and load balancing across multiple tunnels. Overlapping CIDRs are handled by associating each VPC attachment with a separate route table, ensuring traffic is isolated per VPC even if IP ranges conflict.
KKey Concepts to Remember
- Transit Gateway
- VPC Peering
- Direct Connect
- Overlapping CIDR
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Transit Gateway
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — Transit Gateway.
What is the correct answer to this question?
The correct answer is: Set up an AWS Transit Gateway in each region and connect them via a VPN attachment. — The correct answer is A. AWS Transit Gateway supports inter-region peering, but the option specifies a VPN attachment between Transit Gateways. While Transit Gateway peering is the ideal low-latency solution using the AWS global backbone, a VPN attachment (e.g., using a third-party VPN appliance or AWS Site-to-Site VPN) can also provide private connectivity between Transit Gateways in different regions, especially when the VPCs have overlapping CIDRs. Transit Gateways allow you to manage overlapping CIDRs by using separate route tables per VPC attachment. Option B is incorrect because Direct Connect is for on-premises connections, not inter-region. Option C fails because VPC peering does not support overlapping CIDRs. Option D does not meet the multi-region requirement. Therefore, A is the best available choice.
What should I do if I get this ANS-C01 question wrong?
Review transit Gateway, then practise related ANS-C01 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Transit Gateway
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Last reviewed: Jun 11, 2026
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