- A
Create two VPN connections to a single virtual private gateway and advertise the same CIDR from both data centers. Use BGP AS_PATH prepending to influence routing.
Why wrong: Overlapping CIDRs will cause route conflicts; BGP AS_PATH prepending does not solve the overlap.
- B
Create a single VPN connection with two tunnels to the same virtual private gateway. Use different BGP ASNs for each data center.
Why wrong: Still one VPN connection, cannot handle overlapping routes.
- C
Create two separate virtual private gateways, each in a different VPC, and use VPC peering. Alternatively, use a transit gateway with separate VPN attachments and BGP communities to tag routes.
Transit gateway with separate attachments and BGP communities can handle overlapping CIDRs by using route priority or filtering.
- D
Create two VPN connections to the same virtual private gateway. Use static routes with different metric values.
Why wrong: Static routes cannot handle dynamic overlapping routes; BGP required.
Quick Answer
The correct answer is to use a transit gateway with separate VPN attachments and BGP communities to tag routes. This design resolves overlapping CIDR conflicts because a transit gateway allows you to create separate route tables for each VPN attachment, isolating the overlapping 192.168.0.0/16 prefixes from each on-premises data center. BGP communities then tag these routes with unique identifiers, enabling the transit gateway to perform path selection and forward traffic to the correct destination without ambiguity. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how to handle overlapping CIDR with two on-prem VPN BGP connections, a common trap where candidates mistakenly try to use a single virtual private gateway. The key insight is that a VPG cannot differentiate between identical prefixes, but a transit gateway’s separate route tables can. Memory tip: think of the transit gateway as a “traffic cop” with separate lanes—each overlapping route gets its own lane and a colored badge (BGP community) to avoid collisions.
ANS-C01 Network Design Practice Question
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. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. 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 has a VPC with a CIDR of 10.0.0.0/16. They need to connect to two on-premises data centers, each with overlapping CIDR blocks (192.168.0.0/16). The company wants to use AWS Site-to-Site VPN with dynamic routing (BGP). Which design allows the VPC to reach both data centers without route conflicts?
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
Create two separate virtual private gateways, each in a different VPC, and use VPC peering. Alternatively, use a transit gateway with separate VPN attachments and BGP communities to tag routes.
Option C is correct because overlapping CIDR blocks cannot be advertised to the same virtual private gateway (VPG) without causing route conflicts. By using a transit gateway with separate VPN attachments, each data center's routes are isolated in separate route tables, and BGP communities can be used to tag and differentiate routes. This design allows the VPC to reach both data centers by leveraging the transit gateway's ability to manage overlapping prefixes through separate route tables and path selection.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Create two VPN connections to a single virtual private gateway and advertise the same CIDR from both data centers. Use BGP AS_PATH prepending to influence routing.
Why it's wrong here
Overlapping CIDRs will cause route conflicts; BGP AS_PATH prepending does not solve the overlap.
- ✗
Create a single VPN connection with two tunnels to the same virtual private gateway. Use different BGP ASNs for each data center.
Why it's wrong here
Still one VPN connection, cannot handle overlapping routes.
- ✓
Create two separate virtual private gateways, each in a different VPC, and use VPC peering. Alternatively, use a transit gateway with separate VPN attachments and BGP communities to tag routes.
- ✗
Create two VPN connections to the same virtual private gateway. Use static routes with different metric values.
Why it's wrong here
Static routes cannot handle dynamic overlapping routes; BGP required.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that a single virtual private gateway can handle overlapping CIDRs by using BGP attributes like AS_PATH prepending, but the fundamental limitation is that the VPG's route table cannot store duplicate prefixes, regardless of BGP attributes.
Detailed technical explanation
How to think about this question
A transit gateway supports multiple VPN attachments, each with its own BGP session and route table, allowing overlapping CIDRs to be isolated per attachment. BGP communities can be applied to routes from each data center to enable fine-grained route filtering and policy-based routing within the transit gateway. In contrast, a virtual private gateway maintains a single route table and cannot differentiate between identical prefixes from different VPN connections, making it unsuitable for overlapping address spaces.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Create two separate virtual private gateways, each in a different VPC, and use VPC peering. Alternatively, use a transit gateway with separate VPN attachments and BGP communities to tag routes. — Option C is correct because overlapping CIDR blocks cannot be advertised to the same virtual private gateway (VPG) without causing route conflicts. By using a transit gateway with separate VPN attachments, each data center's routes are isolated in separate route tables, and BGP communities can be used to tag and differentiate routes. This design allows the VPC to reach both data centers by leveraging the transit gateway's ability to manage overlapping prefixes through separate route tables and path selection.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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