The correct answer is to attach all VPCs to a Transit Gateway, create a central egress VPC with a NAT gateway, and configure Transit Gateway route tables to send default traffic to that central VPC. This works because the Transit Gateway acts as a hub, allowing you to centrally control routing for all attached VPCs; by pointing the 0.0.0.0/0 route in the Transit Gateway route table toward the central egress VPC, all internet-bound traffic from any attached VPC is forced through the single NAT gateway, achieving centralized egress while preserving inter-VPC connectivity. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of Transit Gateway route table propagation and static routes, often appearing as a design question where a common trap is to place a NAT gateway in each VPC or to rely on VPC peering, which breaks centralized egress. Remember the memory tip: “One NAT to rule them all” — the Transit Gateway route table is the single control point that funnels all default traffic to your egress VPC.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 is planning to connect multiple VPCs in different AWS accounts using AWS Transit Gateway. The VPCs must be able to communicate with each other, but the company wants to centralize egress traffic to the internet through a single VPC that has a NAT gateway. Which configuration meets these requirements?
Attach all VPCs to a Transit Gateway, create a central egress VPC with a NAT gateway, and configure Transit Gateway route tables to send default traffic to the central VPC.
Centralizes internet egress while allowing VPC-to-VPC communication.
B
Use AWS Site-to-Site VPN to connect the VPCs and route traffic through a central VPN endpoint.
Why wrong: Unnecessary complexity and not designed for VPC-to-VPC connectivity.
C
Use VPC peering to connect all VPCs and configure a NAT gateway in one VPC.
Why wrong: VPC peering is not transitive; each pair needs a peering connection.
D
Attach each VPC to a Transit Gateway and configure a NAT gateway in each VPC.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Attach all VPCs to a Transit Gateway, create a central egress VPC with a NAT gateway, and configure Transit Gateway route tables to send default traffic to the central VPC.
Option A is correct because AWS Transit Gateway allows you to attach multiple VPCs from different accounts and centrally manage routing. By configuring the Transit Gateway route tables to send default traffic (0.0.0.0/0) to a central egress VPC that contains a NAT gateway, all other VPCs can route internet-bound traffic through that single NAT gateway, centralizing egress while maintaining inter-VPC communication.
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.
✓
Attach all VPCs to a Transit Gateway, create a central egress VPC with a NAT gateway, and configure Transit Gateway route tables to send default traffic to the central VPC.
Why this is correct
Centralizes internet egress while allowing VPC-to-VPC communication.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use AWS Site-to-Site VPN to connect the VPCs and route traffic through a central VPN endpoint.
Why it's wrong here
Unnecessary complexity and not designed for VPC-to-VPC connectivity.
✗
Use VPC peering to connect all VPCs and configure a NAT gateway in one VPC.
Why it's wrong here
VPC peering is not transitive; each pair needs a peering connection.
✗
Attach each VPC to a Transit Gateway and configure a NAT gateway in each VPC.
Why it's wrong here
This is not centralized egress.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse VPC peering with Transit Gateway, assuming VPC peering supports transitive routing (which it does not), or they think placing a NAT gateway in each VPC is acceptable, missing the explicit requirement for centralized egress.
Detailed technical explanation
How to think about this question
Transit Gateway uses route tables that can be associated with attachments and propagate routes. To centralize egress, you create a dedicated egress VPC attachment, then in the Transit Gateway route table used by all other VPCs, add a static route for 0.0.0.0/0 pointing to the egress VPC attachment. The egress VPC itself must have a default route to its NAT gateway, and the NAT gateway must have an Elastic IP. This design also supports inspection via a firewall appliance in the egress VPC if needed.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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: Attach all VPCs to a Transit Gateway, create a central egress VPC with a NAT gateway, and configure Transit Gateway route tables to send default traffic to the central VPC. — Option A is correct because AWS Transit Gateway allows you to attach multiple VPCs from different accounts and centrally manage routing. By configuring the Transit Gateway route tables to send default traffic (0.0.0.0/0) to a central egress VPC that contains a NAT gateway, all other VPCs can route internet-bound traffic through that single NAT gateway, centralizing egress while maintaining inter-VPC communication.
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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Question Discussion
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