Design a Centralized Egress VPC with NAT Gateways and Direct Connect Backup
A company is deploying a VPC with a public and private subnet in each of three Availability Zones. They need to provide internet access to instances in the private subnets while ensuring that all outbound traffic is logged and that traffic to a particular on-premises CIDR (10.0.0.0/8) is routed via an AWS Direct Connect Virtual Private Gateway. The company has a VPN connection as a backup. Which design should they use?
Quick Answer
The answer is to deploy a centralized egress VPC with NAT Gateways in each Availability Zone, then peer the application VPCs to this egress VPC. This design is correct because it separates outbound internet traffic from on-premises traffic: route tables in the application VPCs send 0.0.0.0/0 to the egress VPC for centralized NAT and logging, while 10.0.0.0/8 is routed directly to the Direct Connect Virtual Private Gateway, with the VPN connection serving as automatic failover. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of transitive routing limitations with VPC peering and the need for a hub-and-spoke egress model—a common trap is trying to route 10.0.0.0/8 through the egress VPC, which would break the Direct Connect path. Remember the memory tip: “Egress for the internet, Direct Connect for the data center; never mix the two in the same route table hop.”
⚠ Common exam trap
Many exam-takers assume NAT Gateways must be placed in the same VPC as the private subnets, overlooking the centralized egress VPC pattern that enables logging and centralized control, and they may incorrectly think a Transit Gateway can directly attach a NAT Gateway.
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 a centralized egress VPC with NAT Gateways in each AZ. Peer the application VPCs to the egress VPC. Configure route tables in the application VPCs to send 0.0.0.0/0 to the egress VPC and 10.0.0.0/8 to the Direct Connect Virtual Private Gateway.
It uses a centralized egress VPC with NAT Gateways in each AZ to provide internet access to private subnets while logging all outbound traffic. By peering the application VPCs to the egress VPC, the route tables can direct 0.0.0.0/0 traffic to the egress VPC for NAT, and 10.0.0.0/8 traffic to the Direct Connect Virtual Private Gateway, ensuring on-premises traffic uses the dedicated connection with VPN backup. This design meets all requirements: internet access, logging, and specific routing for on-premises CIDR.
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 a centralized egress VPC with NAT Gateways in each AZ. Peer the application VPCs to the egress VPC. Configure route tables in the application VPCs to send 0.0.0.0/0 to the egress VPC and 10.0.0.0/8 to the Direct Connect Virtual Private Gateway.
Why this is correct
This design centralizes egress traffic, uses managed NAT Gateways, and allows logging. Route tables direct on-premises traffic through DX.
- ✗
Place a NAT Gateway in each AZ in the application VPC. Configure route tables to send 0.0.0.0/0 to the NAT Gateway. Use a VPN connection for on-premises traffic.
Why it's wrong here
This does not centralize logging and does not use Direct Connect as primary path.
- ✗
Use a Transit Gateway to connect all VPCs and on-premises. Attach a NAT Gateway in one AZ to the Transit Gateway. Configure route tables to send 0.0.0.0/0 to the NAT Gateway and 10.0.0.0/8 to the Direct Connect Virtual Private Gateway.
Why it's wrong here
This is a valid design but the question specifies a Direct Connect Virtual Private Gateway, not a Transit Gateway. Also, a single NAT Gateway in one AZ is a single point of failure.
- ✗
Create a NAT instance in each private subnet and configure each instance's route table to send 0.0.0.0/0 to the NAT instance. For on-premises traffic, use a VPC peering connection to an on-premises network.
Why it's wrong here
NAT instances are not managed, do not scale, and are a single point of failure. Also, VPC peering is not recommended for large-scale routing.
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Variation 1. A company has a hybrid network with multiple VPCs connected via a Transit Gateway. They need to centralize outbound internet traffic through a single VPC. Which architecture should be used?
hard- A.Attach a NAT Gateway to each VPC's private subnets.
- B.Attach an Internet Gateway to each VPC and route 0.0.0.0/0 to the IGW.
- ✓ C.Designate one VPC as an egress VPC with a NAT Gateway and Internet Gateway, and use Transit Gateway route tables to route 0.0.0.0/0 from other VPCs to the egress VPC.
- D.Use a VPN connection to an on-premises data center for internet access.
Why C: It uses a dedicated egress VPC with a NAT Gateway and Internet Gateway, and leverages Transit Gateway route tables to direct 0.0.0.0/0 traffic from all other VPCs to this central point. This architecture centralizes outbound internet traffic, avoids overlapping NAT responsibilities, and maintains a clean hub-and-spoke design where only the egress VPC has direct internet access.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.