ANS-C01 Network Design Practice Question
A company has a multi-VPC architecture connected via AWS Transit Gateway. They have VPCs in different AWS accounts. The network team wants to centralize internet traffic through a single egress VPC that has a NAT Gateway and an Internet Gateway. All other VPCs should route outbound internet traffic through the Transit Gateway to the egress VPC. They have configured route tables accordingly, but instances in non-egress VPCs cannot reach the internet. What is the most likely missing configuration?
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
✓
The Transit Gateway route table does not have a route to the non-egress VPC attachments for the return traffic.
For traffic from non-egress VPCs to reach the internet, the Transit Gateway must have a route table that includes a route to the non-egress VPC attachments for the return traffic. Without this route, return traffic from the internet (via the egress VPC's NAT Gateway) cannot be forwarded back to the originating VPC through the Transit Gateway. The other options are incorrect: B is false because security groups are stateful, so inbound rules do not affect return traffic; C is false because the egress VPC needs a route to the Transit Gateway for outbound traffic, not return traffic; D is false because VPC endpoints are not used for general internet access.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The Transit Gateway route table does not have a route to the non-egress VPC attachments for the return traffic.
Why this is correct
For return traffic from the egress VPC to reach the non-egress VPCs, the TGW route table must have routes to those attachments.
- ✗
The security group for the NAT Gateway in the egress VPC is blocking inbound traffic from other VPCs.
Why it's wrong here
NAT Gateway does not have security groups.
- ✗
The egress VPC's route table does not have a route to the Transit Gateway for the return traffic from the internet.
Why it's wrong here
Return traffic from internet goes to NAT Gateway, which forwards to private instances; no need for route to TGW for return traffic from internet.
- ✗
The non-egress VPCs are using VPC endpoints instead of the Transit Gateway for internet access.
Why it's wrong here
VPC endpoints are for specific services, not general internet.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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