ANS-C01 Network Implementation Practice Question
A company has multiple VPCs connected via AWS Transit Gateway. Each VPC has its own route table in the transit gateway. The company wants to restrict traffic between certain VPCs. For example, VPC A should be able to send traffic to VPC B but not to VPC C. VPC B should be able to send traffic to VPC C. Which configuration should the company use?
⚠ Common exam trap
Watch out — candidates often confuse Transit Gateway route tables with VPC route tables or assume that security groups or network ACLs can be applied to Transit Gateway attachments, when in fact route table isolation is the only native mechanism for controlling inter-VPC traffic at the Transit Gateway level.
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 separate transit gateway route tables for each VPC and add only the desired routes for other VPCs.
AWS Transit Gateway supports multiple route tables, and by creating a separate route table for each VPC attachment, you can control which VPCs can communicate by selectively adding routes to the desired destination VPC CIDRs. This allows VPC A to have a route only to VPC B, while VPC B's route table includes routes to both VPC A and VPC C, achieving the required traffic isolation without relying on stateful or stateless filtering at the attachment level.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use network ACLs on the transit gateway attachments to filter traffic.
Why it's wrong here
Transit gateway does not support network ACLs on attachments.
- ✗
Use security groups on the VPC attachments to allow or deny traffic.
Why it's wrong here
Security groups are per-ENI, not at transit gateway level.
- ✓
Create separate transit gateway route tables for each VPC and add only the desired routes for other VPCs.
Why this is correct
Enables selective communication.
- ✗
Create a single transit gateway route table and associate all VPC attachments with it.
Why it's wrong here
Would allow all-to-all communication.
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Same concept, more angles
2 more ways this is tested on ANS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company is using AWS Transit Gateway to connect multiple VPCs and on-premises networks via Direct Connect and Site-to-Site VPN. The network team wants to ensure that traffic between VPCs does not traverse the on-premises network. Which Transit Gateway feature should be used?
medium- A.Transit Gateway Multicast
- B.Transit Gateway Network Manager
- C.Transit Gateway BGP over AWS Direct Connect
- ✓ D.Transit Gateway route tables with separate associations and propagations
Why D: Transit Gateway route tables with separate associations and propagations allow you to isolate traffic between VPCs and on-premises networks. By associating VPC attachments to one route table and on-premises attachments (Direct Connect, VPN) to another, you can prevent VPC-to-VPC traffic from being routed through the on-premises network. This ensures that inter-VPC traffic stays within AWS, meeting the requirement.
Variation 2. A company is using AWS Transit Gateway to connect multiple VPCs and an on-premises network via Direct Connect. The network team wants to isolate traffic between VPCs while allowing all VPCs to reach the on-premises network. Which TWO configurations should be implemented?
medium- A.Attach the Direct Connect virtual interface to a VPN attachment on the Transit Gateway
- ✓ B.Use a Direct Connect gateway to propagate routes to all VPCs
- ✓ C.Create separate Transit Gateway route tables for each VPC and associate them with the respective VPC attachments
- D.Use a single Transit Gateway route table for all attachments
- E.Create VPC peering connections between each VPC
Why B: Creating separate Transit Gateway route tables for each VPC and associating them with the respective VPC attachments enforces traffic isolation between VPCs: each VPC route table only contains routes to the on-premises network, not to other VPCs, thus achieving the isolation requirement. Additionally, using a Direct Connect gateway to propagate routes to all VPCs ensures that the on-premises network routes are available in these isolated route tables. Option A is incorrect because a Direct Connect virtual interface must be attached to a Direct Connect Gateway, which then associates with the Transit Gateway via a transit gateway attachment. A VPN attachment is for VPN connections, not Direct Connect. Option D is incorrect because a single Transit Gateway route table for all attachments would allow VPC-to-VPC communication via automatically propagated routes. Option E is incorrect because VPC peering connections would create direct connectivity between VPCs, violating the isolation requirement.
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.