Isolating VPC Traffic Using Transit Gateway Route Tables
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?
Quick Answer
The answer is Transit Gateway route tables with separate associations and propagations. This feature enables Transit Gateway route table isolation by allowing you to create distinct routing domains, so traffic between VPCs stays within the AWS backbone and never traverses the on-premises network. By associating VPC attachments with one route table and VPN or Direct Connect attachments with another, you control exactly which paths are available, effectively blocking inter-VPC traffic from leaking to your on-premises environment. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept tests your understanding of how to segment hybrid network traffic using Transit Gateway, often appearing in scenario-based questions where you must prevent hair-pinning through a corporate data center. A common trap is confusing route propagation with route association—remember that propagation populates routes, while association binds an attachment to a specific route table. Memory tip: think of route tables as separate “rooms” for VPCs and VPNs; if they don’t share a door (route table), traffic cannot cross.
⚠ Common exam trap
It's easy for candidates to confuse routing protocol features (like BGP) or management tools (like Network Manager) with the actual routing isolation mechanism, assuming they control traffic paths, when in fact Transit Gateway route tables with separate associations and propagations are the correct solution.
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
✓
Transit Gateway route tables with separate associations and propagations
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Transit Gateway Multicast
Why it's wrong here
Multicast is for group communication, not traffic isolation.
- ✗
Transit Gateway Network Manager
Why it's wrong here
Network Manager is for monitoring, not routing control.
- ✗
Transit Gateway BGP over AWS Direct Connect
Why it's wrong here
BGP is for exchanging routes, not isolating traffic.
- ✓
Transit Gateway route tables with separate associations and propagations
Why this is correct
This allows creating isolated routing domains.
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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 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.
Variation 2. 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?
hard- A.Use network ACLs on the transit gateway attachments to filter traffic.
- B.Use security groups on the VPC attachments to allow or deny traffic.
- ✓ C.Create separate transit gateway route tables for each VPC and add only the desired routes for other VPCs.
- D.Create a single transit gateway route table and associate all VPC attachments with it.
Why C: 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.
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.