Transit Gateway Centralized Traffic Inspection with Gateway Load Balancer
A company is designing a network security architecture for a multi-account environment using AWS Transit Gateway. The security team needs to centralize inspection of all traffic between VPCs using a third-party firewall appliance in a shared services VPC. What is the most scalable and highly available design?
Quick Answer
The answer is to create a Transit Gateway with separate route tables for each VPC and a shared route table for the inspection VPC, using a Gateway Load Balancer to distribute traffic to a fleet of firewall instances. This design is correct because it leverages Transit Gateway centralized inspection with separate route tables to steer all inter-VPC traffic into a shared services VPC, while the Gateway Load Balancer provides horizontal scalability and high availability by transparently distributing flows across multiple firewall appliances using the GENEVE protocol. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how to combine Transit Gateway route tables with Gateway Load Balancer for scalable, fault-tolerant traffic inspection, avoiding common traps like relying on a single firewall instance or using VPC peering meshes. A key memory tip: think of the inspection VPC as a “security hub” with its own route table, and the Gateway Load Balancer as the “traffic distributor” that keeps no single point of failure.
⚠ Common exam trap
Watch out — candidates often confuse Network Load Balancer (NLB) with Gateway Load Balancer (GWLB), assuming NLB can transparently inspect traffic, but GWLB is the only AWS load balancer designed for inline traffic inspection with third-party appliances.
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 Transit Gateway with separate route tables for each VPC and a shared route table for the inspection VPC. Use Gateway Load Balancer to distribute traffic to a fleet of firewall instances.
It combines AWS Transit Gateway with separate route tables per VPC and a shared inspection route table, enabling centralized traffic steering to a fleet of firewall instances behind a Gateway Load Balancer (GWLB). GWLB transparently distributes traffic across multiple firewall instances at the network layer (GENEVE protocol), providing both horizontal scalability and high availability without introducing a single point of failure or requiring complex VPC peering meshes.
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 VPC peering to connect all VPCs to the shared services VPC and route traffic through the firewall.
Why it's wrong here
VPC peering is not transitive and does not scale well for many VPCs.
- ✓
Create a Transit Gateway with separate route tables for each VPC and a shared route table for the inspection VPC. Use Gateway Load Balancer to distribute traffic to a fleet of firewall instances.
Why this is correct
This design is scalable and highly available, with Gateway Load Balancer for inspection and Transit Gateway for routing.
- ✗
Attach all VPCs to a Transit Gateway and configure a Network Load Balancer in the shared services VPC to distribute traffic to firewall instances.
Why it's wrong here
Network Load Balancer does not support packet inspection; a Gateway Load Balancer is needed.
- ✗
Attach all VPCs to a Transit Gateway and use a single firewall instance with elastic network interfaces in each VPC.
Why it's wrong here
Single firewall instance is not highly available.
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Same concept, more angles
3 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 designing a network security architecture for a multi-account environment using AWS Transit Gateway. The company requires that all traffic between VPCs must be inspected by a centralized security appliance in a shared services VPC. The security appliance must receive traffic for both directions (ingress and egress). Which THREE components are required to achieve this? (Choose THREE.)
hard- ✓ A.A shared services VPC containing the security appliances.
- ✓ B.VPC attachments to the Transit Gateway for each VPC.
- C.VPC peering connections between each spoke VPC and the shared services VPC.
- ✓ D.Transit Gateway route tables that route traffic between VPCs through the security appliances.
- E.NAT gateways in each spoke VPC for outbound traffic.
Why A: The shared services VPC hosts the centralized security appliances (e.g., firewalls, intrusion detection systems) that must inspect all inter-VPC traffic. This VPC acts as a hub for traffic inspection, ensuring that both ingress and egress traffic flows through the appliances before reaching its destination.
Variation 2. A company is designing a network security architecture for a multi-account AWS environment. They need to centrally inspect and filter traffic between VPCs using a third-party firewall appliance. Which AWS service should they use to route traffic through the inspection VPC?
easy- A.VPC Peering
- B.AWS Direct Connect
- ✓ C.AWS Transit Gateway
- D.VPC Endpoints
Why C: AWS Transit Gateway allows centralized routing and can forward traffic to a central inspection VPC for firewall inspection. Option A is wrong because VPC Peering does not support transitive routing and is not centralized. Option B is wrong because AWS Direct Connect is for on-premises connectivity, not inter-VPC routing. Option D is wrong because VPC Endpoints are for private access to AWS services, not for routing inter-VPC traffic.
Variation 3. A company is designing a network security architecture for a multi-account AWS environment using AWS Transit Gateway. They need to ensure that all traffic between VPCs must be inspected by a centralized security appliance (e.g., firewall) in a shared services VPC. Which routing design meets this requirement?
hard- A.Attach all VPCs to a single route table and enable VPC peering for inspection VPC.
- B.Use a centralized NAT gateway in the inspection VPC and configure all spokes to route traffic through it.
- ✓ C.Attach the inspection VPC and all spoke VPCs to the same Transit Gateway route table, and add a static route for the spoke VPC CIDRs pointing to the inspection VPC attachment, with blackhole routes for the same CIDRs.
- D.Create separate route tables for each VPC attachment and propagate routes from all VPCs.
Why C: By attaching both the inspection VPC and the spoke VPCs to the same route table with blackhole routes, traffic between spokes is forced to go through the inspection VPC. Option A is incorrect because it uses separate route tables, which would allow direct routing. Option B allows direct traffic. Option D is not a standard practice.
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.