ANS-C01 Network Design Practice Question
A financial services company is designing a hybrid network architecture using AWS Direct Connect. They have a Direct Connect connection with a public VIF and a private VIF. The private VIF is associated with a Direct Connect gateway that is attached to a Transit Gateway in us-east-1. The Transit Gateway has attachments to a production VPC and a shared services VPC. The company wants to ensure that all traffic from the on-premises network to the production VPC flows through a centralized inspection appliance in the shared services VPC for security compliance. Additionally, traffic from the production VPC to the internet must use a NAT gateway in the shared services VPC. The inspection appliance in the shared services VPC performs stateful inspection and must see both directions of traffic. The network engineer configured the following route tables: In the Transit Gateway route table associated with the Direct Connect gateway attachment, a static route for 0.0.0.0/0 points to the shared services VPC attachment. In the Transit Gateway route table associated with the production VPC attachment, a static route for the on-premises CIDR (10.0.0.0/8) points to the shared services VPC attachment. In the Transit Gateway route table associated with the shared services VPC attachment, a static route for the on-premises CIDR points to the Direct Connect gateway attachment, and a static route for 0.0.0.0/0 points to the Direct Connect gateway attachment (for outbound internet traffic, the shared services VPC has its own internet gateway and NAT gateway). The production VPC has a default route (0.0.0.0/0) pointing to the Transit Gateway. The shared services VPC has a default route pointing to the NAT gateway. However, traffic from on-premises to the production VPC is not being inspected; it goes directly to the production VPC. What is the most likely reason?
⚠ Common exam trap
AWS often tests the misconception that static routes always take precedence over propagated routes in Transit Gateway route tables, but in reality, prefix length (specificity) determines priority, not whether the route is static or propagated.
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 associated with the Direct Connect gateway attachment has propagation enabled, which installs a more specific route for the production VPC CIDR directly to the production VPC attachment, bypassing the shared services VPC.
When propagation is enabled on the Transit Gateway route table associated with the Direct Connect gateway attachment, the production VPC CIDR is automatically learned as a more specific route (e.g., 10.1.0.0/16) pointing directly to the production VPC attachment. This more specific route overrides the static 0.0.0.0/0 route that was intended to force traffic through the shared services VPC, causing on-premises traffic to bypass the inspection appliance.
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 associated with the Direct Connect gateway attachment has propagation enabled, which installs a more specific route for the production VPC CIDR directly to the production VPC attachment, bypassing the shared services VPC.
Why this is correct
Propagation from the production VPC attachment installs a route for the production VPC CIDR pointing to the production VPC attachment, which is more specific than the 0.0.0.0/0 route to shared services. This causes traffic to bypass inspection.
- ✗
The shared services VPC has a default route pointing to the NAT gateway, which is not a valid next hop for traffic coming from the Transit Gateway.
Why it's wrong here
The NAT gateway is for outbound traffic. For traffic from Transit Gateway to the internet, the shared services VPC needs a route to the internet gateway. But that is not related to inspection bypass.
- ✗
The production VPC route table has a default route pointing to the Transit Gateway, but the NAT gateway is in the shared services VPC, causing asymmetric routing.
Why it's wrong here
The default route to Transit Gateway is correct for sending internet traffic to shared services. Asymmetric routing is not the issue here.
- ✗
The Transit Gateway route table associated with the shared services VPC attachment does not have a route for the production VPC CIDR pointing to the production VPC attachment, so return traffic cannot reach production.
Why it's wrong here
While return traffic routing is important, the symptom is that inspection is bypassed, not that traffic fails. The return path may work via the default route or other routes.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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