Question 412 of 1,621
ANS-C01 Network Design Practice Question
A company is using AWS Transit Gateway to connect multiple VPCs and on-premises networks. The network team notices that traffic between two VPCs is taking a suboptimal path through the on-premises network instead of staying within AWS. What is the MOST likely cause?
⚠ Common exam trap
Many candidates assume suboptimal routing is caused by asymmetric routing or security group misconfigurations, but the real issue is a missing specific route in the Transit Gateway route table, causing fallback to a default route that sends traffic on-premises.
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 tables do not have a route for the destination VPC CIDR, so traffic is sent to the on-premises network via a default route.
When a Transit Gateway route table lacks a specific route for the destination VPC CIDR, traffic destined for that VPC is evaluated against the remaining routes. If a default route (0.0.0.0/0) exists pointing to an on-premises network attachment (e.g., a VPN or Direct Connect), the traffic will be forwarded over that path, resulting in a suboptimal route that leaves AWS and re-enters through the on-premises network. This is the most likely cause because the Transit Gateway performs destination-based routing, and missing a specific VPC route forces fallback to the default route.
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 tables do not have a route for the destination VPC CIDR, so traffic is sent to the on-premises network via a default route.
Why this is correct
Correct: Missing route causes blackhole or fallback to default route.
- ✗
The subnet route tables in each VPC have a route pointing to the other VPC's CIDR via the Internet Gateway.
Why it's wrong here
Subnets route to Transit Gateway, not IGW.
- ✗
The VPCs are peered directly, bypassing the Transit Gateway.
Why it's wrong here
If peered, traffic would go directly, not through on-premises.
- ✗
Security groups are blocking traffic between the VPCs.
Why it's wrong here
Security groups do not affect routing.
Visual reference
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Last reviewed: Jun 24, 2026
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