The answer is missing route tables in the Transit Gateway as the most likely cause of suboptimal path troubleshooting. When a Transit Gateway route table lacks a specific route for the destination VPC CIDR, traffic defaults to any existing default route (0.0.0.0/0), which often points to an on-premises attachment like VPN or Direct Connect. This forces traffic to leave AWS, traverse the on-premises network, and re-enter the destination VPC, creating a suboptimal path. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of Transit Gateway route table evaluation order and the danger of overly broad default routes. A common trap is assuming VPC peering or a direct connection will automatically take precedence, but the Transit Gateway performs exact-match routing first. Remember the memory tip: “No specific route? Traffic takes the default detour.” Always verify that each VPC’s CIDR is explicitly added to the relevant Transit Gateway route table to keep traffic within AWS.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The subnet route tables in each VPC have a route pointing to the other VPC's CIDR via the Internet Gateway.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often 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.
Detailed technical explanation
How to think about this question
Under the hood, AWS Transit Gateway uses route tables associated with each attachment to determine the next hop. When a route table contains a more specific route (e.g., 10.1.0.0/16 for a VPC) and a less specific default route (0.0.0.0/0 pointing to on-premises), the longest prefix match ensures the specific route is preferred. If the specific route is missing, the default route is used, sending traffic over the VPN or Direct Connect. This behavior is analogous to BGP route propagation where missing specific prefixes cause traffic to follow a default route, a common misconfiguration in hybrid cloud designs.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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