- A
The VPN attachment is in the wrong TGW route table.
Why wrong: Attachment is associated with the correct route table.
- B
The VPC route table does not have a route pointing to the Transit Gateway for on-premises traffic.
Return traffic needs a route to TGW.
- C
The VPC CIDR overlaps with the on-premises CIDR.
Why wrong: Overlap is problematic but not the direct cause here.
- D
The Transit Gateway route table does not have a route for the VPC CIDR.
Why wrong: TGW has a route for 10.0.0.0/8, which covers the VPC.
Question 203 of 1,705
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 uses AWS Transit Gateway to connect multiple VPCs and on-premises networks. They have a VPC with a CIDR of 10.0.0.0/16 and an on-premises network with CIDR 10.0.0.0/8. The Transit Gateway route table has a static route for 10.0.0.0/8 pointing to the VPN attachment. However, traffic from on-premises to the VPC is not working. 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 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 VPC route table does not have a route pointing to the Transit Gateway for on-premises traffic.
Option B is correct because for traffic from on-premises to reach the VPC, the VPC's route table must have a route pointing to the Transit Gateway (TGW) attachment as the target for the on-premises CIDR (10.0.0.0/8). Without this route, the VPC subnet will not forward return traffic to the TGW, causing a one-way communication failure. The TGW route table already has a static route for 10.0.0.0/8 pointing to the VPN attachment, so the issue is on the VPC side, not the TGW.
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 VPN attachment is in the wrong TGW route table.
Why it's wrong here
Attachment is associated with the correct route table.
- ✓
The VPC route table does not have a route pointing to the Transit Gateway for on-premises traffic.
Why this is correct
Return traffic needs a route to TGW.
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 VPC CIDR overlaps with the on-premises CIDR.
Why it's wrong here
Overlap is problematic but not the direct cause here.
- ✗
The Transit Gateway route table does not have a route for the VPC CIDR.
Why it's wrong here
TGW has a route for 10.0.0.0/8, which covers the VPC.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often focus on the TGW route table configuration and overlook the requirement for a return route in the VPC route table, assuming that the TGW handles all routing decisions unilaterally.
Detailed technical explanation
How to think about this question
Under the hood, AWS Transit Gateway uses separate route tables for each attachment, and traffic forwarding requires symmetric routing: the TGW must have a route to the destination, and the source VPC must have a route back to the TGW for return traffic. In this scenario, the on-premises network sends traffic to the VPC via the VPN attachment, but the VPC's subnet route table lacks a route for 10.0.0.0/8 pointing to the TGW attachment, so the VPC's ENI drops the return packets. This is a classic asymmetric routing issue that can be resolved by adding a static route in the VPC route table for the on-premises CIDR with the TGW attachment ID as the target.
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.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The VPC route table does not have a route pointing to the Transit Gateway for on-premises traffic. — Option B is correct because for traffic from on-premises to reach the VPC, the VPC's route table must have a route pointing to the Transit Gateway (TGW) attachment as the target for the on-premises CIDR (10.0.0.0/8). Without this route, the VPC subnet will not forward return traffic to the TGW, causing a one-way communication failure. The TGW route table already has a static route for 10.0.0.0/8 pointing to the VPN attachment, so the issue is on the VPC side, not the TGW.
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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Last reviewed: Jul 4, 2026
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