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 has a large AWS environment with hundreds of VPCs connected via a Transit Gateway. The network team is implementing a new hub-and-spoke architecture where all traffic between VPCs must be inspected by a centralized firewall appliance in a shared services VPC. The firewall appliance is a third-party virtual appliance that supports VRF-like segmentation. The network engineer has configured the Transit Gateway with separate route tables for each VPC, and the shared services VPC is associated with all route tables. The firewall appliance is deployed in the shared services VPC with two ENIs: one in a 'trust' subnet and one in an 'untrust' subnet. The trust subnet is used for traffic coming from spoke VPCs, and the untrust subnet is used for traffic going to other spoke VPCs. The firewall appliance performs stateful inspection and returns traffic to the Transit Gateway via the correct ENI. However, after implementation, traffic between two spoke VPCs (VPC A and VPC B) is being dropped. The engineer verifies that the Transit Gateway route tables have static routes for each spoke VPC CIDR pointing to the shared services VPC attachment. The spoke VPCs have routes to each other's CIDR via the Transit Gateway. The firewall logs show that traffic from VPC A reaches the trust ENI, but the firewall is unable to send traffic to VPC B because it does not have a route to VPC B's CIDR. 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 route table for the untrust subnet does not have a route to VPC B's CIDR pointing to the Transit Gateway.
Option D is correct because the firewall's untrust subnet route table must have a route pointing to the Transit Gateway for VPC B's CIDR. The firewall receives traffic from VPC A on its trust ENI, processes it, and then sends it out the untrust ENI. Without a route in the untrust subnet's route table directing traffic for VPC B's CIDR to the Transit Gateway, the firewall cannot forward the traffic back to the Transit Gateway for delivery to VPC B, causing the drop.
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 shared services VPC attachment is not propagating routes to the Transit Gateway route tables.
Why it's wrong here
Propagation is not required; static routes are used.
✗
The security group for the firewall's untrust ENI is blocking outbound traffic.
Why it's wrong here
Security groups are stateful; if inbound traffic is allowed, outbound return traffic is automatically allowed.
✗
The Transit Gateway route table for VPC A does not have a route for VPC B's CIDR.
Why it's wrong here
The engineer verified that static routes are in place.
✓
The route table for the untrust subnet does not have a route to VPC B's CIDR pointing to the Transit Gateway.
Why this is correct
The untrust subnet needs a route to return traffic to the Transit Gateway. Without it, the firewall cannot send traffic to VPC B.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often focus on Transit Gateway route tables and forget that the subnet route tables within the shared services VPC must also be configured to route traffic back to the Transit Gateway, especially when using a firewall with separate ENIs for trust and untrust subnets.
Detailed technical explanation
How to think about this question
In a hub-and-spoke architecture with Transit Gateway, the spoke VPCs route traffic to each other via the Transit Gateway, which forwards it to the shared services VPC attachment. The firewall appliance in the shared services VPC must have proper routing in both the trust and untrust subnet route tables: the trust subnet routes incoming traffic to the firewall, and the untrust subnet must have a route for all spoke VPC CIDRs pointing to the Transit Gateway attachment. This is analogous to VRF-lite where each subnet acts as a separate routing domain, and the firewall performs inter-VRF routing. Without the return route, the firewall's untrust ENI cannot send packets back to the Transit Gateway, resulting in asymmetric routing and packet drops.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 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 route table for the untrust subnet does not have a route to VPC B's CIDR pointing to the Transit Gateway. — Option D is correct because the firewall's untrust subnet route table must have a route pointing to the Transit Gateway for VPC B's CIDR. The firewall receives traffic from VPC A on its trust ENI, processes it, and then sends it out the untrust ENI. Without a route in the untrust subnet's route table directing traffic for VPC B's CIDR to the Transit Gateway, the firewall cannot forward the traffic back to the Transit Gateway for delivery to VPC B, causing the drop.
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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