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 set up a transit gateway with attachments to VPC-A and VPC-B. The transit gateway route table shows routes to both VPCs and a blackhole for 0.0.0.0/0. VPC-A's public subnet route table sends 10.1.0.0/16 traffic to the transit gateway. However, an EC2 instance in VPC-A's public subnet cannot reach an instance in VPC-B. 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
✓
VPC-B's route table does not have a route to VPC-A's CIDR via the transit gateway.
Option A is correct because for traffic to flow from VPC-A to VPC-B via a transit gateway, both VPCs must have routes in their route tables pointing to the transit gateway for the other VPC's CIDR. Since VPC-A's route table sends 10.1.0.0/16 (VPC-B's CIDR) to the transit gateway, but VPC-B's route table lacks a return route to VPC-A's CIDR via the transit gateway, the return traffic from VPC-B is dropped, causing connectivity failure.
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.
✓
VPC-B's route table does not have a route to VPC-A's CIDR via the transit gateway.
Why this is correct
For bidirectional communication, VPC-B must also have a route back to VPC-A via the transit gateway.
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.
✗
VPC-A's route table does not have a route to the transit gateway.
Why it's wrong here
It has a route to 10.1.0.0/16 via the transit gateway.
✗
The transit gateway route table does not have a route for 10.0.0.0/16.
Why it's wrong here
It does have a route for 10.0.0.0/16 via VPC-A attachment.
✗
The blackhole route in the transit gateway is blocking traffic between VPCs.
Why it's wrong here
Blackhole is for 0.0.0.0/0, not for specific VPC CIDRs.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that a transit gateway route table alone ensures bidirectional connectivity, when in fact each VPC's subnet route tables must have explicit routes for the other VPC's CIDR to enable return traffic.
Detailed technical explanation
How to think about this question
Transit gateway route tables propagate routes from attached VPCs, but each VPC's subnet route tables must be manually configured with static routes pointing to the transit gateway for other VPC CIDRs. Without a symmetric route in VPC-B's route table for VPC-A's CIDR, the transit gateway will forward the initial packet to VPC-B, but the return packet from VPC-B's instance will be dropped because VPC-B's subnet route table does not have a route back to VPC-A via the transit gateway, leading to asymmetric routing failure.
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 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: VPC-B's route table does not have a route to VPC-A's CIDR via the transit gateway. — Option A is correct because for traffic to flow from VPC-A to VPC-B via a transit gateway, both VPCs must have routes in their route tables pointing to the transit gateway for the other VPC's CIDR. Since VPC-A's route table sends 10.1.0.0/16 (VPC-B's CIDR) to the transit gateway, but VPC-B's route table lacks a return route to VPC-A's CIDR via the transit gateway, the return traffic from VPC-B is dropped, causing connectivity failure.
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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Question Discussion
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