The answer is that the VPC route tables are missing explicit routes for the other VPCs’ CIDRs and the on-premises CIDR pointing to the Transit Gateway. Even though the Transit Gateway route table has propagation enabled for all attachments, propagation only populates the Transit Gateway’s own route table—it does not automatically add routes back into the individual VPC route tables. Each VPC must have static routes or propagated routes from the Transit Gateway that explicitly direct traffic for the other VPCs’ CIDRs (e.g., 10.2.0.0/16) and the on-premises CIDR (10.0.0.0/8) to the Transit Gateway attachment. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the separation between Transit Gateway routing and VPC subnet routing; a common trap is assuming that enabling propagation on the Transit Gateway alone is sufficient for cross-VPC communication. Remember the memory tip: “Propagation fills the hub, but spokes need their own signs”—the Transit Gateway learns routes, but each VPC must still point to the hub for those destinations.
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 has multiple VPCs in the same AWS region that need to communicate with each other and with an on-premises data center. The company currently uses VPC peering connections between each VPC pair, which has become difficult to manage as the number of VPCs grows. The company wants to simplify the network architecture and implement a hub-and-spoke model using AWS Transit Gateway. The on-premises data center is connected to AWS via a Direct Connect connection with a private VIF. The company has already created a Transit Gateway and attached all VPCs to it. They have also created a Direct Connect gateway and associated it with the Transit Gateway. The on-premises router is advertising the on-premises CIDR (10.0.0.0/8) over BGP. However, after the migration, the VPCs cannot communicate with each other, and the on-premises network cannot reach the VPCs. The VPC route tables have been updated to route all traffic to the Transit Gateway. The Transit Gateway route table has propagation enabled for all VPC attachments and the Direct Connect gateway attachment. What is the most likely missing configuration?
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 VPC route tables do not have routes for the other VPCs' CIDRs and the on-premises CIDR pointing to the Transit Gateway.
Even though the Transit Gateway route table has propagation enabled, the VPCs might not have routes that point to the Transit Gateway for the other VPCs' CIDRs and the on-premises CIDR. The VPC route tables need explicit routes for the other VPCs' CIDRs (e.g., 10.2.0.0/16) pointing to the Transit Gateway. Alternatively, the Transit Gateway route table might not have routes for the on-premises CIDR because the Direct Connect gateway propagation might not be working if the on-premises prefixes are not being advertised correctly. The most common issue is that the VPC route tables do not have routes for the other VPCs' CIDRs. Option A is correct. Option B is incorrect because the Transit Gateway route table has propagation enabled. Option C is incorrect because the Direct Connect gateway association is in place. Option D is incorrect because the on-premises router is advertising the CIDR.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 Direct Connect gateway is not associated with the Transit Gateway.
Why it's wrong here
It is associated, as stated.
✗
The Transit Gateway route table does not have propagation enabled for the VPC attachments.
Why it's wrong here
Propagation is enabled for all attachments, so routes should be propagated.
✓
The VPC route tables do not have routes for the other VPCs' CIDRs and the on-premises CIDR pointing to the Transit Gateway.
Why this is correct
Without explicit routes in each VPC's route table for the other VPCs and on-premises CIDRs, traffic will not be forwarded to the Transit Gateway.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The VPC route tables do not have routes for the other VPCs' CIDRs and the on-premises CIDR pointing to the Transit Gateway. — Even though the Transit Gateway route table has propagation enabled, the VPCs might not have routes that point to the Transit Gateway for the other VPCs' CIDRs and the on-premises CIDR. The VPC route tables need explicit routes for the other VPCs' CIDRs (e.g., 10.2.0.0/16) pointing to the Transit Gateway. Alternatively, the Transit Gateway route table might not have routes for the on-premises CIDR because the Direct Connect gateway propagation might not be working if the on-premises prefixes are not being advertised correctly. The most common issue is that the VPC route tables do not have routes for the other VPCs' CIDRs. Option A is correct. Option B is incorrect because the Transit Gateway route table has propagation enabled. Option C is incorrect because the Direct Connect gateway association is in place. Option D is incorrect because the on-premises router is advertising the CIDR.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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