- A
The VPC route tables for the subnets that cannot be reached do not have a route to the Transit Gateway for the on-premises prefixes.
Without a route in the VPC subnet route table pointing to the Transit Gateway, traffic from the VPC to on-premises will not be forwarded.
- B
The Direct Connect gateway association with the Transit Gateway is misconfigured.
Why wrong: If the association were misconfigured, no traffic would flow.
- C
The Transit Gateway route table does not have a static route or propagation for the on-premises prefixes.
Why wrong: If some traffic works, the Transit Gateway likely has the routes; the issue is on the VPC side.
- D
The on-premises router is advertising different prefixes over BGP than expected.
Why wrong: The question states the on-premises router is advertising the same prefixes, so this is not the issue.
Quick Answer
The answer is that the VPC route tables for the unreachable subnets lack a route pointing to the Transit Gateway for the on-premises prefixes. This is the most likely cause of Transit Gateway Direct Connect partial connectivity because, even when the Transit Gateway and Direct Connect gateway are correctly configured and BGP routes are propagated, each VPC subnet must have an explicit route in its route table directing traffic destined for on-premises networks to the Transit Gateway attachment. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the data path flow: traffic must be routed from the subnet to the TGW, then to the Direct Connect gateway, and finally to on-premises. A common trap is assuming that a working Transit Gateway route table automatically fixes all subnets, but the missing subnet-level route creates a silent black hole for specific CIDRs. Remember the memory tip: “Subnets need a signpost to the TGW—without it, traffic gets lost.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 has a Direct Connect connection with a private VIF to a VPC. They have set up a Transit Gateway and attached the VPC and the Direct Connect gateway. The on-premises network can reach some VPC resources but not others. The VPC has multiple subnets with different CIDR blocks. The on-premises router is advertising the same prefixes over BGP. What is the most likely cause of the partial connectivity?
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 tables for the subnets that cannot be reached do not have a route to the Transit Gateway for the on-premises prefixes.
The most common cause of partial connectivity in this scenario is that the VPC route tables do not have routes pointing to the Transit Gateway for the on-premises prefixes. Even if the Transit Gateway has routes, the VPC subnets must send traffic to the Transit Gateway. Option A is correct. Option B is incorrect because if the Transit Gateway route table had incorrect propagation, it would affect all prefixes equally. Option C is incorrect because the on-premises router advertising the same prefixes would not cause partial connectivity. Option D is incorrect because the Direct Connect gateway association is fine if some traffic works.
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 VPC route tables for the subnets that cannot be reached do not have a route to the Transit Gateway for the on-premises prefixes.
- ✗
The Direct Connect gateway association with the Transit Gateway is misconfigured.
Why it's wrong here
If the association were misconfigured, no traffic would flow.
- ✗
The Transit Gateway route table does not have a static route or propagation for the on-premises prefixes.
Why it's wrong here
If some traffic works, the Transit Gateway likely has the routes; the issue is on the VPC side.
- ✗
The on-premises router is advertising different prefixes over BGP than expected.
Why it's wrong here
The question states the on-premises router is advertising the same prefixes, so this is not the issue.
Common exam traps
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.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
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 for the subnets that cannot be reached do not have a route to the Transit Gateway for the on-premises prefixes. — The most common cause of partial connectivity in this scenario is that the VPC route tables do not have routes pointing to the Transit Gateway for the on-premises prefixes. Even if the Transit Gateway has routes, the VPC subnets must send traffic to the Transit Gateway. Option A is correct. Option B is incorrect because if the Transit Gateway route table had incorrect propagation, it would affect all prefixes equally. Option C is incorrect because the on-premises router advertising the same prefixes would not cause partial connectivity. Option D is incorrect because the Direct Connect gateway association is fine if some traffic works.
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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Last reviewed: Jun 20, 2026
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