This ANS-C01 practice question tests your understanding of network design. 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 is designing a hybrid network architecture using AWS Direct Connect. They have a single Direct Connect connection with a private virtual interface (VIF) to a VPC in us-east-1. The on-premises network uses BGP to advertise a prefix (10.0.0.0/8) to AWS. The VPC has a CIDR of 10.1.0.0/16. The company wants to add a second VPC (10.2.0.0/16) in the same region and allow on-premises to communicate with both VPCs. They plan to use a Transit Gateway to connect the VPCs and the Direct Connect gateway. The Direct Connect gateway is associated with the Transit Gateway. The on-premises router is advertising 10.0.0.0/8. After configuration, the on-premises network can communicate with 10.1.0.0/16 but not with 10.2.0.0/16. The network engineer verifies that the Transit Gateway route table has routes for both VPC attachments and that the Direct Connect gateway is associated with the Transit Gateway. What is the MOST likely issue?
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 Direct Connect gateway is not associated with the Transit Gateway route table that contains the 10.2.0.0/16 route
When using a Direct Connect gateway with a Transit Gateway, the Direct Connect gateway propagates routes to the Transit Gateway. The Transit Gateway route table must have a route for the on-premises prefix (10.0.0.0/8) pointing to the Direct Connect gateway attachment. Also, the Transit Gateway must propagate the VPC CIDRs to the Direct Connect gateway so that on-premises can learn them. The issue is likely that the 10.2.0.0/16 route is not being propagated to the Direct Connect gateway. This can happen if the Transit Gateway route table does not have the VPC attachment associated or if the route propagation is disabled for that VPC attachment. The engineer verified routes in the Transit Gateway route table, so the issue might be that the Direct Connect gateway is not receiving the route for 10.2.0.0/16. Typically, the Transit Gateway automatically propagates routes from attachments to the Direct Connect gateway if the route table is associated. However, if the VPC attachment is not associated with the same route table as the Direct Connect gateway attachment, propagation may not happen. The most likely fix is to ensure that both VPC attachments and the Direct Connect gateway attachment are in the same Transit Gateway route table.
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 route table that contains the 10.2.0.0/16 route
Why this is correct
If the Direct Connect gateway attachment is in a different route table, it won't propagate the VPC CIDR to on-premises.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The Transit Gateway route table does not have a route for 10.2.0.0/16
Why it's wrong here
The engineer verified that routes exist.
✗
The private virtual interface is not configured to support multiple VPCs
Why it's wrong here
Private VIF can support multiple VPCs via Transit Gateway.
✗
The on-premises router is not advertising the 10.2.0.0/16 prefix
Why it's wrong here
On-premises does not need to advertise the VPC CIDR; it needs to receive it.
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.
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The Direct Connect gateway is not associated with the Transit Gateway route table that contains the 10.2.0.0/16 route — When using a Direct Connect gateway with a Transit Gateway, the Direct Connect gateway propagates routes to the Transit Gateway. The Transit Gateway route table must have a route for the on-premises prefix (10.0.0.0/8) pointing to the Direct Connect gateway attachment. Also, the Transit Gateway must propagate the VPC CIDRs to the Direct Connect gateway so that on-premises can learn them. The issue is likely that the 10.2.0.0/16 route is not being propagated to the Direct Connect gateway. This can happen if the Transit Gateway route table does not have the VPC attachment associated or if the route propagation is disabled for that VPC attachment. The engineer verified routes in the Transit Gateway route table, so the issue might be that the Direct Connect gateway is not receiving the route for 10.2.0.0/16. Typically, the Transit Gateway automatically propagates routes from attachments to the Direct Connect gateway if the route table is associated. However, if the VPC attachment is not associated with the same route table as the Direct Connect gateway attachment, propagation may not happen. The most likely fix is to ensure that both VPC attachments and the Direct Connect gateway attachment are in the same Transit Gateway route table.
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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