The answer is that the VPC attachments are using the default transit gateway route table instead of custom route tables. This is the most likely cause because when you create a Transit Gateway, it comes with a default route table that is automatically shared by all attachments unless you explicitly override it. If the network team created custom route tables for each VPC but forgot to associate the VPC attachments with those custom tables, the attachments remain in the default route table, which contains all propagated routes—including the Direct Connect routes—allowing unintended inter-VPC traffic to flow. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of Transit Gateway route table isolation and the critical distinction between route table association and route propagation. A common trap is assuming that creating a custom route table automatically isolates traffic; in reality, you must manually associate each attachment with its specific route table. Memory tip: think of the default route table as a public pool—every attachment jumps in unless you assign it a private lane.
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 VPC with an AWS Transit Gateway connecting multiple VPCs and an on-premises network via AWS Direct Connect. The network team needs to ensure that only specific VPCs can communicate with each other. They create a transit gateway route table for each VPC and attach the VPC to the route table. They also propagate routes from the Direct Connect virtual interface. However, after configuration, traffic between two VPCs that should not communicate is still flowing. 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 VPC attachments are using the default transit gateway route table instead of custom route tables.
If the transit gateway has a default route table that is shared, all attachments may be associated with it by default. If the team did not explicitly associate each VPC with its own route table, they might be using the default route table, which allows all routes. Option B is correct. Option A is not possible because route tables are not associated with attachments; attachments are associated with route tables. Option C would affect on-premises connectivity, not inter-VPC. Option D is the opposite of what is needed.
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 have a route for the other VPC CIDR pointing to the transit gateway.
Why it's wrong here
This is necessary for communication; but does not explain unintended traffic.
✗
The transit gateway route tables are not associated with the VPC attachments.
Why it's wrong here
Associations are required; but the issue is using default route table.
✓
The VPC attachments are using the default transit gateway route table instead of custom route tables.
The Direct Connect virtual interface is propagating routes into all route tables.
Why it's wrong here
Propagation is controlled per route table.
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 Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The VPC attachments are using the default transit gateway route table instead of custom route tables. — If the transit gateway has a default route table that is shared, all attachments may be associated with it by default. If the team did not explicitly associate each VPC with its own route table, they might be using the default route table, which allows all routes. Option B is correct. Option A is not possible because route tables are not associated with attachments; attachments are associated with route tables. Option C would affect on-premises connectivity, not inter-VPC. Option D is the opposite of what is needed.
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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