The answer is to verify that route propagation is enabled on the VPC's route tables associated with the subnets. This is the first check because even when BGP successfully advertises the 10.0.0.0/24 route from on-premises to the virtual private gateway (VGW), the VPC route tables will not automatically use that route unless the "Propagate routes" option is explicitly enabled for the VGW attachment. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of the separation between BGP peering and VPC routing—a common trap is assuming that a healthy BGP session or a functioning virtual interface (VIF) automatically populates subnet route tables. Remember that the VGW learns the routes, but route propagation is the manual toggle that pushes them into the route tables. A simple memory tip: "BGP learns, propagation earns"—the VGW learns the route, but only propagation earns it a place in the subnet's routing decisions.
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 a Direct Connect connection with a private virtual interface (VIF) to a VPC. The VPC uses a virtual private gateway (VGW). The on-premises network advertises a route to a specific subnet (10.0.0.0/24) via BGP. However, traffic from the VPC to that subnet is failing. What should the network engineer check first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Verify that route propagation is enabled on the VPC's route tables associated with the subnets.
Option B is correct because the route propagation from the VGW to the VPC route tables must be enabled for the learned BGP routes to be used. Option A is wrong because the VGW is the correct attachment. Option C is wrong because the VIF state being up doesn't guarantee route propagation. Option D is wrong because the customer gateway (CGW) is configured on the AWS side; the on-premises router is separate.
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.
✗
Verify that the customer gateway device is advertising the subnet route.
Why it's wrong here
The on-premises router advertises; but the issue is on the AWS side propagation.
✗
Verify that the virtual interface is in the 'available' state and BGP session is established.
Why it's wrong here
BGP session might be up but routes not propagated to VPC route tables.
✗
Verify that the VGW is attached to the VPC.
Why it's wrong here
Attachment is required but doesn't ensure routes are propagated into the route tables.
✓
Verify that route propagation is enabled on the VPC's route tables associated with the subnets.
Why this is correct
Without route propagation, the BGP routes from Direct Connect are not added to the route tables.
Clue confirmation
The clue word "first" 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: Verify that route propagation is enabled on the VPC's route tables associated with the subnets. — Option B is correct because the route propagation from the VGW to the VPC route tables must be enabled for the learned BGP routes to be used. Option A is wrong because the VGW is the correct attachment. Option C is wrong because the VIF state being up doesn't guarantee route propagation. Option D is wrong because the customer gateway (CGW) is configured on the AWS side; the on-premises router is separate.
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: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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