The correct answer is to configure the on-premises router to only accept the VPC CIDR route from AWS and not a default route. This prevents internet leakage because a Direct Connect private VIF can propagate a default route (0.0.0.0/0) from AWS to the on-premises network, causing all non-VPC traffic to be routed over the Direct Connect link instead of the internet. On the ANS-C01 exam, this scenario tests your understanding of BGP route advertisement control and the default route propagation behavior of a private VIF with a virtual private gateway. A common trap is assuming that disabling route propagation on the VPC subnet level will block the default route, but BGP advertisements are controlled at the router level, not the subnet level. Memory tip: think of the private VIF as a one-way street for your VPC CIDR only—if AWS sends a default route, your on-premises router will take the wrong exit, so filter inbound BGP routes to accept only your specific prefix.
ANS-C01 Network Design Practice Question
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 uses AWS Direct Connect with a private VIF and a virtual private gateway (VGW) to connect its on-premises data center to a VPC. The on-premises network uses BGP to advertise routes to AWS. The VPC has multiple subnets. The company wants to ensure that only traffic destined for the VPC CIDR (10.0.0.0/16) is sent over Direct Connect, and all other traffic uses the internet. However, after configuration, on-premises users can access the internet through the Direct Connect link, which is unintended. What change should be made to restrict traffic?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure the on-premises router to only accept the VPC CIDR route from AWS and not a default route.
Option B is correct. On-premises routers should only advertise the VPC CIDR (10.0.0.0/16) to AWS, and AWS should not advertise a default route to on-premises. By default, Direct Connect private VIF can propagate a default route if configured. To prevent on-premises from using Direct Connect for internet, ensure that the on-premises router does not accept a default route from AWS or that AWS does not advertise one. Option A is incorrect because removing the VGW disconnects the VPC. Option C is incorrect because adding a static route would not fix the issue. Option D is incorrect because disabling route propagation for the specific subnet does not affect BGP advertisements.
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.
✗
Add a static route in the VPC route table to send all traffic to the internet gateway.
Why it's wrong here
This does not affect on-premises routing.
✗
Disable route propagation for the private subnet route table.
Why it's wrong here
Route propagation affects VPC routing, not on-premises.
✓
Configure the on-premises router to only accept the VPC CIDR route from AWS and not a default route.
Why this is correct
This prevents the on-premises network from using Direct Connect for internet traffic.
Remove the virtual private gateway and use a VPN connection instead.
Why it's wrong here
This would break the Direct Connect connection.
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: Configure the on-premises router to only accept the VPC CIDR route from AWS and not a default route. — Option B is correct. On-premises routers should only advertise the VPC CIDR (10.0.0.0/16) to AWS, and AWS should not advertise a default route to on-premises. By default, Direct Connect private VIF can propagate a default route if configured. To prevent on-premises from using Direct Connect for internet, ensure that the on-premises router does not accept a default route from AWS or that AWS does not advertise one. Option A is incorrect because removing the VGW disconnects the VPC. Option C is incorrect because adding a static route would not fix the issue. Option D is incorrect because disabling route propagation for the specific subnet does not affect BGP advertisements.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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