The answer is that the on-premises router is not advertising the prefix via BGP over the Direct Connect. This is the most likely cause because for traffic to flow through the Direct Connect private VIF, the on-premises router must advertise the VPC’s CIDR block to the AWS side over the BGP session; if that advertisement is missing or blocked—for instance, by a BGP community policy that rejects the prefix—the virtual private gateway never learns the route, so the VPC route table’s static entry pointing to the virtual gateway remains unused, and traffic defaults to the internet. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of BGP route propagation and the critical role of prefix advertisement in Direct Connect routing, often appearing as a trick where candidates assume a static route alone suffices. A common trap is to blame security groups or more specific routes, but remember: routing decisions happen before security group evaluation. Memory tip: “No BGP ad, no Direct Connect path—if the prefix isn’t spoken, the internet route is broken.”
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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 an AWS Direct Connect connection with a private VIF to a VPC. They notice that traffic from the on-premises network to the VPC is being routed through the internet instead of the Direct Connect. The VPC route table has a route pointing to the virtual private gateway for the on-premises CIDR. 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 on-premises router is not advertising the prefix via BGP over the Direct Connect
Option D is correct because if the on-premises router is advertising the prefix with a community that is not accepted by the Direct Connect BGP peer, the route will not be installed. Option A is incorrect because a more specific route would not cause traffic to go to the internet. Option B is incorrect because the virtual private gateway is the correct target. Option C is incorrect because security groups do not affect routing.
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 virtual private gateway is not attached to the VPC
Why it's wrong here
If not attached, the route would be invalid.
✗
The security group on the VPC resources blocks the traffic
Why it's wrong here
Security groups do not affect routing decisions.
✗
The on-premises CIDR is more specific than the VPC route table entry
Why it's wrong here
More specific routes would still use the virtual private gateway.
✓
The on-premises router is not advertising the prefix via BGP over the Direct Connect
Why this is correct
Without BGP advertisement, the route is not propagated to the VPC.
Clue confirmation
The clue word "most likely" 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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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 on-premises router is not advertising the prefix via BGP over the Direct Connect — Option D is correct because if the on-premises router is advertising the prefix with a community that is not accepted by the Direct Connect BGP peer, the route will not be installed. Option A is incorrect because a more specific route would not cause traffic to go to the internet. Option B is incorrect because the virtual private gateway is the correct target. Option C is incorrect because security groups do not affect routing.
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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