The answer is a missing specific route on the on-premises router for the VPC CIDR via the Direct Connect. When the on-premises network lacks a precise route directing traffic for the VPC’s IP range to the Direct Connect virtual interface, the router falls back to its default route, often sending traffic over the internet—creating a suboptimal path. This scenario tests your understanding of how routing tables control traffic flow in hybrid networks, a core concept for the AWS Certified Advanced Networking Specialty ANS-C01 exam. A common trap is assuming the issue lies with AWS-side configurations like prefix lists or ASN prepending, but the root cause is almost always on-premises routing. Remember the memory tip: “On-premises routes decide the path; if the VPC CIDR is missing, the internet is kissing.”
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 uses AWS Direct Connect with a private VIF to connect to a VPC. The network team notices that traffic from on-premises to an EC2 instance in the VPC is taking a suboptimal path through the internet instead of the Direct Connect. 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 does not have a specific route for the VPC CIDR via the Direct Connect.
Option A is correct. If the on-premises network does not have a specific route pointing to the Direct Connect for the VPC CIDR, traffic may use the internet. Option B is wrong because prefix lists are not required. Option C is wrong because ASN prepending would affect outbound traffic, not inbound. Option D is wrong because VPC route tables are not visible to on-premises.
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 on-premises router does not have a specific route for the VPC CIDR via the Direct Connect.
Why this is correct
Without a specific route, traffic may default to internet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The VPC route table does not include a route to the Direct Connect gateway.
Why it's wrong here
VPC routing does not affect on-premises routing decisions.
✗
The Direct Connect virtual interface has BGP ASN prepending configured.
Why it's wrong here
ASN prepending affects outbound path selection, not inbound.
✗
The VPC route table does not have a prefix list for the on-premises CIDR.
Why it's wrong here
Prefix lists are for AWS, not for on-premises routing.
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 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 does not have a specific route for the VPC CIDR via the Direct Connect. — Option A is correct. If the on-premises network does not have a specific route pointing to the Direct Connect for the VPC CIDR, traffic may use the internet. Option B is wrong because prefix lists are not required. Option C is wrong because ASN prepending would affect outbound traffic, not inbound. Option D is wrong because VPC route tables are not visible to on-premises.
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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