The correct answer is that the VPC’s local route takes precedence over the Direct Connect route when the VPC CIDR and the on-premises prefix overlap. This happens because AWS always considers a local route—the implicit route for the VPC’s own CIDR block—as more specific than any propagated or static route, even if the on-premises prefix is advertised via BGP with the same or a longer prefix length. In this scenario, traffic from the VPC destined for 10.0.0.0/16 is treated as local traffic, so it never leaves the VPC to reach the Direct Connect connection. On the ANS-C01 exam, this concept tests your understanding of route precedence in overlapping CIDR scenarios, a common trap where candidates mistakenly think BGP attributes or more specific prefixes override the local route. Remember: local always wins—think of it as the VPC’s “home field advantage” that cannot be beaten by any external route.
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 VIF to a VPC. The on-premises network uses BGP to advertise a specific prefix (10.0.0.0/16) to AWS. The VPC CIDR is 10.0.0.0/16. The company wants to ensure that traffic from the VPC to on-premises uses the Direct Connect connection. However, traffic is going over the internet instead. 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 CIDR overlaps with the on-premises prefix, and the local route in the VPC route table takes precedence.
Option D is correct because when the VPC CIDR and the on-premises prefix overlap, the VPC's local route takes precedence over any other route, including Direct Connect. Traffic destined for the on-premises network would be considered local to the VPC. Option A is wrong because longer prefix match does not apply to overlapping local routes. Option B is wrong because AWS always prefers more specific routes; the issue is the local route. Option C is wrong because BGP ASN prepending does not affect route preference over the local route.
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 BGP ASN is prepended, causing the route to be less preferred.
Why it's wrong here
AS path length does not override the local route.
✗
The on-premises prefix is less specific than the VPC CIDR, so the VPC route takes precedence.
Why it's wrong here
Actually, more specific routes are preferred, but the local route for exact match is always preferred.
✓
The VPC CIDR overlaps with the on-premises prefix, and the local route in the VPC route table takes precedence.
Why this is correct
Overlapping CIDRs cause the local route to be used, ignoring Direct Connect.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The BGP route is not being propagated into the VPC route table.
Why it's wrong here
If the VPC CIDR overlaps, the local route wins regardless of propagation.
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 Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The VPC CIDR overlaps with the on-premises prefix, and the local route in the VPC route table takes precedence. — Option D is correct because when the VPC CIDR and the on-premises prefix overlap, the VPC's local route takes precedence over any other route, including Direct Connect. Traffic destined for the on-premises network would be considered local to the VPC. Option A is wrong because longer prefix match does not apply to overlapping local routes. Option B is wrong because AWS always prefers more specific routes; the issue is the local route. Option C is wrong because BGP ASN prepending does not affect route preference over the local route.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company has a Direct Connect connection with a private VIF to a VPC. The on-premises network uses BGP to advertise its prefixes. Recently, the on-premises router started advertising a more specific route (10.0.0.0/24) that overlaps with the VPC's CIDR (10.0.0.0/16). What is the impact on traffic destined to 10.0.0.5?
medium
A.The instance becomes unreachable from on-premises.
✓ B.Return traffic from the instance to on-premises is routed through Direct Connect instead of staying within the VPC.
C.Traffic destined to the instance is blackholed.
D.Traffic is load-balanced between the VPC and Direct Connect.
Why B: Option C is correct because when a Direct Connect virtual interface advertises a more specific route that overlaps with the VPC CIDR, AWS prefers the more specific route from Direct Connect, causing traffic from the VPC to the on-premises network to be routed via Direct Connect instead of locally. Option A is wrong because the instance remains reachable from on-premises. Option B is wrong because BGP does not cause route flapping in this scenario. Option D is wrong because the traffic is not blackholed; it goes to on-premises.
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This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.
Question Discussion
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