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Question 533 of 1,705

Network Implementation →mediumMultiple ChoiceObjective-mapped

Troubleshooting Direct Connect: When On-Premises Subnet Overlaps with VPC CIDR

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 uses AWS Direct Connect with a private VIF to connect to a VPC. The VPC has a virtual private gateway (VGW). The on-premises network uses BGP to exchange routes with the VGW. The company wants to route traffic from the VPC to an on-premises subnet 192.168.1.0/24. The on-premises router advertises 192.168.1.0/24 over BGP. However, instances in the VPC cannot reach that subnet. The VPC route table has the local route and a route to the VGW for 0.0.0.0/0. 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.

A
BGP route propagation is disabled on the VPC route table
Why wrong: If disabled, routes wouldn't be in route table; but route propagation is enabled by default for VGW.
B
The 192.168.1.0/24 subnet overlaps with the VPC CIDR, causing the local route to take precedence
Local routes have higher priority than propagated routes.
C
The VPC route table does not have a specific route for 192.168.1.0/24 pointing to the virtual private gateway
Why wrong: Even if it did, the local route might override.
D
The Direct Connect virtual interface does not have the allowed prefixes configured
Why wrong: Allowed prefixes are for Direct Connect Gateway, but the route is advertised.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The 192.168.1.0/24 subnet overlaps with the VPC CIDR, causing the local route to take precedence

The correct answer is B. If the on-premises subnet 192.168.1.0/24 overlaps with the VPC CIDR (e.g., VPC CIDR 192.168.0.0/16), the VPC's local route takes precedence over any propagated BGP routes. Traffic destined for 192.168.1.0/24 stays within the VPC and never reaches the virtual private gateway, even though the on-premises router is advertising the route. Option A is incorrect because BGP route propagation is enabled by default for Direct Connect private VIFs, and even if disabled, a static route to the VGW would be needed. Option C is incorrect because if the subnet overlaps, a specific route to the VGW would be ignored due to the local route's higher priority. Option D is incorrect because allowed prefixes are configured on the Direct Connect gateway, not the VIF, and the route is being advertised successfully.

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.

✗
BGP route propagation is disabled on the VPC route table
Why it's wrong here
If disabled, routes wouldn't be in route table; but route propagation is enabled by default for VGW.
✓
The 192.168.1.0/24 subnet overlaps with the VPC CIDR, causing the local route to take precedence
Why this is correct
Local routes have higher priority than propagated routes.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The VPC route table does not have a specific route for 192.168.1.0/24 pointing to the virtual private gateway
Why it's wrong here
Even if it did, the local route might override.
✗
The Direct Connect virtual interface does not have the allowed prefixes configured
Why it's wrong here
Allowed prefixes are for Direct Connect Gateway, but the route is advertised.

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.

Visual reference

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.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

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 192.168.1.0/24 subnet overlaps with the VPC CIDR, causing the local route to take precedence — The correct answer is B. If the on-premises subnet 192.168.1.0/24 overlaps with the VPC CIDR (e.g., VPC CIDR 192.168.0.0/16), the VPC's local route takes precedence over any propagated BGP routes. Traffic destined for 192.168.1.0/24 stays within the VPC and never reaches the virtual private gateway, even though the on-premises router is advertising the route. Option A is incorrect because BGP route propagation is enabled by default for Direct Connect private VIFs, and even if disabled, a static route to the VGW would be needed. Option C is incorrect because if the subnet overlaps, a specific route to the VGW would be ignored due to the local route's higher priority. Option D is incorrect because allowed prefixes are configured on the Direct Connect gateway, not the VIF, and the route is being advertised successfully.

What should I do if I get this ANS-C01 question wrong?

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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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.