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.
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 VPC route table does not have a specific route for 192.168.1.0/24 pointing to the VGW. The default route (0.0.0.0/0) goes to the VGW, but if the on-premises subnet is part of the VPC CIDR (e.g., if VPC CIDR is 192.168.0.0/16), then the local route takes precedence and traffic never goes to VGW. Since the subnet is 192.168.1.0/24, it could be within a VPC CIDR like 192.168.0.0/16. The local route would match and traffic stays in VPC. Option B is wrong because allowed prefixes are configured in Direct Connect Gateway, not VPC route table. Option C is wrong because if the subnet is within VPC CIDR, propagation doesn't help. Option D is wrong because the route is advertised.
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.
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.
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 192.168.1.0/24 subnet overlaps with the VPC CIDR, causing the local route to take precedence — The VPC route table does not have a specific route for 192.168.1.0/24 pointing to the VGW. The default route (0.0.0.0/0) goes to the VGW, but if the on-premises subnet is part of the VPC CIDR (e.g., if VPC CIDR is 192.168.0.0/16), then the local route takes precedence and traffic never goes to VGW. Since the subnet is 192.168.1.0/24, it could be within a VPC CIDR like 192.168.0.0/16. The local route would match and traffic stays in VPC. Option B is wrong because allowed prefixes are configured in Direct Connect Gateway, not VPC route table. Option C is wrong because if the subnet is within VPC CIDR, propagation doesn't help. Option D is wrong because the route is advertised.
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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