- A
The virtual private gateway is not attached to the VPC
Why wrong: The VGW is attached (given), so this is not the issue.
- B
The VPC has a VPC endpoint for S3 that is causing a routing conflict
Why wrong: VPC endpoints do not interfere with on-premises routing.
- C
The VPC route tables lack a route for the on-premises CIDR pointing to the virtual private gateway
Without a route, VPC instances do not know to send traffic to the VGW for on-premises destinations.
- D
The BGP session is not advertising the on-premises CIDR to the VGW
Why wrong: Given that BGP is established and on-premises can reach VPC, the route is likely advertised, but VPC routing is missing.
Question 1,159 of 1,705
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 is implementing a hybrid network using AWS Direct Connect. They have a virtual private gateway (VGW) attached to their VPC and a Direct Connect gateway (DXGW) with a private virtual interface (VIF) to their on-premises router. They have established a BGP session between the on-premises router and the VGW. The on-premises network can reach EC2 instances in the VPC, but the VPC instances cannot reach on-premises resources. 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 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 VPC route tables lack a route for the on-premises CIDR pointing to the virtual private gateway
The correct answer is C. For instances in a VPC to reach on-premises resources over Direct Connect, the VPC route tables must have a route for the on-premises CIDR that points to the virtual private gateway (VGW). Without this route, traffic from the VPC to the on-premises network will not be forwarded to the VGW, even though the BGP session is established and the on-premises side can reach the VPC. Option A is incorrect because the VGW is attached to the VPC (otherwise on-premises could not reach EC2 instances). Option B is incorrect because a VPC endpoint for S3 does not cause routing conflicts for on-premises traffic. Option D is incorrect because the BGP session is established and the on-premises CIDR is being advertised (otherwise on-premises could not reach the VPC).
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
The VGW is attached (given), so this is not the issue.
- ✗
The VPC has a VPC endpoint for S3 that is causing a routing conflict
Why it's wrong here
VPC endpoints do not interfere with on-premises routing.
- ✓
The VPC route tables lack a route for the on-premises CIDR pointing to the virtual private gateway
Why this is correct
Without a route, VPC instances do not know to send traffic to the VGW for on-premises destinations.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The BGP session is not advertising the on-premises CIDR to the VGW
Why it's wrong here
Given that BGP is established and on-premises can reach VPC, the route is likely advertised, but VPC routing is missing.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
Visual reference
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
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 VPC route tables lack a route for the on-premises CIDR pointing to the virtual private gateway — The correct answer is C. For instances in a VPC to reach on-premises resources over Direct Connect, the VPC route tables must have a route for the on-premises CIDR that points to the virtual private gateway (VGW). Without this route, traffic from the VPC to the on-premises network will not be forwarded to the VGW, even though the BGP session is established and the on-premises side can reach the VPC. Option A is incorrect because the VGW is attached to the VPC (otherwise on-premises could not reach EC2 instances). Option B is incorrect because a VPC endpoint for S3 does not cause routing conflicts for on-premises traffic. Option D is incorrect because the BGP session is established and the on-premises CIDR is being advertised (otherwise on-premises could not reach the VPC).
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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Last reviewed: Jun 20, 2026
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