The answer is a missing route in the VPC route table for the on-premises CIDR pointing to the virtual private gateway. When troubleshooting a Direct Connect private VIF return route missing issue, the core concept is that AWS requires an explicit route in the VPC’s route table to direct return traffic back to the on-premises network via the VGW; without it, the VPC instances have no path to respond, even though the BGP session is established and inbound traffic from on-premises succeeds. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of asymmetric routing and the distinction between BGP route propagation and VPC route table configuration—a common trap is assuming that a working BGP session automatically creates the return path. Remember the memory tip: “BGP brings routes in, but the VPC route table lets them out.”
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 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
Option B is correct because the VPC route table must have a route for the on-premises CIDR pointing to the virtual private gateway. Option A is wrong because the VGW is already attached. Option C is wrong because BGP is established. Option D is wrong because VPC endpoints are not relevant.
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.
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.
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 route tables lack a route for the on-premises CIDR pointing to the virtual private gateway — Option B is correct because the VPC route table must have a route for the on-premises CIDR pointing to the virtual private gateway. Option A is wrong because the VGW is already attached. Option C is wrong because BGP is established. Option D is wrong because VPC endpoints are not relevant.
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
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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 single private virtual interface (VIF) to a virtual private gateway (VGW) attached to a VPC. The VPC CIDR is 10.0.0.0/16. The on-premises CIDR is 172.16.0.0/12. The BGP session is established, and the on-premises router is advertising the 172.16.0.0/12 route to the VGW. The VGW is configured to propagate routes to the VPC route tables. However, instances in the VPC cannot reach on-premises resources. The VPC route table shows a propagated route for 172.16.0.0/12 with a target of the VGW. What is the most likely issue?
medium
A.The security groups for the VPC instances do not allow outbound traffic to the on-premises network
B.The VPC route table does not have a route for the on-premises CIDR
✓ C.The on-premises router does not have a route to the VPC CIDR via the Direct Connect interface
D.The BGP session is not advertising the VPC CIDR to the on-premises router
Why C: Option C is correct because the on-premises router must also have a route back to the VPC CIDR via the Direct Connect interface. Option A is wrong because the VPC route table has the route. Option B is wrong because the BGP session is established. Option D is wrong because security groups are stateful and outbound traffic is allowed by default.
Last reviewed: Jun 20, 2026
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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.
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