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 set up a site-to-site VPN connection between its on-premises network and AWS. The tunnel status shows 'UP' on both sides, but traffic from on-premises cannot reach EC2 instances in the VPC. 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 do not have a route pointing to the virtual private gateway for the on-premises CIDR.
The correct answer is C because the tunnel being up indicates Layer 3 connectivity, but traffic may be blocked by VPC route tables or security group rules. Option A is incorrect because tunnel status is up. Option B is incorrect because a mismatch in pre-shared keys would prevent the tunnel from establishing. Option D is incorrect because the VPNgateway is the target, not a route table entry.
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 pre-shared keys are mismatched.
Why it's wrong here
A pre-shared key mismatch would prevent the tunnel from establishing.
✓
The VPC route tables do not have a route pointing to the virtual private gateway for the on-premises CIDR.
Why this is correct
Without a route to the virtual private gateway, traffic from the VPC to on-premises will not be forwarded.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The VPN tunnel has been idle for too long and needs to be re-initiated.
Why it's wrong here
The tunnel status shows 'UP', so it is not idle.
✗
The security group associated with the EC2 instances does not allow inbound traffic from the VPN gateway.
Why it's wrong here
Security groups are stateful and evaluate inbound rules; however, the primary issue is likely routing.
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.
Trap categories for this question
Command / output trap
The tunnel status shows 'UP', so it is not idle.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 do not have a route pointing to the virtual private gateway for the on-premises CIDR. — The correct answer is C because the tunnel being up indicates Layer 3 connectivity, but traffic may be blocked by VPC route tables or security group rules. Option A is incorrect because tunnel status is up. Option B is incorrect because a mismatch in pre-shared keys would prevent the tunnel from establishing. Option D is incorrect because the VPN gateway is the target, not a route table entry.
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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