- A
Ensure the VPN connection is attached to a transit gateway instead of a virtual private gateway.
Why wrong: VPN can be attached to either; both work. The attachment type is not the first check.
- B
Check the network ACL associated with the EC2 instance's subnet to ensure it allows inbound traffic from on-premises.
Why wrong: While important, routing is the first thing to check; network ACLs are stateless and may block traffic, but routing is the more common issue.
- C
Verify that the VPC route table includes a route to the on-premises CIDR with the VPN gateway as target.
Traffic from on-premises enters the VPC via VPN, but the return traffic needs a route back to the VPN gateway.
- D
Confirm that BGP session is established and exchanging routes.
Why wrong: Tunnel status up indicates BGP may be working, but routes might not be propagated to the VPC route table.
Quick Answer
The answer is to verify that the VPC route table includes a route to the on-premises CIDR with the VPN gateway as the target. When troubleshooting site-to-site VPN connectivity, the route table is the first place to check because even with a healthy tunnel, traffic will not be forwarded from the VPC to the on-premises network without a specific route pointing to the virtual private gateway. This scenario is a classic trap on the AWS Certified Advanced Networking Specialty ANS-C01 exam, testing your ability to isolate routing issues from security group or BGP problems—many candidates mistakenly check network ACLs or tunnel status first. The key insight is that a tunnel being “up” only confirms Layer 3 connectivity, but the VPC’s route table controls Layer 3 forwarding decisions. For a quick memory tip: think “Tunnel Up, Route Missing” — if traffic fails despite a green tunnel, always check the route table before anything else.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 an AWS Site-to-Site VPN connection between its on-premises network and a VPC. The tunnel status is up, but traffic from on-premises cannot reach an EC2 instance in the VPC. The instance's security group allows inbound traffic from the on-premises CIDR. Which configuration should be checked first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"first"Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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
Verify that the VPC route table includes a route to the on-premises CIDR with the VPN gateway as target.
Option A is correct. The VPC's route table must have a route pointing to the VPN gateway for the on-premises CIDR. If missing, traffic will not be forwarded to the VPN tunnel. Option B is wrong because network ACLs are stateless and need both inbound and outbound rules; but first check routing. Option C is wrong because the VPN connection uses a virtual private gateway, not a transit gateway, unless specifically designed. Option D is wrong because the VPN tunnel being up indicates BGP is working; the issue is likely routing.
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.
- ✗
Ensure the VPN connection is attached to a transit gateway instead of a virtual private gateway.
Why it's wrong here
VPN can be attached to either; both work. The attachment type is not the first check.
- ✗
Check the network ACL associated with the EC2 instance's subnet to ensure it allows inbound traffic from on-premises.
Why it's wrong here
While important, routing is the first thing to check; network ACLs are stateless and may block traffic, but routing is the more common issue.
- ✓
Verify that the VPC route table includes a route to the on-premises CIDR with the VPN gateway as target.
- ✗
Confirm that BGP session is established and exchanging routes.
Why it's wrong here
Tunnel status up indicates BGP may be working, but routes might not be propagated to the VPC route table.
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 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.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Verify that the VPC route table includes a route to the on-premises CIDR with the VPN gateway as target. — Option A is correct. The VPC's route table must have a route pointing to the VPN gateway for the on-premises CIDR. If missing, traffic will not be forwarded to the VPN tunnel. Option B is wrong because network ACLs are stateless and need both inbound and outbound rules; but first check routing. Option C is wrong because the VPN connection uses a virtual private gateway, not a transit gateway, unless specifically designed. Option D is wrong because the VPN tunnel being up indicates BGP is working; the issue is likely routing.
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: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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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