- A
The VGW is not configured with the correct BGP ASN
Why wrong: The tunnel is established, so BGP configuration is likely correct.
- B
The VGW is not attached to the VPC
Why wrong: The VPN tunnel is established, indicating the VGW is attached.
- C
The VPN tunnel is not in the UP state
Why wrong: The problem states the tunnel is established.
- D
The private subnet route table does not have a route for the on-premises CIDR pointing to the VGW
Without this route, traffic from the instance to on-premises is dropped.
Question 427 of 1,705
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 a VPC with a public subnet and a private subnet. An EC2 instance in the private subnet needs to communicate with an on-premises server using an IPsec VPN. The company has set up a Virtual Private Gateway (VGW) and a Customer Gateway (CGW) with a Site-to-Site VPN connection. The VPN tunnel is established. However, the EC2 instance cannot ping the on-premises server. The security groups and network ACLs allow all traffic. 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 private subnet route table does not have a route for the on-premises CIDR pointing to the VGW
The most likely cause is that the private subnet's route table lacks a route for the on-premises CIDR pointing to the Virtual Private Gateway (VGW). Even though the VPN tunnel is established, traffic from the EC2 instance in the private subnet will be dropped at the subnet level if there is no explicit route directing the on-premises destination to the VGW. Security groups and network ACLs are permissive, so the issue is routing, not filtering.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 VGW is not configured with the correct BGP ASN
Why it's wrong here
The tunnel is established, so BGP configuration is likely correct.
- ✗
The VGW is not attached to the VPC
Why it's wrong here
The VPN tunnel is established, indicating the VGW is attached.
- ✗
The VPN tunnel is not in the UP state
Why it's wrong here
The problem states the tunnel is established.
- ✓
The private subnet route table does not have a route for the on-premises CIDR pointing to the VGW
Why this is correct
Without this route, traffic from the instance to on-premises is dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the misconception that a VPN tunnel being 'UP' automatically allows traffic, but in AWS, you must also configure the route tables in the VPC to direct traffic to the VGW for the remote network.
Detailed technical explanation
How to think about this question
In AWS Site-to-Site VPN, the VGW acts as the target for routes in the VPC route tables. For traffic from a private subnet to reach an on-premises network, the private subnet's route table must contain a route for the on-premises CIDR block with the VGW as the target. Without this route, the EC2 instance will attempt to send traffic to the internet gateway (if present) or drop it, even though the VPN tunnel is active. This is a common misconfiguration where engineers focus on tunnel status and security rules but overlook subnet routing.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Visual reference
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
What to study next
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The private subnet route table does not have a route for the on-premises CIDR pointing to the VGW — The most likely cause is that the private subnet's route table lacks a route for the on-premises CIDR pointing to the Virtual Private Gateway (VGW). Even though the VPN tunnel is established, traffic from the EC2 instance in the private subnet will be dropped at the subnet level if there is no explicit route directing the on-premises destination to the VGW. Security groups and network ACLs are permissive, so the issue is routing, not filtering.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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