ANS-C01 Network Design Practice Question
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?
⚠ Common exam trap
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.
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.
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.
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.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.