ANS-C01 Network Implementation Practice Question
A network engineer is setting up a site-to-site VPN connection between an on-premises network and an AWS VPC. The engineer configures the customer gateway device with the correct parameters. However, the VPN tunnel status remains 'DOWN'. What is the most likely cause?
⚠ Common exam trap
A common mix-up: candidates confuse security groups (which apply to VPC resources) with VPN tunnel authentication, leading them to incorrectly select Option D, when in fact security groups have no bearing on IPsec tunnel state.
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 pre-shared key on the customer gateway device does not match the pre-shared key configured in the VPN connection.
The most likely cause is a mismatch in the pre-shared key (PSK) between the customer gateway device and the VPN connection configuration in AWS. IPsec tunnel establishment requires both ends to authenticate using identical PSK values; if they differ, the Internet Key Exchange (IKE) phase 1 negotiation fails, leaving the tunnel status 'DOWN'. This is a common configuration error that directly prevents the VPN from coming up.
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 customer gateway.
Why it's wrong here
VGW is attached to VPC, not customer gateway.
- ✗
The VPN connection is not associated with a subnet.
Why it's wrong here
VPN connection is associated with a VGW, not a subnet.
- ✓
The pre-shared key on the customer gateway device does not match the pre-shared key configured in the VPN connection.
Why this is correct
Mismatched PSK prevents IKE negotiation.
- ✗
The security group associated with the VPN endpoint is blocking the traffic.
Why it's wrong here
Security groups do not apply to VPN tunnel endpoints; they apply to VPC resources.
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.