- A
The virtual private gateway is not attached to the customer gateway.
Why wrong: VGW is attached to VPC, not customer gateway.
- B
The VPN connection is not associated with a subnet.
Why wrong: VPN connection is associated with a VGW, not a subnet.
- C
The pre-shared key on the customer gateway device does not match the pre-shared key configured in the VPN connection.
Mismatched PSK prevents IKE negotiation.
- D
The security group associated with the VPN endpoint is blocking the traffic.
Why wrong: Security groups do not apply to VPN tunnel endpoints; they apply to VPC resources.
Question 680 of 1,705
VPN Tunnel DOWN? Pre-Shared Key Mismatch Is the Likely Cause
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 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?
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 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.
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 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.
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.
- ✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that 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.
Detailed technical explanation
How to think about this question
IPsec VPNs use IKE to establish a secure tunnel; IKE phase 1 authenticates peers using a pre-shared key or certificates. If the PSK mismatches, the IKE SA (Security Association) cannot be formed, and the tunnel remains in a DOWN state. AWS VPN logs in CloudWatch can show IKE negotiation failures with 'NO_PROPOSAL_CHOSEN' or 'INVALID_KEY_INFORMATION' messages, helping diagnose PSK mismatches.
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 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.
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 Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
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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