300-410 Infrastructure Security Practice Question
A network engineer is troubleshooting an IPsec VPN tunnel between two Cisco IOS routers. The tunnel fails to establish, and the engineer sees the debug output: 'ISAKMP: Unable to find a valid preshared key'. The engineer verifies that the preshared key is identical on both peers. Which additional configuration is most likely causing the issue?
⚠ Common exam trap
The trap here is assuming that identical key strings are sufficient; the key must also be associated with the correct peer IP address.
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 crypto isakmp key command is configured with the wrong peer address.
The error 'Unable to find a valid preshared key' typically occurs when the router cannot match the preshared key to the peer's IP address. This is often due to the 'crypto isakmp key' command specifying an incorrect peer address, even if the key string itself is correct.
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 crypto isakmp key command is configured with the wrong peer address.
Why this is correct
If the preshared key is configured for a peer address that does not match the actual source IP of the remote peer, the router cannot find a valid key for that peer. This results in the 'Unable to find a valid preshared key' error even if the key string is correct. The engineer should verify that the peer address in the 'crypto isakmp key' command matches the remote peer's IP.
- ✗
The crypto map is applied to the wrong interface.
Why it's wrong here
If the crypto map is applied to the wrong interface, the router may not attempt to establish the tunnel at all, or traffic may not be encrypted. However, the specific error about preshared key indicates that the router is attempting ISAKMP negotiation but cannot find a matching key, which is more directly related to the key configuration than the interface.
- ✗
The transform set is not configured with the correct encryption algorithm.
Why it's wrong here
A mismatch in the transform set would cause Phase 2 (IPsec) negotiation to fail, but the error occurs during Phase 1 (ISAKMP), which is before the transform set is used. The preshared key is used in Phase 1, so the transform set is not the cause of this specific error.
- ✗
The ISAKMP policy priority numbers are different on the two peers.
Why it's wrong here
Different ISAKMP policy priority numbers do not cause a preshared key lookup failure. The priority determines the order of policy evaluation, but as long as there is a matching policy with compatible attributes, the tunnel can establish. The error is specifically about the preshared key, not policy mismatch.
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.
Go deeper
Related to this question
About these practice questions
This 300-410 question is part of Courseiva's 1,401-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
This 300-410 practice question is part of Courseiva's free Cisco 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 300-410 exam.