hardMultiple ChoiceObjective-mapped
350-701 Practice Question: Is troubleshooting a site-to-site IPsec VPN that…
A network engineer is troubleshooting a site-to-site IPsec VPN that fails to establish. The IKE phase 1 completes successfully, but phase 2 fails. The debug output shows 'IPSEC(validate_proposal): transform set proposal mismatch'. Both peers have the same transform set configured. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between IKE phase 1 and phase 2 failures, and the trap here is that candidates assume identical transform set names mean identical algorithms, ignoring that default values (like AES key length) can differ between devices or IOS versions.
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
✓
Mismatched encryption/authentication algorithms in the transform set
The error 'IPSEC(validate_proposal): transform set proposal mismatch' indicates that the IPsec transform sets proposed by the two peers do not match during IKE phase 2 negotiation. Even if the transform sets appear identical in configuration, a mismatch in the encryption algorithm (e.g., AES-256 vs AES-128) or authentication algorithm (e.g., SHA-1 vs SHA-256) will cause this failure. Since IKE phase 1 completed successfully, the pre-shared key and routing are not the issue, and lifetime mismatches typically generate a different error.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Mismatched IPsec lifetime values
Why it's wrong here
Can cause mismatch but debug shows transform set, not lifetime.
- ✗
Missing route for the remote subnet
Why it's wrong here
Would cause traffic not to trigger VPN, not a transform set mismatch.
- ✓
Mismatched encryption/authentication algorithms in the transform set
Why this is correct
Even if both sets are named the same, the actual algorithms might differ; 'transform set proposal mismatch' indicates algorithm mismatch.
- ✗
Incorrect pre-shared key
Why it's wrong here
PSK mismatch would fail phase 1, not phase 2.
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 350-701 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 350-701 exam.