PCNSE Secure Access and VPN Practice Question
A network administrator is troubleshooting an IPsec site-to-site VPN that fails to establish. IKE phase 1 completes successfully, but phase 2 fails with a 'no proposal chosen' message. Both sides have identical IKE and IPsec crypto profiles, and the pre-shared key is correct. What is the most likely cause of the failure?
⚠ Common exam trap
Candidates often assume identical crypto profiles guarantee phase 2 success, overlooking that proxy IDs (traffic selectors) are a separate, critical parameter that must be mirrored exactly on both peers.
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 proxy IDs (local/remote subnets) do not match between peers
In IPsec site-to-site VPNs, IKE phase 1 establishes the secure management channel using parameters like encryption, authentication, and Diffie-Hellman groups. Phase 2 negotiates the IPsec security associations (SAs) for actual data traffic, and the 'no proposal chosen' error indicates a mismatch in the phase 2 parameters. Since both sides have identical crypto profiles and the pre-shared key is correct, the most likely cause is that the proxy IDs (local and remote subnets) do not match between peers. Proxy IDs define the traffic selectors that each peer expects to protect; if they are misaligned, the IPsec SA negotiation fails even if all other settings are identical.
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 proxy IDs (local/remote subnets) do not match between peers
Why this is correct
IKE phase 1 succeeding confirms peer authentication and proposal agreement. Phase 2 'no proposal chosen' with identical crypto profiles points to proxy ID mismatch, since the firewall selects the IPsec SA based on matching local and remote subnet selectors.
- ✗
The tunnel is configured as route-based instead of policy-based
Why it's wrong here
Phase 2's 'no proposal chosen' means the IPsec proxy-IDs or traffic selectors do not match, not the tunnel type; route-based and policy-based both negotiate phase 2 identically. Route-based is chosen when the design needs VTI interfaces for dynamic routing or multiple subnets, so it is a design decision made before phase 2 negotiation.
- ✗
The IKE gateway's local interface is down
Why it's wrong here
A down local interface would prevent IKE phase 1 from completing at all, so it cannot explain a phase 2 proposal mismatch. It is tempting because interface state is a common VPN fault, but the stem confirms phase 1 already succeeded.
- ✗
Dead peer detection is not enabled on the IKE gateway
Why it's wrong here
Dead peer detection affects liveness checking of an established tunnel, not phase 2 proposal matching, so it cannot produce 'no proposal chosen'. It is tempting because DPD misconfiguration does cause tunnels to drop, but only after IKE and IPsec SAs have already negotiated.
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 PCNSE practice question is part of Courseiva's free Palo Alto Networks 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 PCNSE exam.