hardMultiple ChoiceObjective-mapped
PCNSA Practice Question: A security administrator is troubleshooting a…
A security administrator is troubleshooting a site-to-site IPsec VPN between two Palo Alto Networks firewalls. The Phase 1 proposal includes AES-256, SHA-256, and DH Group 14 with a lifetime of 28800 seconds. The Phase 2 proposal includes AES-256, SHA-256, and PFS with DH Group 14. The tunnel is established and traffic is flowing, but intermittently the tunnel drops and re-establishes. The logs show the following error: 'Phase 2 negotiation failed because no suitable proposal found.' Both firewalls have IKE gateway and IPsec crypto profile configurations that match for encryption, authentication, and DH groups. Which option is the most likely cause of this issue?
⚠ Common exam trap
A common mix-up: candidates assume matching IKE gateway and IPsec crypto profile proposals guarantee matching Phase 2 parameters, but they overlook that the IPsec SA lifetime is a separate parameter that must be explicitly set to the same value on both peers, and a default value mismatch is a common cause of intermittent rekey failures.
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 IPsec SA lifetime is not configured on one of the firewalls, causing a mismatch.
The error 'Phase 2 negotiation failed because no suitable proposal found' occurs when the IPsec SA parameters do not match during rekey. Although both firewalls have matching IKE gateway and IPsec crypto profiles for encryption, authentication, and DH groups, the IPsec SA lifetime is a separate parameter that might not be identical if not explicitly configured. If one firewall has a different default lifetime or an explicitly configured different lifetime, the mismatch causes rekey failures. This is the most likely cause because the tunnel initially establishes but fails intermittently when the lifetime timer expires and renegotiation occurs.
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 DH group used in Phase 2 is not supported by the firewall model.
Why it's wrong here
Incorrect: DH Group 14 is widely supported.
- ✓
The IPsec SA lifetime is not configured on one of the firewalls, causing a mismatch.
Why this is correct
Correct: If the IPsec SA lifetime is not explicitly set, the firewall uses a default value that may differ from the peer's configured value, leading to proposal mismatch.
- ✗
The Phase 2 proposal uses a different DH group than Phase 1.
Why it's wrong here
Incorrect: PFS uses DH group 14, but the error is not about DH group mismatch.
- ✗
The Phase 1 lifetime is shorter than the Phase 2 lifetime.
Why it's wrong here
Incorrect: Phase 1 lifetime is typically longer than Phase 2; the error is not lifetime-related.
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 PCNSA 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 PCNSA exam.