PCNSE Manage, Monitor and Operate Practice Question
Exhibit
Refer to the exhibit. Apr 12 14:32:15 PA-500: IKE phase 1 negotiation failed for peer 203.0.113.10 [reason: No Proposal Chosen] Apr 12 14:32:17 PA-500: IKE phase 1 negotiation failed for peer 203.0.113.10 [reason: No Proposal Chosen]
The firewall log shows repeated IKE phase 1 negotiation failures. The remote peer is a third-party VPN device. Which of the following is the most likely cause?
⚠ Common exam trap
A common mix-up: candidates assume pre-shared key mismatch is the most common cause of IKE phase 1 failures, but in reality, parameter mismatches (especially encryption algorithms) are more frequent and cause negotiation failures before authentication even begins.
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 IKE encryption algorithm settings do not match between the local firewall and the remote peer.
IKE phase 1 negotiation failures are most commonly caused by mismatched IKE parameters, particularly the encryption algorithm (e.g., AES-256 vs AES-128), hash algorithm, DH group, or lifetime. Since the remote peer is a third-party device, the local firewall and remote peer must agree on all IKE phase 1 proposals; if even one parameter (like encryption) does not match, the negotiation fails. Option C directly addresses this core requirement.
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 remote peer's firewall is blocking UDP port 500.
Why it's wrong here
Port blocking would lead to no response or timeout, not a 'No Proposal Chosen' error.
- ✗
The pre-shared key is incorrect.
Why it's wrong here
Incorrect PSK leads to authentication failure after proposal is selected, not 'No Proposal Chosen'.
- ✓
The IKE encryption algorithm settings do not match between the local firewall and the remote peer.
Why this is correct
Mismatched IKE parameters cause the 'No Proposal Chosen' error during phase 1 negotiation.
- ✗
The IKE version (v1 vs v2) is mismatched between the two devices.
Why it's wrong here
Version mismatch usually results in a 'No acceptable proposal' or 'Version not supported' error.
Quick reference
Symmetric Encryption Algorithm Comparison
| Algorithm | Key Size | Block Size | Status | Notes |
|---|---|---|---|---|
| AES-128 | 128-bit | 128-bit | Current standard | NIST approved; WPA3, TLS |
| AES-256 | 256-bit | 128-bit | Current standard | Preferred for sensitive / govt data |
| 3DES | 112-bit effective | 64-bit | Deprecated (2023) | Replaced by AES |
| DES | 56-bit | 64-bit | Broken | Cracked in < 24 h; never deploy |
| ChaCha20 | 256-bit | Stream cipher | Current | TLS 1.3, WireGuard |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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