PCNSE Secure Access and VPN Practice Question
An organization has two sites connected via IPSec VPN. The tunnel is up, but ICMP traffic between sites fails. No other traffic works. The firewall policy allows any-any. What is the most likely issue?
⚠ Common exam trap
Watch out — candidates often assume a tunnel being 'up' means all traffic should work, but in Palo Alto Networks, the tunnel state only reflects IKE and IPsec SA establishment, not the correctness of proxy IDs which control traffic selection.
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 (interesting traffic) are not configured correctly.
When the IPSec tunnel is up but no traffic passes, the most common cause is misconfigured proxy IDs (also called interesting traffic selectors). Proxy IDs define which source/destination subnets are permitted through the tunnel; if they don't match on both peers, the tunnel may establish (IKE and IPsec SAs are created) but the firewall will not encrypt or forward traffic because it does not match the defined selectors. Since the firewall policy allows any-any, the issue is not a policy block, pointing directly to proxy ID mismatch.
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 IKE phase 1 proposal is mismatched.
Why it's wrong here
A mismatched IKE phase 1 proposal prevents the tunnel from establishing at all, so it cannot be up. It is tempting because proposal mismatches are a common VPN fault, and would be correct when phase 1 negotiations fail and no security association forms.
- ✓
The proxy IDs (interesting traffic) are not configured correctly.
Why this is correct
With route-based or policy-based tunnels, mismatched proxy IDs mean phase 2 selectors never match, so the firewall drops traffic even though IKE is up. Correctly aligned local and remote proxy IDs restore ICMP and all other traffic.
- ✗
The IPSec crypto profile uses AES-256 and the peer uses 3DES.
Why it's wrong here
Encryption algorithm mismatch is negotiated during IKE and IPSec setup; if the peers disagreed, no tunnel would form. It is tempting because crypto profile mismatches are frequent VPN faults, and would be correct when phase 2 proposals fail and the tunnel never reaches the up state.
- ✗
The tunnel interface MTU is set too low.
Why it's wrong here
An MTU set too low would fragment or drop large packets, yet small ICMP echoes would still traverse the tunnel; total failure of all traffic including ICMP points elsewhere. Low MTU values are legitimately tuned to prevent fragmentation when the tunnel overhead exceeds the path MTU, typically alongside MSS clamping.
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.