PCNSE Troubleshoot Practice Question
A company with multiple branch offices connects to headquarters using IPSec VPN tunnels terminated on PA-220 firewalls. Users at one branch report intermittent connectivity issues when accessing critical applications hosted at HQ. Ping tests to HQ servers succeed consistently, but TCP-based applications (e.g., file transfers, web access) frequently drop connections after a few seconds, particularly when transferring large data. The VPN tunnel status shows 'active' with no rekeys. Security policies are configured to allow all required application traffic. Interface statistics show no discards or errors. Which action should be taken to resolve the issue?
⚠ Common exam trap
The trap here is that candidates see 'VPN tunnel active' and 'no errors' and assume the issue is application-layer or security-policy-related, overlooking the classic symptom of PMTUD failure where pings succeed (small packets) but TCP transfers fail (large packets).
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
✓
Reduce the MTU on the branch firewall's WAN interface to 1400.
The symptoms—consistent pings but TCP drops on large transfers—strongly point to a Path MTU Discovery (PMTUD) failure. When the VPN tunnel encapsulates packets with IPsec headers, the effective MTU shrinks. If the WAN interface MTU remains at the default 1500, large TCP segments get fragmented or dropped, causing TCP connections to stall. Reducing the MTU on the branch firewall's WAN interface to 1400 ensures that the total packet size (including IPsec overhead) stays within the path's physical MTU, allowing PMTUD to work correctly and preventing silent packet drops.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Disable TCP checksum offloading on the clients.
Why it's wrong here
Checksum offloading might cause corruption but not typical symptom of intermittent mid-transfer drops.
- ✗
Change the IPSec encryption algorithm from AES-256 to AES-128.
Why it's wrong here
Encryption algorithm does not affect packet size significantly and is unlikely to fix fragmentation.
- ✗
Increase the TCP timeout value in the security policy.
Why it's wrong here
Timeout would not cause failures mid-transfer but after idle periods.
- ✓
Reduce the MTU on the branch firewall's WAN interface to 1400.
Why this is correct
MTU mismatch across VPN can cause packet fragmentation and reassembly issues, leading to drops for large packets. Reducing MTU ensures packets fit within the tunnel.
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.