- A
Disable TCP checksum offloading on the clients.
Why wrong: Checksum offloading might cause corruption but not typical symptom of intermittent mid-transfer drops.
- B
Change the IPSec encryption algorithm from AES-256 to AES-128.
Why wrong: Encryption algorithm does not affect packet size significantly and is unlikely to fix fragmentation.
- C
Increase the TCP timeout value in the security policy.
Why wrong: Timeout would not cause failures mid-transfer but after idle periods.
- D
Reduce the MTU on the branch firewall's WAN interface to 1400.
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.
Question 295 of 516
Resolve PMTUD Failure Causing TCP Timeouts in IPsec VPN
This PCNSE practice question tests your understanding of troubleshoot. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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).
Detailed technical explanation
How to think about this question
IPsec VPNs encapsulate the original IP packet with an ESP header, trailer, and authentication data, adding 50–60 bytes of overhead. When the original packet is already 1500 bytes, the encapsulated packet exceeds the Ethernet MTU of 1500, forcing fragmentation. Many firewalls and routers drop fragments or send ICMP Fragmentation Needed messages that are blocked by security policies, causing TCP to stall. Reducing the WAN interface MTU to 1400 ensures the encapsulated packet fits within 1500 bytes, allowing PMTUD to succeed. The standard recommendation is to set the MTU to 1400 for IPsec tunnels, as it accommodates common overhead without sacrificing throughput.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the PCNSE exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this PCNSE question test?
Troubleshoot — This question tests Troubleshoot — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this PCNSE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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.
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