PCNSE Core Concepts and Architecture Practice Question
A firewall administrator notices that traffic from a specific subnet is being unexpectedly dropped. The firewall log shows a 'flow_drop' reason of 'packet too long for interface MTU'. The interface MTU is set to 1500, and the packets are 1500 bytes. What is the most likely cause?
⚠ Common exam trap
Watch out — candidates often assume the firewall drops the packet before encapsulation because the original packet matches the MTU, but the drop occurs after encapsulation adds overhead, making the final frame too large.
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 packet is being encapsulated (e.g., IPsec) after routing, increasing its size beyond 1500 bytes.
When a packet is encapsulated (e.g., by IPsec) after the routing decision, the original packet's size remains 1500 bytes, but the encapsulation adds overhead (e.g., IPsec ESP headers/trailers, typically 50–60 bytes). This causes the resulting frame to exceed the interface MTU of 1500, triggering a 'packet too long for interface MTU' drop. The firewall logs the drop at the physical interface after encapsulation, not before.
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 route lookup for the destination requires a larger MTU.
Why it's wrong here
Route lookup does not set interface MTU; the drop occurs because the 1500-byte packet plus headers exceeds the 1500-byte interface MTU. Larger-MTU routes are tempting since path MTU does affect fragmentation, but here the egress interface itself rejects the frame.
- ✗
The firewall is not performing TCP MSS clamping on the traffic.
Why it's wrong here
MSS clamping adjusts TCP segment size for tunnelled or lower-MTU paths, but the drop is a 1500-byte packet against a 1500-byte interface MTU, so clamping would not prevent it. Clamping is correct when tunnel overhead reduces the effective path MTU.
- ✗
The firewall is using jumbo frames on the internal interface.
Why it's wrong here
A 1500-byte packet plus 14-byte Ethernet header exceeds a 1500 MTU, so the drop is normal framing overhead, not jumbo frames. Jumbo frames raise MTU to 9000 and would prevent this drop; they suit storage or vMotion traffic, not this diagnosis.
- ✓
The packet is being encapsulated (e.g., IPsec) after routing, increasing its size beyond 1500 bytes.
Why this is correct
IPsec encapsulation adds outer headers after the original packet is routed, pushing a 1500-byte frame past the interface MTU and triggering the drop. This matches the logged reason exactly, since the original packet size alone equals the MTU.
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.