PCNE Practice Question: Managing, Monitoring, and Optimising Network Operations
An engineer notices that some packets sent from a Compute Engine VM in GCP to an on-premises server via a VPN tunnel are being fragmented. The on-premises server is not receiving the fragmented packets. What is the most likely cause?
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 VM's MTU is set to 1500
GCP uses an MTU of 1460 bytes on VMs, while the internet standard is 1500. If the VPN encapsulates packets, the additional overhead reduces effective MTU. Fragmentation may occur but is often problematic. The issue is typically due to an MTU mismatch, and setting the VM's MTU to 1460 or lower (e.g., 1400) ensures no fragmentation.
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 VM's MTU is set to 1500
Why this is correct
VMs with MTU 1500 may generate packets too large after VPN encapsulation, causing fragmentation. The correct step is to set MTU to 1460 or lower.
- ✗
Cloud NAT is not configured
Why it's wrong here
Cloud NAT handles outbound connectivity for private VMs lacking external IPs, not packet fragmentation. Fragmentation occurs due to a mismatch between the VPN tunnel’s Maximum Transmission Unit (MTU) and the packet size; the correct fix involves adjusting the tunnel’s MTU or enabling Path MTU Discovery. Cloud NAT is tempting because it manages source NAT for internet-bound traffic, which could appear relevant if the on-premises server were unreachable due to missing public IP routing, but that is not the fragmentation issue described.
- ✗
The VPN tunnel is using UDP encapsulation
Why it's wrong here
UDP encapsulation is normal for IPsec and does not inherently cause fragmentation issues.
- ✗
The on-premises firewall is dropping ICMP
Why it's wrong here
While ICMP unreachable messages can help, they are not the root cause.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCNE practice question is part of Courseiva's free Google Cloud 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 PCNE exam.