# 📚 Making MTU 9000 Persistent on XCP-ng (OVS)
## 📚 Guide Overview
**Title:** Making MTU 9000 Persistent on XCP-ng (OVS)
**Author:** MSLS Partners LLC
**Last Updated:** April 2025
**Description:** This guide explains how to configure MTU 9000 for Open vSwitch (OVS) inside XCP-ng environments. It ensures that jumbo frames remain active and survive reboots, greatly improving network performance for storage and virtual machines.
**Difficulty Level:** Intermediate 🛠️
**Estimated Time:** 15–20 minutes
## 📖 Table of Contents
- [Purpose](#bkmrk-%F0%9F%93%96-purpose)
- [Requirements](#bkmrk-%F0%9F%93%96-requirements)
- [Background](#bkmrk-%F0%9F%93%96-background)
- [Steps](#bkmrk-%F0%9F%93%96-steps)
- [🛠 Step 1: Create the MTU Fix Bash Script](#bkmrk-%F0%9F%9B%A0-step-1%3A-create-the)
- [🛠 Step 2: Create the systemd Service](#bkmrk-%F0%9F%9B%A0-step-2%3A-create-the)
- [🛠 Step 3: Enable the Service](#bkmrk-%F0%9F%9B%A0-step-3%3A-enable-the)
- [🛠 Step 4: Validate](#bkmrk-%F0%9F%9B%A0-step-4%3A-validate)
- [Expected Results](#bkmrk-%F0%9F%93%96-expected-results)
- [Troubleshooting](#bkmrk-%F0%9F%93%96-troubleshooting)
- [Bonus: Quick MTU Validation Script](#bkmrk-%F0%9F%93%96-bonus%3A-quick-mtu-v)
- [Final Notes](#bkmrk-%F0%9F%93%96-final-notes)
## 📖 Purpose
This guide explains how to correctly configure **MTU 9000** on **Open vSwitch (OVS)** inside **XCP-ng** to ensure jumbo frames survive across reboots.
## 📖 Requirements
- XCP-ng 8.x or newer
- Open vSwitch in use (default in XCP-ng)
- SSH access to the hypervisor
- Root privileges
## 📖 Background
- XCP-ng uses Open vSwitch (OVS) to manage VM networking.
- Setting MTU at the Linux NIC level is **not enough**; OVS controls the real MTU behavior.
- XCP-ng resets network bridges at boot, so MTU settings must be **reapplied automatically**.
## 📖 Steps
### 🛠 Step 1: Create the MTU Fix Bash Script
```
sudo nano /usr/local/bin/fix-ovs-mtu.sh
```
Paste this:
```
#!/bin/bash
sleep 20
ovs-vsctl set interface eth4 mtu_request=9000
ovs-vsctl set interface xenbr4 mtu_request=9000
echo "$(date) - MTU 9000 applied to eth4 and xenbr4" >> /var/log/fix-ovs-mtu.log
```
Make it executable:
```
sudo chmod +x /usr/local/bin/fix-ovs-mtu.sh
```
### 🛠 Step 2: Create the systemd Service
```
sudo nano /etc/systemd/system/fix-ovs-mtu.service
```
Paste this:
```
[Unit]
Description=Fix OVS Interfaces MTU to 9000 After Boot
After=network-online.target openvswitch-switch.service
[Service]
ExecStart=/usr/local/bin/fix-ovs-mtu.sh
Type=oneshot
RemainAfterExit=yes
[Install]
WantedBy=multi-user.target
```
### 🛠 Step 3: Enable the Service
```
sudo systemctl daemon-reload
sudo systemctl enable fix-ovs-mtu.service
sudo systemctl start fix-ovs-mtu.service
sudo systemctl status fix-ovs-mtu.service
```
### 🛠 Step 4: Validate
- Check service status:
```
sudo systemctl status fix-ovs-mtu.service
```
- Check MTU settings:
```
ip link show eth4
ip link show xenbr4
```
- Verify inside OVS:
```
ovs-vsctl list interface eth4 | grep mtu
ovs-vsctl list interface xenbr4 | grep mtu
```
## 📖 Expected Results
- eth4 MTU = 9000
- xenbr4 MTU = 9000
- VMs' VIFs inherit MTU 9000
- Jumbo frames supported end-to-end
- Performance improvement (storage, VM transfers)
## 📖 Troubleshooting
| Symptom | Likely Cause | Solution |
|---|
| MTU resets to 1500 after reboot | Systemd service not enabled | Check service status |
| Ping fragmentation on large packets | Switch ports not configured for MTU 9000 | Enable jumbo frames on switch ports |
| iperf3 speed low | MTU not properly set at all layers | Double-check NIC, bridge, VM, and switch |
## 📖 Bonus: Quick MTU Validation Script
```
#!/bin/bash
echo "Checking eth4:"
ip link show eth4 | grep mtu
echo "Checking xenbr4:"
ip link show xenbr4 | grep mtu
echo "Checking VMs VIFs:"
ip link | grep vif | grep mtu
```
## 📖 Final Notes
- Always verify MTU from storage to VM.
- Setting `mtu_request` inside OVS and a systemd fix ensures full persistence.
- Better performance, lower CPU usage for 10GbE network operations.
---
## ✅ Quick Steps Checklist
- Create the MTU fix script at `/usr/local/bin/fix-ovs-mtu.sh`
- Make the script executable
- Create the systemd service at `/etc/systemd/system/fix-ovs-mtu.service`
- Reload systemd
- Enable and start the service
- Verify MTU settings on `eth4` and `xenbr4` after boot
- Validate VMs inherit MTU 9000
- Test with `iperf3` and `ping -M do -s 8972`
---
Created and maintained by **MSLS Partners LLC** 📚
Empowering IT with clear documentation and professional best practices.
Version 1.0 | Last Reviewed: April 2025