Technical Tip: How to adjust the Maximum Transmission Unit (MTU) value

[acp]

Description
This article describes how to adjust the Maximum Transmission Unit (MTU) value on a FortiGate interface.

The MTU is the largest physical packet size, measured in bytes, that a network can transmit. Any packets larger than the MTU are divided into smaller packets before they are sent.
The default MTU is 1500 on a FortiGate interface. 

Solution

The MTU value can only be changed through CLI.

To change the value from the CLI (For example on WAN2):

#config system interface
    edit "wan2"
        set vdom "root"
        set mtu-override enable
        set mtu 9000
    next
end

To verify interface MTU from CLI:

# diag netlink interface list <interface-name>

Note: Changing the MTU value might affect the internet access for a while. Hence it is suggested to change the value by taking a downtime.

Related Articles

Technical Tip: Finding the MTU of the FortiGate interface