Technical Note : How to adjust the Maximum Transmission Unit (MTU) value on a FortiGate interface

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MTU definition : 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.

• To change the MTU on a given interface from the GUI proceed as follows :
  

1. Go to System> Network> Interface.
2. Select the Edit icon for the interface.
3. Select Override default MTU value (1500).
4. Enter the new MTU value.
5. Click OK.

• To change the value from the CLI  (example on an AMC module - NP2)  :

config system interface

    edit "amc-sw1/2"

        set vdom "root"

        set mtu-override enable

        set mtu 9000

    next

end

See also the related article  "Technical Note : Jumbo frames support on FortiGate devices"

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