Technical Tip: How to change MTU of modem/LTE interfaces

This article describes how to change the MTU value for LTE/modem interfaces.

FortiOS 6.4.4 and above.

1) A new CLI option is available on FortiOS 6.4.4 and later to allow modification of MTU values for modem/LTE over the 'wwan' interface.

# config system lte-modem
    set allow-modify-mtu-size <enable/disable>
end

2) The default option for this setting is enabled, which means that whatever MTU size being set for WWAN interface will be ignored and the lte daemon will enforce the MTU size obtained from carrier network for ideal throughput.

3) If the option is set to disabled, the MTU size configured for WWAN interface will be used.

# config system interface
    edit “wwan”
        set mtu-override enable
        set mtu 1000
    end

# config system lte-modem
  set allow-modify-mtu-size disable
end

# diag netlink interface list wwan
if=wwan family=00 type=1 index=24 mtu=1000 link=0 master=0