Interface MTU packet size

You can change the maximum transmission unit (MTU) of the packets that FortiGate transmits to improve network performance. Ideally, the MTU should be the same as the smallest MTU of all the networks between FortiGate and the destination of the packets. If the packets that the FortiGate sends are larger than the smallest MTU, they're broken up or fragmented, which slows down transmission. You can easily experiment by lowering the MTU to find an MTU size for optimum network performance.

• 68 to 1500 bytes for static mode
• 576 to 1500 bytes for DHCP mode
• 576 to 1492 bytes for PPPoE mode
• Larger frame sizes (if supported by the FortiGate model), up to 9216 bytes for NP2, NP4, and NP6-accelerated interfaces

This option is available only for physical interfaces. Virtual interfaces associated with a physical interface inherit the physical interface MTU size.

Interfaces on some FortiGate models support frames larger than the traditional 1500 bytes. Jumbo frames are supported on FortiGate models that have either a SOC2 or NP4lite (except for the FortiGate 30D), and on FortiGate 100D series models. For information about your FortiGate model’s hardware, see FortiOS Hardware Acceleration. To find out the maximum frame size that's supported for other models, visit the Fortinet Support website.

If you need to send larger frames over a route, all Ethernet devices on that route must support the larger frame size. Otherwise, the larger frames won't be recognized and will be dropped.

If you have standard size and larger size frame traffic on the same interface, routing alone can't route them to different routes based only on frame size. However, you can use VLANs to make sure the larger frame traffic is routed over network devices that support the larger size. VLANs inherit the MTU size from the parent interface. You must configure the VLAN to include both ends of the route, as well as all switches and routers along the route.

You can configure the MTU packet size. If you select an MTU size larger than your FortiGate model supports, an error message will indicate this. In this situation, try configuring a smaller MTU size until the value is supported.

In transparent mode, if you change the MTU of an interface, you must change the MTU of all interfaces on FortiGate to match the new MTU.

To change the MTU size - CLI:

config system interface

edit <interface name>

set mtu-override enable

set mtu <byte size>

next

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