Interface MTU packet size

You can change the maximum transmission unit (MTU) of the packets that the FortiGate unit transmits to improve network performance. Ideally, the MTU should be the same as the smallest MTU of all the networks between the FortiGate unit and the destination of the packets. If the packets that the FortiGate unit sends are larger than the smallest MTU, they are 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.

To change the MTU, select Override default MTU value (1500) and enter the MTU size based on the addressing mode of the interface

• 68 to 1 500 bytes for static mode
• 576 to 1 500 bytes for DHCP mode
• 576 to 1 492 bytes for PPPoE mode
• larger frame sizes if supported by the FortiGate model – up to 9216 bytes for NP2, NP4, and NP6-accelerated interfaces

Only available on physical interfaces. Virtual interfaces associated with a physical interface inherit the physical interface MTU size.

Interfaces on some 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, as well as on FortiGate-100D series models (for information about your FortiGate unit’s hardware, see the Hardware Acceleration guide). For other models, please contact Fortinet Customer Support for the maximum frame size that is supported.

If you need to enable sending larger frames over a route, you need all Ethernet devices on that route to support that larger frame size, otherwise your larger frames will not be recognized and are dropped.

If you have standard size and larger size frame traffic on the same interface, routing alone cannot 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 that larger size. VLANs will inherit the MTU size from the parent interface. You will need to configure the VLAN to include both ends of the route as well as all switches and routers along the route.

MTU packet size is changed in the CLI. If you select an MTU size larger than your FortiGate unit supports, an error message will indicate this. In this situation, try 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 the FortiGate unit to match the new MTU.

To change the MTU size, use the following CLI commands:

config system interface edit <interface_name>

set mtu-override enable set mtu <byte_size>

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