Changing the maximum transmission unit (MTU) on FortiGate interfaces changes the size of transmitted packets. Most FortiGate device's physical interfaces support jumbo frames that are up to 9216 bytes, but some only support 9000 or 9204 bytes.
To avoid fragmentation, the MTU should be the same as the smallest MTU in all of the networks between the FortiGate and the destination. If the packets sent by the FortiGate are larger than the smallest MTU, then they are fragmented, slowing down the transmission. Packets with the DF flag set in the IPv4 header are dropped and not fragmented .
On many network and endpoint devices, the path MTU is used to determine the smallest MTU and to transmit packets within that size.
ASIC accelerated FortiGate interfaces, such as NP6, NP7, and SOC4 (np6xlite), support MTU sizes up to 9216 bytes.
FortiGate VMs can have varying maximum MTU sizes, depending on the underlying interface and driver.
Virtual interfaces, such as VLAN interfaces, inherit their MTU size from their parent interface.
config system interface edit <interface> set mtu-override enable set mtu ? <integer> Maximum transmission unit (<min>-<max>) next end
config system interface edit <interface> set mtu-override enable set mtu <max bytes> next end
To manually test the maximum MTU size on a path, you can use the ping command on a Windows computer.
For example, you can send ICMP packets of a specific size with a DF flag, and iterate through increasing sizes until the ping fails.
-foption specifies the Do not Fragment (DF) flag.
-loption specifies the length, in bytes, of the Data field in the echo Request messages. This does not include the 8 bytes for the ICMP header and 20 bytes for the IP header. Therefore, if the maximum MTU is 1500 bytes, then the maximum supported data size is: 1500 - 8 - 20 = 1472 bytes.
In Windows command prompt, try a likely MTU size:
>ping 188.8.131.52 -l 1472 -f Pinging 184.108.40.206 with 1472 bytes of data: Reply from 220.127.116.11: bytes=1472 time=41ms TTL=52 Reply from 18.104.22.168: bytes=1472 time=42ms TTL=52 Reply from 22.214.171.124: bytes=1472 time=103ms TTL=52 Reply from 126.96.36.199: bytes=1472 time=38ms TTL=52 Ping statistics for 188.8.131.52: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 38ms, Maximum = 103ms, Average = 56ms