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 22.214.171.124 -l 1472 -f
Pinging 126.96.36.199 with 1472 bytes of data: Reply from 188.8.131.52: bytes=1472 time=41ms TTL=52 Reply from 184.108.40.206: bytes=1472 time=42ms TTL=52 Reply from 220.127.116.11: bytes=1472 time=103ms TTL=52 Reply from 18.104.22.168: bytes=1472 time=38ms TTL=52 Ping statistics for 22.214.171.124: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 38ms, Maximum = 103ms, Average = 56ms
Increase the size and try the ping again:
>ping 126.96.36.199 -l 1473 -f Pinging 188.8.131.52 with 1473 bytes of data: Request timed out. Ping statistics for 184.108.40.206: Packets: Sent = 1, Received = 0, Lost = 1 (100% loss),
The second test fails, so the maximum MTU size on the path is 1472 bytes + 8-byte ICMP header + 20-byte IP header = 1500 bytes
The TCP maximum segment size (MSS) is the maximum amount of data that can be sent in a TCP segment. The MSS is the MTU size of the interface minus the 20 byte IP header and 20 byte TCP header. By reducing the TCP MSS, you can effectively reduce the MTU size of the packet.
The TCP MSS can be configured in a firewall policy, or directly on an interface.
config firewall policy edit <policy ID> set srcintf "internal" set dstintf "wan1" set srcaddr "10.10.10.6" set dstaddr "all" set schedule "always" set service "ALL" set tcp-mss-sender 1448 set tcp-mss-receiver 1448 next end
config system interface edit "wan2" set vdom "root" set mode dhcp set allowaccess ping fgfm set type physical set tcp-mss 1448 set role wan next end