Forti 40F problem with MTU.

[Mikeost]

Hello, I have problem with MTU. My configuration on the interface:

FortiWiFi-40F # show system interface wan config system interface edit “wan” set vdom “root” set ip 10.10.10.1 255.255.255.0 set allowaccess ping https ssh http set type physical set monitor-bandwidth enable set role wan set snmp-index 1 set mtu-override enable set mtu 3000 next end

but I can ping this address  (10.10.10.1) from other router with mtu 9000 and don’t fragment set. Why?

I'm sure, the router send packets 9000B.

[Toshi_Esumi]

First, I strongly suggest you run sniffing on wan interface at 40F to make sure you get 9000 byte packets w/ DF bit on. And you would be able to see how the 40F is responding back.

I don't know the answer because I haven't tested it before. But my guess is the MTU and fragmentation apply only packets coming from another interface and going out this interface. You can test it by pinging through it if you have another device/router supports jumbo packets connected on another interface.

And the sniffing would give you most of the answers.