configure MTU for routing on 200D?

[thund31]

I mean, i have two physical switch connecting to a fortigate 200D and these two switches are using differnt VLAN IP subnets therefore the fortigate is acting as a router(static routing, not using dynamic).

The two switches had MTU 9000 jumbo configured on all ports and I want to make the transmission between the two switches consistent so the ports on the fortigate that are connected to the 2 switches had MTU 9000 configured too.

now, there's a confusing doubt about routing MTU. I checked this document but still not sure: https://docs.fortinet.com/document/fortigate/6.0.0/handbook/822669/interface-mtu-packet-size

Do I need to configure the routing MTU or something on this fortigate firewall(to ensure that the correct frame size will be consistent between the 2 switches after the routing happened)?

thanks in advance!

[Toshi_Esumi]

If ethernet frames comes through the FGT over the ethernet cable between the switches and the FGT and you want jumbo packets to go/come through without fragmentation, you need to configure the MTU on the FGT interfaces as in the handbook.

[thund31]

@toshi esumi

thanks for the answer!

i forgot to say...

i doubt that whether the routing mtu should be configure on the fortigate to avoid packet fragmentation or not cause i need a consistent MTU size throughout the routing and connection between end point.

is the default routing mtu on fortigate 1500 or not?

[ede_pfau]

Yes, it is 1500 bytes.

I don't see where the cited handbook chapter is ambiguous. If you need a non-standard MTU you have to set it on all interfaces involved (MTU is set on the physical interface on a FGT, a VLAN port inherits it) across the FGT and all switches.

I am not aware of a "routing MTU", a value especially set for routing traffic.

[Toshi_Esumi]

If you have doubt, you can verify with "fnsysctl ifconfig -a <interface_name>".

[emnoc]

What he said and " diag netlink interface list | grep <interface name> "

As far as routing-mtu that is a feature seen in  linux/unix where you can set route and the mtu per that route. That is not doable from within fortios

Ken Felix