MTU Settings

[Keeper_of_the_Keys]

Hi,

When you have an LACP aggregated link and/or VLAN interfaces in a fortigate at what "level" are you supposed to set the MTU?

On our different generations of switches I have seen different behavior and I don't know which applies to Fortigate.

I have seen:

- Jumbo frames are set per vlan

- Jumbo frames are set per port (on the port level and not the lag level)

The FortiOS system interfaces table contains items for each port, vlan and lag so where am I supposed to set the mtu?

Thanks!

[Keeper_of_the_Keys]

Anyone?

[Kangming]

Hi   Maybe you can refer to these documents on kb:   Technical Note: MTU size and Jumbo frames support on FortiGate devices

https://kb.fortinet.com/kb/microsites/search.do?cmd=displayKC&docType=kc&externalId=FD30914&sliceId=...

Technical Note : How to adjust the Maximum Transmission Unit (MTU) value on a FortiGate interface

https://kb.fortinet.com/kb/documentLink.do?popup=true&externalID=11745&languageId=

Technical Tip: Define MTU size larger than 1500 on VLAN interfaces

 https://kb.fortinet.com/kb/microsites/search.do?cmd=displayKC&docType=kc&externalId=FD49051&sliceId=...

[Keeper_of_the_Keys · Accepted solution]

Hey thanks for the reply!

The point of the question is that none of these documents address what the priority/order of defining custom MTUs should be.

So should I be defining on the port + lag + vlan? port + vlan? lag + vlan? vlan only?

It is not clear from the docs.

The documentation you linked to suggests port + vlan, however they are not using an LACP aggregate.

From some experimenting it seems to be lag first then vlans under the lag and no need to set on the ports that make up the lag.

But tbh I still don't know 100% if I wasn't supposed to also set this at the port level and I would much rather *know* how it is supposed to work than try to establish it through trial and error.

Thanks again!