FortiGate-VM MTU setting
For optimal performance you can set the MTU of your FortiGate VM interfaces to be compatible with the OpenStack 10 environment, which by default, has an MTU of 1446.
To set the MTU of a FortiGate interface to 1400:
config system interface
set mtu-override enable
set mtu 1400
See these pages for more information on RedHat OpenStack networks and MTU values: