Troubleshooting Tip: Error 'object set operator error,-54' while configuring 'mtu-override' option in the IPsec interface

To check the current MTU value of the Interface on FortiGate, use any one commands:

diagnose netlink interface list <intf_name>

fnsysctl ifconfig <intf_name>

For this example, use 'diagnose netlink interface list vpn-ipsec' or 'fnsysctl ifconfig vpn-ipsec'.

When configuring the mtu-override option in the IPsec VPN interface, it shows the following error:

FGT #config system interface

FGT (interface) # edit vpn-ipsec

FGT (vpn-ipsec) # set mtu-override enable
FGT (vpn-ipsec) # set mtu 1340
FGT (vpn-ipsec) # next
object set operator error, -54 discard the setting
Command fail. Return code 1
FGT (interface) # 

This error appears when the VPN interface has IP and remote-ip configured. Removing the IP and remote-ip in the vpn interface configuration would resolve the issue.

config system interface
    edit vpn-ipsec
        unset ip
        unset remote-ip
        set mtu-override enable
        set mtu 1340
    next
end

Related articles :

Technical Tip: MTU size and Interface related Errors on FortiGate

Technical Tip: How to adjust the Maximum Transmission Unit (MTU) value