Fortigate 100f (downgrade from 6.4.6 to 6.2.3 ) this error appears ....

Hello,

Downgrading FortiGate is not supported and is usually not recommended. If you would like to use an older version, then you should format the FortiGate and then install the image using TFTP. 

https://community.fortinet.com/t5/FortiWeb/Technical-Tip-How-to-format-and-load-software-version-via-TFTP/ta-p/198014

Are you able to see the BIOS menu when the FortiGate boots up?

Kind regards,

Kavin

You may have to format reload the box

https://community.fortinet.com/t5/FortiGate/Technical-Tip-Formatting-and-loading-FortiGate-firmware-image/ta-p/191946?externalID=FD51468

Did you just reflash or did you also format boot device before flashing? If you only reflashed the config is still there...

In my experience, If you downgrade the firmware, the only way you can make things work is to apply the config of the same version of the config as the version firmware. Downgrading never keeps all the config and there are always config import errors present. 

If you have FortiCloud enabled, maybe you have a backup for 6.2.3 there?

If not, I would suggest resseting to factory default and pasting the config file manualy, from serial port or sepparate managmed interface. 

good luck, 

Jernej

yeah either what nejcs wrote or do a factory reset before downgrade. Newer config with older firmware always causes conflicts. 

Since you did not do a factory reset before downgrade you now only have the option left to format boot device plus reflash via TFTP to get the FGT back for factory defaults and downgrade it.

Note that we have the upgrade path, but it does not say that path can be followed reverse.

The path is made to make sure section and config migration is properly done across versions. Guess that is because not every version can be aware of every other older version's settings and sections.

any permanent solution for this issue I need a solution for this formated and installed but still it's facing the same issues.

same here.. Format and TFTP reflash but the box doesnt come up.

Similar issue here.
When downgrading to 6.0.14, it appears the same error while booting, but boot completes.
However, any of the interfaces can't be accesible

We had a similar discussion earlier this month.
https://community.fortinet.com/t5/Fortinet-Forum/FortiGate-100F-does-not-pass-any-traffic-after-firmware/m-p/213837#M194828

Toshi