Technical Tip: 'npu_vlink' interface lost after upgrade from 5.4 to 5.6 or above

Description

This article describes the reason of 'npu_vlink' interface being removed after upgrading from 5.4 to 5.6 or above.

Scope
FortiGate with NP ASIC chip.

Solution
1) When upgrading from 5.4 to 5.6, sometimes it is noticed that there are config file error related to 'npu_vlink' interface being removed.

2) In 5.4, when VDOM is not enabled, 'npu_vlink' is still created, while in v5.6 and above, when VDOM is not enabled, 'npu_vlink' is not created.

3) Therefore, if the FortiGate is upgraded when VDOM is not enabled, the 'npu_vlink' interfaces will be removed after the upgrade and generate an error during startup '# diagnose debug config-error-log read'.