Fail to save external lookup table mode to nvram on slot <SLOT>: Device could not be opened
After a failover, the following message is seen in the log: Fail to save external lookup table mode to nvram on slot <SLOT>: Device could not be opened
The message itself indicate that the configuration to slot <SLOT> is not able to program the settings how the external tables are being used. If the switch is not capable of using the right external table settings, we can expect that the tables are running out-of-sync. That can result in unpredictable behavior.
If this is a mixed stack with X480 as master and another Summit as backup: It is expected behavior after a failover. Only the X480 has a choice of external TCAM tables
If this is a pure X480 stack: Remove slot <SLOT> from the stack and run diagnostics after it has completely powered down. The reboot will reinitialize the NVRAM and if not it will fail diagnostics and the unit need to be replaced.