The running config is held in the DRAM of the router. It is only stored in nvram once it has been written / saved to startup config. If the config had never been saved it would be flushed once the box rebooted.
As Martin points out the config is normally stored in NVRAM rather than flash (though there is a configuration option to store the config in flash). But this is a detail and probably your issue is something different.
As Martin points out one possible explanation for this issue is that the config was not saved after being created and was lost at the reboot. In my experience there is another thing that frequently causes this symptom. If the config register is set to 0x2142 then the router boots and ignores the startup config. The symptom looks like the startup config is lost, but if you do show startup it will clearly still be there. Setting the config register is part of password recovery and some other similar maintenance activities and if the config register is not set back to normal at the completion of the maintenance it will produce exactly the symptom that you describe. So please check the value of the config register (it is displayed at the bottom of the show version display).
We are pleased to announce availability of Beta software for 16.6.3.
16.6.3 will be the second rebuild on the 16.6 release train targeted
towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are
looking for early feedback from customers befor...
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