When the router is powered on, it performs a power on self test (POST) which checks its hardware configuration. Then it goes on to find an IOS from the system. There are three places where the router searches for IOS:
1-Flash [The default NVRAM in the router equivalent of hard drive in a computer]
2-RX-Boot [A network location typically a TFTP server]
3-ROM [It contains basic router commands for troubleshooting .. equivalent to safe mode]
Configuration Files
There are two types of configuration files in IOS:
1-startup-configuration [Resides in NVRAM]
2-running-configuration [Resides in RAM]
The startup-configuration file is loaded when the router starts up while running-configuration is volatile and is erased once the router is powered off.
So if you want to save your running-configuration into the NVRAM so that your changes you made to the configuration are permanent, you should use the following command:
#copy running-config startup-config
[This will copy running-config into startup-config.]
