Routers use a configuration register to find some configuration settings at boot time
16 bits (4 hex digits)
For example, the command
config-register 0x2100 sets the value to hexadecimal 2100, , which causes the router to load the ROMMON OS rather than IOS next time the router is reloaded.
Cisco routers automatically save the new configuration register value when you press Enter at the end of the config-register command.
Configuration register’s new value has no effect until the next time the router is reloaded.
Engineers set the configuration register to different values for many reasons, but the most common are to help tell the router what IOS image to load.
The configuration register can be used to change router behavior in several ways, such as:
– How the router boots (into ROMmon, NetBoot)
– Options while booting (ignore configuration, disable boot messages)
– Console speed (baud rate for a terminal emulation session)