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What is Configuration Register in Cisco Router/Switch

Configuration register is used to control how the router boots up, it tells the router to load the IOS from flash memory as well as to load the startup configuration from NVRAM. Config register  is a 16 bit (2 byte) value.     Default configuration register value is 0×2102 (represented in Hexadecimal Number).
Main uses of Config register Value are
  • To force the system into the ROM monitor mode
  • To select a boot source (Flash or TFTP) and default boot filename
  • To select the console terminal baud rate
  • To load operating software from ROM
  • To enable or disable the Break Function
To check the current config-register value of your Router, use this Cisco CLI command

Router#Show Version 
To change config register Value, used this Cisco CLI command
Router(config)#Config-register 0×2101
This config register value 0×2101, instructs the router to load small IOS from ROM and then show the current configuration register value.
This config-register value is also modified to recover lost passwords on a cisco Router, and for password recovery of a router the config register value is set to 0×2142.

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