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Configuration register

Configuration register
5.1.4
This page will explain how a router uses the configuration register. Students will also learn how to change the boot field.
The order in which the router looks for system bootstrap information depends on the boot field setting in the configuration register. The default configuration register setting can be changed with the global configuration mode command config-register. Use a hexadecimal number as the argument for this command.
The configuration register is a 16-bit register in NVRAM that is represented as 4 hexadecimal digits. The lowest four bits of the configuration register form the boot field. To ensure that the upper 12 bits are not changed, first use the show version command to retrieve the current values of the configuration register. Then use the config-register command and change only the value of the last hexadecimal digit.
To change the boot field in the configuration register, follow these guidelines: 
  • To enter the ROM monitor mode, set the configuration register value to 0xnnn0, where nnn represents the previous value of the non-boot field digits. This value sets the boot field bits to 0000 binary. After a reload or power cycle, the device will boot to ROM monitor prompt. Use the b command to boot the operating system manually.
  • To boot from the first image in Flash or to boot to the IOS in ROM (platform dependant), set the configuration register to 0xnnn1, where nnn represents the previous value of the non-boot field digits. This value sets the boot field bits to 0001 binary. Older platforms, such as Cisco 1600 and 2500 routers, will boot to a limited IOS in ROM. Newer platforms, such as Cisco 1700, 2600 and high end routers, will boot from the first image in Flash.
  • To configure the system to use the boot system commands in NVRAM, set the configuration register to any value from 0xnnn2 to 0xnnnF, where nnn represents the previous value of the non-boot field digits. These values set the boot field bits to a value between 0010 and 1111 binary. The router sequentially processes each boot system command in NVRAM until the process is successful or the end of the list is reached. If there are no boot system commands in the startup configuration file, the router attempts to boot the first file in flash memory.
The Lab Activity will require students to change the boot process of a router.
The next page will teach students how to troubleshoot IOS boot failures.

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