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Routers use a 16-bit software configuration register, with which you can set specific system parameters. Settings for the software configuration register are written into nonvolatile random access memory (NVRAM).Some reasons for changing the software configuration register settings are as follows:
The lowest four bits of the configuration register (bits 3, 2, 1, and 0) form the boot field. The order in which the switch/router looks for system bootstrap information depends on the boot field setting in the configuration register.
When the boot field is set to either 0 or 1 (0-0-0-0 or 0-0-0-1), the system ignores any boot instructions in the system configuration file and the following occurs:
You can enter the boot command only, or include additional boot instructions with the command, such as the name of a file stored in Flash memory or a file that you specify for booting from a network tftp server.
If you use the boot command without specifying a file or any other boot instructions, the system boots from the default Flash image (the first image in onboard Flash memory). Otherwise, you can instruct the system to boot from a specific Flash image (using the boot system flash filename command), or boot from a network server by sending broadcast TFTP requests (using the boot system filename command), or by sending a direct TFTP request to a specific server (using the boot system filename ip address command).
You can also use the boot command to boot images stored in the Personal Computer Memory Card International Association (PCMCIA) Flash memory cards in the route processor. If you set the boot field to any bit pattern other than 0 or 1, the system uses the resulting number to form a filename for booting over the network. The system uses the filename to invoke the system image by booting over the net. However, if the configuration file contains any boot instructions, the system uses those boot instructions instead of the filename it computed from the configuration register settings.
You must set the boot field for the boot functions you require. For more detailed information on the software configuration register features, refer to the following sections.
Changing the Software Configuration Register
Settings for the software configuration register are written into NVRAM. The factory default value for Cisco router is 0x2102.
You can change the default configuration register setting with the enabled config-mode command config-register. Use a hexadecimal number as the argument to this command. For example, the command
Router(config)# config-register 0x2100
configures the router to boot to ROM monitor mode.A Summary of Boot Tasks and Their Register Values
To change the boot field and leave all the other bits set to their default values, follow these guidelines:
Understanding the Boot Field Commands
The lowest four bits of the software configuration register (bits 3, 2, 1, and 0) form the boot field. The boot field specifies a number in binary form which you have to convert to Hexadecimal to use in the configuration register. If you set the boot field value to 0, you must boot the operating system manually by entering the boot command at the bootstrap prompt ( either > or for newer platforms rommon> ).
Changing Register Settings While Running System Software
To change the configuration register while running the system software, follow these steps:
Step 1 At the privileged EXEC prompt (Router#), enter the configure terminal command to enter global configuration mode.
Router#configure terminal Router(config)#
Step 2 Set the contents of the configuration register by entering the config-register value configuration command, where value is a hexadecimal number preceded by 0x as in the following example:
Router(config)# config-register 0x2142
Step 3 Press Ctrl-Z to exit Global Configuration mode.
Step 4 Display the current configuration register value, which will be used at the next system reload, by entering the show version command.
The value is displayed on the last line of the screen display, as in the following example:
Configuration register is 0x2102 (will be 0x2142 at next reload)
Step 5 Restart the router.
Changes to the configuration register take effect only when the system reloads.
Router# reload Proceed with reload? [confirm] %SYS-5-RELOAD: Reload Requested
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