Cisco 4000 Series Virtual Configuration Register

Take the following steps to change the configuration register while running the Cisco IOS software:

Step 1 Enter the enable command and your password to enter the privileged level, as follows:

router> enable

P assword:

router#

Step 2 At the privileged-level system prompt (router #), enter the command configure terminal. You will be prompted as shown in the following example:

router# configure term

Enter configuration commands, one per line.

Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z

Step 3 To set the contents of the configuration register, enter the config-register value configuration command where value is a hexadecimal number preceded by 0x
(see Table D-3), as in the following:

config-register 0x value

(The virtual configuration register is stored in nonvolatile memory.)

Step 4 Press CTRL-Z to exit the configuration mode. The new value settings will be saved to memory; however, the new settings do not take effect until the system software is reloaded by rebooting the router.

Step 5 Enter the show version EXEC command to display the configuration register value currently in effect and the value that will be used at the next reload. The value will be displayed on the last line of the screen display as in the following example:

Configuration register is 0x142 (will be 0x102 at next reload)

Step 6 Reboot the router. The new value takes effect. Configuration register changes take effect only when the server restarts, for example, when you switch the power OFF and ON or when you issue a reload command from the console.

Configuring the Boot Field

The lowest four bits of the processor configuration register (bits 3, 2, 1, and 0) form the boot field. (See Table D-2.)

Table D-2 : Explanation of Boot Field (Configuration Register Bits 00--03)

Boot Field	Meaning
00	Stays at the system bootstrap prompt (ROM monitor) on a reload or power cycle
01	Boots the boot helper image as a system image
02-F	Specifies a default netboot filename Enables default booting from system Flash memory Enables boot system commands that override the default netboot filename1

¹ Values of the boot field are 215 in the form cisco<n>-processor_name, where 2 < n < 15.

The boot field specifies a number in binary. If you set the boot field value to 0, you must have console port access to boot the operating system manually. Boot the operating system manually by entering the b command at the bootstrap prompt as follows:

        > b [tftp] flash filename

Definitions of the various command options follow:

b---Boots the default system software from ROM

b flash---Boots the first file in Flash memory

b filename [host]---Boots over the network using TFTP

b flash [filename]---Boots the file (filename) from Flash memory

For more information about the b [tftp] flash filename command, see the appropriate Cisco IOS software publications.

If you set the boot field value to a value of 2 through F, and there is a valid system boot command stored in the configuration file, the router boots the system software as directed by that value. (See Table D-3.) If you set the boot field to any other bit pattern, the router uses the resulting number to form a default boot filename for booting from a network (TFTP) server.

If there are no boot commands in the configuration file, the router attempts to boot the first file in system Flash memory. If no file is found in system Flash memory, the router attempts to netboot a default file whose name is derived from the value of the boot field (for example: cisco2-4500). If the attempt to boot from a network (TFTP) server fails, the boot helper image in boot Flash will boot up.

If boot commands are in the configuration file, the router software processes each boot command in sequence until the process is successful or the end of the list is reached. If the end of the list is reached without a file being successfully booted, the router will retry the netboot commands up to six times unless the boot default ROM software if netboot fails bit (bit 13 of the virtual configuration register) is set. If bit 13 is set, the system boots the boot helper image found in boot Flash memory without any retries.

In the following example, the virtual configuration register is set to boot the router automatically from Flash memory and to ignore Break at the next reboot of the router:

router# configure terminal        
Enter configuration commands, one per line.
Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z 
config-register 0x102
Ctrl-Z
router# 

The server creates a default boot filename as part of the automatic configuration processes. To form the boot filename, the server starts with cisco and links the octal equivalent of the boot field number, a dash, and the processor-type name. Table D-3 lists the default boot filenames or actions for the processor.

Gateway# configure         
Configuring from terminal, memory, or network [terminal]? term
Enter configuration commands, one per line.
Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z
boot system flash filename

        config-reg 0x102
CTRL/Z