Table of Contents

Software Configuration Register

Software Configuration Register

This appendix describes the router software configuration register, the factory default settings, and the procedures for changing those settings.

Software Configuration Register Settings

The router has a 16-bit software register, which is written into the nonvolatile memory. Use the processor configuration register information contained in this appendix to do the following:

Table B-1 lists the meaning of each of the software configuration memory bits, and Table B-2 defines the boot field names.

Software Configuration Bit Meanings
Bit No. Hex Meaning
00-03 0x0000-0x000F Boot Field (see Table B-2)
06 0x0040 Ignore NVM contents
07 0x0080 OEM bit enabled
08 0x0100 Break disabled
10 0x0400 IP broadcast with all zeros
11-12 0x0800-0x1000 Console line speed
13 0x2000 Boot default ROM software if network boot fails
14 0x4000 IP broadcasts do not have net numbers
15 0x8000 Enable diagnostic messages and ignore NVM contents

Explanation of Boot Field (Configuration Register Bits 00-03)
Boot Field Meaning
00 Stays at the system bootstrap prompt
01 Boots system image on EPROM
02-F Specifies a default netboot filename

Enables boot system commands that override default netboot filename1

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

Changing Configuration Register Settings

To change software configuration register settings when the operating system is running, use the config-register command following and restart the server. Configuration register changes take effect only when the server restarts: that is, when you switch the power off and on or when you issue a reload command from the console.

In order to issue the configure or reload commands, you must first enable the privileged mode of operation. At the router> prompt, enter enable. The system will prompt you for the privileged password. After you enter the password, the prompt will change to a pound sign (router#). Enter the user configuration dialogue with the config-terminal command.

To set the contents of the configuration register, use the config-register value configuration command. Value is a hexadecimal number preceded by 0x. The software configuration register is stored in nonvolatile memory. For example, the default switch register contents can be set with the command config-register 0x1. To exit the user configuration dialogue, press the Ctrl-Z key combination. To exit the privileged mode, enter disable at the prompt.

The new value settings will be saved to memory; however, the new settings do not take effect until the system software is reloaded.

To display the configuration register value currently in effect and the value that will be used at the next reload, if the two values are different, use the show version command.

The lowest four bits of the processor configuration register (bits 3, 2, 1, and 0) form the boot field. (See Table B-2.) The boot field specifies a number in binary. If you set the boot field value to 0, you must boot the operating system manually by entering the b command at the bootstrap prompt. If you set the boot field value to 1 (the factory default), the router boots using the default ROM software. If you set the boot field to any other bit pattern, the router uses the resulting number to form a default boot filename for netbooting. (See Table B-3.)

Note A boot system configuration command in the router configuration in NVRAM will override the default netboot filename.

The server creates a 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 B-3 lists the default boot filenames or actions for the processor.

Default Boot Filenames
Action/Filename Bit 3 Bit 2 Bit 1 Bit 0
Bootstrap monitor 0 0 0 0
ROM software 0 0 0 1
cisco2-igs 0 0 1 0
cisco3-igs 0 0 1 1
cisco4-igs 0 1 0 0
cisco5-igs 0 1 0 1
cisco6-igs 0 1 1 0
cisco7-igs 0 1 1 1
cisco10-igs 1 0 0 0
cisco11-igs 1 0 0 1
cisco12-igs 1 0 1 0
cisco13-igs 1 0 1 1
cisco14-igs 1 1 0 0
cisco15-igs 1 1 0 1
cisco16-igs 1 1 1 0
cisco17-igs 1 1 1 1

Bit 8 controls the console Break key. Setting bit 8 (the factory default) causes the processor to ignore the console Break key. Clearing bit 8 causes the processor to interpret Break as a command to force the system into the bootstrap monitor, halting normal operation. A Break can be sent in the first
60 seconds while the system reboots, regardless of the configuration settings.

Bit 10 controls the host portion of the Internet broadcast address. Setting bit 10 causes the processor to use all zeros; clearing bit 10 (the factory default) causes the processor to use all ones. Bit 10 interacts with bit 14, which controls the network and subnet portions of the broadcast address. Table B-4 shows the combined effect of bits 10 and 14.

Configuration Register Settings for Broadcast Address Destination
Bit 14 Bit 10 Address (<net> <host>)
off off <ones> <ones>
off on <zeros> <zeros>
on on <net> <zeros>
on off <net> <ones>

Bit 13 determines the server response to a bootload failure. Setting bit 13 causes the server to load operating software from ROM after five unsuccessful attempts to load a boot file from the network. Clearing bit 13 causes the server to continue attempting to load a boot file from the network indefinitely. By factory default, bit 13 is cleared to 0.

Bits 11 and 12 in the configuration register determine the baud rate of the console terminal. Table B-5 shows the bit settings for the four available baud rates. (The factory-set default baud rate is 9600.)

System Console Terminal Baud Rate Settings
Baud Bit 12 Bit 11
9600 0 0
4800 0 1
1200 1 0
2400 1 1

To enable booting from the standalone bootstrap mode, set bits 3, 2, 1, and 0 to a value between 2 and 15 in conjunction with the software configuration command boot system filename. To disable break and enable the boot system command while in the system software image, enter the following commands at the # prompt:

conf 0x010F

To exit configuration editor, press the CtrL-Z key combination.

If you break to the system bootstrap monitor, use the o/r command, followed by the i command to reboot the router with the default configuration register and ignore NVRAM. This is commonly used as a foolproof method to load an image and set your configuration registers.You do not need to use the write memory command to save the changed configuration register values.

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