Configuration Register
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:
- To set and display the configuration register value
- To force the system into the ROM monitor or boot ROM
- To select a boot source and default boot filename
- To enable or disable the Break function
- To control broadcast addresses
- To set the console terminal baud rate
- To load operating software from Flash memory
- To enable booting from a Trivial File Transfer Protocol (TFTP)
server
- To recover a lost password
- To manually boot the system using the boot command at the
bootstrap program prompt.
- To force the switch router to boot automatically from the
system bootstrap software (boot image) or from its default
system image in onboard Flash memory, and read any boot system
commands that are stored in the configuration file in NVRAM
Boot Field
| Config Register # |
2 |
1 |
0 |
2 |
| Bit Number |
15 14 13 12 |
11 10 9 8 |
7 6 5 4 |
3 2 1 0 |
| Binary |
0 0 1
0 |
0 0 0 1 |
0 0 0 0 |
0 0 1 0 |
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:
- When the boot field is set to 1 (the factory default), you
will boot from an IOS image stored in ROM.
- When the boot field is set to 0, you must boot the operating
system manually by giving a boot command to the system
bootstrap program, or ROM monitor.
Boot Command
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:
- To enter the ROM monitor (primarily a programmer's
environment), set the configuration register value to 0x2100.
This value sets the boot field bits to 0-0-0-0.
- From the ROM monitor, boot the operating system manually by
issuing the boot command at the ROM monitor prompt.
- To boot the system automatically from ROM, set the
configuration register to 0x2101. This value sets the boot field
bits to 0-0-0-1.
- To configure the system to use the boot system commands in
NVRAM, set the configuration register to any value from 0x2102
to 0x210F. These values set the boot field bits to 0-0-1-0
through 1-1-1-1.
Software Configuration Register
| Bit Number |
Hexadecimal |
Meaning |
| 00 to 03 |
0x0000 to 0x000F |
Boot field |
| 06 |
0x0040 |
Causes system software to ignore NVRAM
contents |
| 07 |
0x0080 |
OEM bit enabled |
| 08 |
0x0100 |
Break disabled |
| 09 |
0x0200 |
Use secondary bootstrap |
| 10 |
0x0400 |
IP broadcast with all zeros |
| 11 to 12 |
0x0800 to 0x1000 |
Console line speed (default is 9600 baud) |
| 13 |
0x2000 |
Boot default Flash software if network boot
fails |
| 14 |
0x4000 |
IP broadcasts do not have network numbers |
| 15 |
0x8000 |
Enable diagnostic messages and ignore NVRAM
contents |
Configuration Register Boot Field
| Boot Field |
Meaning |
Used For: |
| 00 |
ROM monitor mode |
To boot to ROM monitor mode, set the
configuration register to 2100. You must then manually boot
the router with the b command. The router will show a
rommon> prompt. |
| 01 |
Boot image from ROM |
To boot an IOS image stored in ROM, set the
configuration register to 2101. The router will show the
router(boot)> prompt. |
| 02 to 0F |
Specifies a default boot filename |
Any value from 2102 to 210F tells the
router to use the boot commands specified in NVRAM. |
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> ).
Definitions of the Boot Command Options
| Command |
Function |
| boot |
Boots the default system software from
Flash memory. |
| boot flash [filename] |
Boots the first file in onboard Flash
memory. The optional filename argument is the name of
the system image file to boot from onboard Flash memory. |
| boot filename [ip address] |
Boots from server host using TFTP.
IP address of the TFTP server on which the system image
resides. If omitted, this value defaults to the IP broadcast
address of 255.255.255.255 |
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
|