Changing the Configuration Register Settings

The following sections describe the 16-bit configuration register in NVRAM in the Cisco 3900 series, Cisco 2900 series, and Cisco 1900 series integrated services routers (ISRs):

About the Configuration Register

The router has a 16-bit configuration register in NVRAM. Each bit has value 1 (on or set) or value 0 (off or clear), and each bit setting affects the router behavior upon the next reload power cycle.

You can use the configuration register to

  • Force the router to boot into the ROM monitor (bootstrap program)
  • Select a boot source and default boot filename
  • Enable or disable the Break function
  • Control broadcast addresses
  • Recover a lost password
  • Change the console line speed

Table D-1 describes the configuration register bits.

 

Table D-1 Configuration Register Bit Descriptions

Bit
Number
Hexadecimal
Meaning

00–03

0x0000–0x000F

Boot field. The boot field setting determines whether the router loads an operating system and where it obtains the system image.

See Table D-2 for details.

06

0x0040

Causes the system software to ignore the contents of NVRAM.

07

0x0080

OEM1 bit enabled.

08

0x0100

Controls the console Break key:

  • (Factory default) Setting bit 8 causes the processor to ignore the console Break key.
  • Clearing bit 8 causes the processor to interpret Break as a command to force the router into the ROM monitor mode, halting normal operation.

Break can always be sent in the first 60 seconds while the router is rebooting, regardless of the configuration register settings.

09

0x0200

This bit controls the system boot:

  • Setting bit 9 causes the system to use the secondary bootstrap.
  • (Factory default) Clearing bit 9 causes the system to boot from flash memory.
  • This bit is typically not modified.

10

0x0400

Controls the host portion of the IP broadcast address:

  • Setting bit 10 causes the processor to use all zeros.
  • (Factory default) Clearing bit 10 causes the processor to use all ones.

Bit 10 interacts with bit 14, which controls the network and subnet portions of the IP broadcast address. See Table D-3 for the combined effects of bits 10 and 14.

05, 11, 12

0x0020, 0x0800, 0x1000

Controls the console line speed. See Table D-4 for the eight available bit combinations and console line speeds.

Factory default is 9600 baud, where bits 5, 11, and 12 are all zero (clear).

Note You cannot change the console line speed configuration register bits from the Cisco IOS CLI2. You can, however, change these bits from the ROM monitor. Or, instead of changing the configuration register settings, you can set the console line speed through other Cisco IOS commands.

13

0x2000

Determines how the router responds to a network boot failure:

  • Setting bit 13 causes the router to boot the default ROM software after 6 unsuccessful network boot attempts.
  • (Factory default) Clearing bit 13 causes the router to indefinitely continue network boot attempts.

14

0x4000

Controls the network and subnet portions of the IP broadcast address:

  • Setting bit 10 causes the processor to use all zeros.
  • (Factory default) Clearing bit 10 causes the processor to use all ones.

Bit 14 interacts with bit 10, which controls the host portion of the IP broadcast address. See Table D-3 for the combined effect of bits 10 and 14.

15

0x8000

Enables diagnostic messages and ignores the contents of NVRAM.

1.OEM = Original Equipment Manufacturer

2.CLI = command-line interface

Table D-2 describes the boot field, which is the lowest four bits of the configuration register (bits 3, 2, 1, and 0). The boot field setting determines whether the router loads an operating system and where the router obtains the system image.

 

Table D-2 Boot Field Configuration Register Bit Descriptions

Boot Field
(Bits 3, 2, 1, and 0)
Meaning

0000

(0x0)

At the next power cycle or reload, the router boots to the ROM monitor (bootstrap program). To use the ROM monitor, you must use a terminal or PC that is connected to the router console port. For information about connecting the router to a PC or terminal, see the hardware installation guide for your router.

In ROM monitor mode, you must manually boot the system image or any other image by using the boot ROM monitor command.

0001

(0x01)

Boots the first image in flash memory as a system image.

0010 - 1111

(0x02 - 0xF)

At the next power cycle or reload, the router sequentially processes each boot system command in global configuration mode that is stored in the configuration file until the system boots successfully.

If no boot system commands are stored in the configuration file, or if executing those commands is unsuccessful, then the router attempts to boot the first image file in flash memory.

Table D-3 shows how each setting combination of bits 10 and 14 affects the IP broadcast address.

 

Table D-3 Broadcast Address Configuration Register Bit Combinations

Bit 10
Bit 14
Broadcast Address (<net> <host>)

0

0

<ones> <ones>

1

0

<ones> <zeros>

1

1

<zeros> <zeros>

0

1

<zeros> <ones>

Table D-4 shows the console line speed for each setting combination of bits 5, 11, and 12.

 

Table D-4 Console Line Speed Configuration Register Bit Combinations

Bit 5
Bit 11
Bit 12
Console Line Speed (baud)

1

1

1

115200

1

0

1

57600

1

1

0

38400

1

0

0

19200

0

0

0

9600

0

1

0

4800

0

1

1

2400

0

0

1

1200

Changing the Configuration Register Settings

You can change the configuration register settings from either the ROM monitor or the Cisco IOS CLI. This section describes how to modify the configuration register settings from the Cisco IOS CLI.

To change the configuration register using the ROM monitor, see Appendix C, “Using ROM Monitor,” in this guide.

To change the configuration register settings from the Cisco IOS CLI, complete the following steps:

Step 1 Connect a terminal or PC to the router console port. If you need help, see the hardware installation guide for your router.

Step 2 Configure your terminal or terminal emulation software for 9600 baud (default), 8 data bits, no parity, and 2 stop bits.

Step 3 Power on the router.

Step 4 If you are asked whether you would like to enter the initial dialog, answer no:

Would you like to enter the initial dialog? [yes]: no
 

After a few seconds, the user EXEC prompt ( Router>) appears.

Step 5 Enter privileged EXEC mode by typing enable and, if prompted, enter your password:

Router> enable
Password: password
Router#
 

Step 6 Enter global configuration mode:

Router# configure terminal
 
Enter configuration commands, one per line.
Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z
 

Step 7 To change the configuration register settings, enter the config-register value command, where value is a hexadecimal number preceded by 0x :

Router(config)# config-register 0xvalue

Note The Cisco IOS software does not allow you to change the console speed bits directly with the config-register command. To change the console speed from the Cisco IOS CLI, see the “Configuring the Console Line Speed (Cisco IOS CLI)” section.

Step 8 Exit global configuration mode:

Router(config)# end
Router#
 

Step 9 Save the configuration changes to NVRAM:

Router# copy run start

The new configuration register settings are saved to NVRAM, but they do not take effect until the next router reload or power cycle.

 

Displaying the Configuration Register Settings

To display the configuration register settings that are currently in effect and the settings that will be used at the next router reload, enter the show version command in privileged EXEC mode.

The configuration register settings are displayed in the last line of the show version command output:

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

Configuring the Console Line Speed (Cisco IOS CLI)

The combined setting of bits 5, 11, and 12 determines the console line speed. You can modify these particular configuration register bits only from the ROM monitor.

To change the configuration register using the ROM monitor, see Appendix C, “Using ROM Monitor” .

To configure the console line speed from the Cisco IOS command-line interface, complete the following steps.

SUMMARY STEPS

1. enable

2. configure terminal

3. line console 0

4. speed baud

DETAILED STEPS

 

Command or Action
Purpose

Step 1

enable

 

Router> enable

Password: password

Router#

Enables privileged EXEC mode. Enter your password if prompted.

Step 2

configure terminal

 

Router# configure terminal

Router(config)#

Enters global configuration mode.

Step 3

line console 0

 

Router(config)# line console 0
Router(config-line)#

Specifies the console line and enters line configuration mode.

Step 4

speed baud

 

Router(config-line)# speed baud

Specifies the console line speed. Possible values (in baud): 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200.