Configuration Register

This chapter describe the 16-bit configuration register in NVRAM in the Cisco 4000 series integrated services routers (ISRs).

Information About Configuration Register

The Cisco 4000 Series ISR platforms have 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 ROMMON (bootstrap program)

  • Select a boot source and default boot filename

  • Recover a lost password

  • Change the console line speed

Table 1 describes the configuration register bits.

Table 1. Configuration Register Bit Descriptions

Bit Number

Hexadecimal

Description

00–03

0x0000–0x00

0F

Boot field. The boot field setting determines whether the router loads an operating system.

See table for more information.

06

0x0040

Causes the system software to ignore the contents of NVRAM.

07

0x0080

This bit is defined as OEM1 enabled.

Note

 

This bit is deprecated on the Cisco 4000 Series ISR platforms.

08

0x0100

This bit controls the console break key.

Note

 

This bit is deprecated on the Cisco 4000 Series ISR platforms.

09

0x0200

This bit controls the system boot.

Note

 

This bit is deprecated on the Cisco 4000 Series ISR platforms.

10

0x0400

This bit controls the host portion of the IP broadcast address.

Note

 

This bit is deprecated on the Cisco 4000 Series ISR platforms.

05, 11, 12

0x0020,

0x0800,

0x1000

This bit controls the console line speed. See Table 3 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 CLI. 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

0x0200

This bit determines how the router responds to a network boot failure.

Note

 

This bit is deprecated on the Cisco 4000 Series ISR platforms.

14

0x0400

This bit controls the network and subnet portions of the IP broadcast address.

Note

 

This bit is deprecated on the Cisco 4000 Series ISR platforms.

15

0x0800

This bit enables diagnostic messages and ignores the contents of NVRAM.
1 Original Equipment Manufacturer

Note

Deprecated bits will not take effect, regardless of the value.

Table 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 2. Boot Field Configuration Register Bit Descriptions

Boot Field

(Bits 3, 2, 1, and 0)

Description

0000

(0x0)

At the next power cycle or reload, the router boots to the ROMmon (bootstrap program). To use the ROMmon, you must use a terminal or PC that isconnected 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 ROMmon mode, you must manually boot the system image or any otherimage by using the boot ROMmon command.

0001 - 1111

(0x01- 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 execution of these commands is unsuccessful, the router attempts to boot the first image file in flash memory.

Table 3 shows the console line speed for each setting combination of bits 5, 11, and 12.

Table 3. 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

This section describes how to modify the configuration register settings from the Cisco IOS CLI.

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

Procedure

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 enableand, 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 0x1

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 section.

Step 8

Exit global configuration mode:

Router(config)# end
Router#

Step 9

Save the configuration changes to NVRAM:

Router# copy run start

What to do next

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

Configuring the Console Line Speed

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 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

Example:

Router> enable 
Password: password 
Router#

Enables privileged EXEC mode. Enter your password if prompted.

Step 2

configure terminal

Example:

Router# configure terminal 
Router(config)#

Enters global configuration mode.

Step 3

line console 0

Example:

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

Specifies the console line and enters line configuration mode.

Step 4

speed baud

Example:

Router(config-line)# speed baud

Specifies the console line speed. Possible values (in baud) are:

  • 1200

  • 2400

  • 4800

  • 9600

  • 19200

  • 38400

  • 57600

  • 115200

Verifying 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)