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.
| Number | | |
|---|---|---|
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. | ||
Controls the console Break key:
Break can always be sent in the first 60 seconds while the router is rebooting, regardless of the configuration register settings. | ||
Controls the host portion of the IP broadcast address:
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. | ||
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. | ||
Determines how the router responds to a network boot failure: | ||
Controls the network and subnet portions of the IP broadcast address:
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. | ||
Enables diagnostic messages and ignores the contents of NVRAM. |
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-3 shows how each setting combination of bits 10 and 14 affects the IP broadcast address.
| | | |
|---|---|---|
Table D-4 shows the console line speed for each setting combination of bits 5, 11, and 12.
| | | | |
|---|---|---|---|
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![Cisco 3900 Series, 2900 Series, and 1900 Series Software Configuration Guide - Changing the Configuration Register Settings [Cisco 1900 Series Integrated Services Routers] (1)](https://i0.wp.com/www.cisco.com/c/dam/en/us/td/i/templates/blank.gif "Cisco 3900 Series, 2900 Series, and 1900 Series Software Configuration Guide - Changing the Configuration Register Settings [Cisco 1900 Series Integrated Services Routers] (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 4 If you are asked whether you would like to enter the initial dialog, answer 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:
Step 6 Enter global configuration mode:
Step 7 To change the configuration register settings, enter the config-register value command, where value is a hexadecimal number preceded by 0x :
![Cisco 3900 Series, 2900 Series, and 1900 Series Software Configuration Guide - Changing the Configuration Register Settings [Cisco 1900 Series Integrated Services Routers] (8)](https://i0.wp.com/www.cisco.com/c/dam/en/us/td/i/templates/note.gif "Cisco 3900 Series, 2900 Series, and 1900 Series Software Configuration Guide - Changing the Configuration Register Settings [Cisco 1900 Series Integrated Services Routers] (8)")
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# Step 9 Save the configuration changes to NVRAM:
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:
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.
![Cisco 3900 Series, 2900 Series, and 1900 Series Software Configuration Guide - Changing the Configuration Register Settings [Cisco 1900 Series Integrated Services Routers] (2024)](data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mP8/h8AAvMB+NzmkbcAAAAASUVORK5CYII=)