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Virtual Configuration Register
This appendix describes the router virtual configuration register, the factory default settings, and the procedures for changing those settings.
Following is the information included in this appendix:
The router has a 16-bit virtual configuration register, which is written into the nonvolatile random-access memory (NVRAM). Following are some reasons for changing the virtual configuration register settings:
Table B-1 lists the meaning of each of the virtual configuration memory bits and Table B-2 defines the boot field names.
Table B-1 Virtual Configuration Register Bit Meanings
| Bit No.(1) | Hexadecimal | Meaning |
|---|---|---|
| 00 to 03 | 0x0000 to 0x000F | Boot field (see Table B-2) |
| 06 | 0x0040 | Causes system software to ignore NVRAM contents |
| 07 | 0x0080 | OEM bit enabled |
| 08 | 0x0100 | Break disabled |
| 10 | 0x0400 | IP broadcast with all zeros |
| 11 to 12 | 0x0800 to 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 NVRAM contents |
Table B-2 Explanation of Boot Field (Configuration Register Bits 00 to 03)
| Boot Field | Meaning |
|---|---|
| 0x0 | Stays at the system bootstrap prompt |
| 0x1 | Boots system image in system ROM |
| 0x2 to 0xF | Specifies a default netboot filenameEnables boot system commands that override default netboot filename |
Some common reasons to modify the value of the virtual configuration register follow:
To change the configuration register while running the system software, follow these steps:
router> enable Password: router#
router# configure terminal Enter configuration commands, one per line. Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z
config-register 0xvalue
Configuration register is 0x142 (will be 0x102 at next reload)
The lowest four bits of the virtual 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 as follows:
> b [tftp] flash filename
Definitions of the various b command options follow:
For more information about the b [tftp] flash filename command, refer to the router products configuration publication. The configuration publication is available on Cisco Connection Documentation CD-ROM or a printed copy can be ordered separately.
If you set the boot field value to a value in the range of 0x2 through 0xF, and there is a valid system boot command stored in the configuration file, then the router boots the system software as directed by that value. 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.)
Table B-3 Default Boot Filenames
| Action/Filename | Bit 3 | Bit 2 | Bit 1 | Bit 0 |
|---|---|---|---|---|
| bootstrap mode | 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 |
In the following example, the virtual configuration register is set to boot the router from Flash memory and to ignore Break at the next reboot of the router:
router# configure terminal Enter configuration commands, one per line. Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z config-register 0x102 boot system flash [filename] ^Z router#
The server creates a default 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.
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 the Break key as a command to force the system into the bootstrap monitor, thereby 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.
Table B-4 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> |
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.)
Table B-5 System Console Terminal Baud Rate Settings
| Baud | Bit 12 | Bit 11 |
|---|---|---|
| 9600 | 0 | 0 |
| 4800 | 0 | 1 |
| 1200 | 1 | 0 |
| 2400 | 1 | 1 |
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 set to 1.
To enable booting from Flash memory, set configuration register bits 3, 2, 1, and 0 to a value between 2 and 15 in conjunction with the boot system flash [filename] configuration command.
To enter the configuration mode, while in the system software image, specify a Flash filename from which to boot, enter the configure terminal command at the enable prompt as in the following example:
router# configure terminal Enter configuration commands, one per line. Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z boot system flash [filename]
To disable break and enable the boot system flash command, enter the config-register command with the value shown in the following example:
router# configure terminal Enter configuration commands, one per line. Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z config-reg 0x2102 ^Z router#
Copying to Flash memory might be required whenever a new image or maintenance release becomes available. If the boot ROMs for your system support Flash load helper, you can use Flash load helper to download a new image. Otherwise, you must manually download a new image to Flash.
Flash load helper is a software option that enables you to upgrade system software on run-from-Flash systems that have a single bank of Flash memory. Or you can use Flash load helper on systems with dual-bank Flash memory that support Flash memory partitioning. Flash load helper is an automated procedure that reloads the ROM-based image, downloads the software to Flash memory, and reboots with the system image in Flash memory. Refer to the chapter "Loading System Images, Microcode Images, and Configuration Files" in the Router Products Configuration Guide for detailed instructions about using Flash load helper.
Following is the sample output for reloading the router and then copying a file (called IJ09140Z) to Flash memory from a TFTP server (called server1):
router# configure terminal Enter configuration commands, one per line. Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z config-reg 0x2101 ^Z
The configuration register setting 0x2101 tells the system to boot from ROM, but does not reset the break disable or check for a default netboot filename.
router# reload ... router(boot)# copy tftp flash IP address or name of remote host [255.255.255.255]? server1 Name of tftp filename to copy into flash []? IJ09140Z copy IJ09140Z from 131.131.101.101 into flash memory? [confirm] <Return> xxxxxxxx bytes available for writing without erasure. erase flash before writing? [confirm] <Return> Clearing and initializing flash memory (please wait)####... Loading from 101.2.13.110: !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!... [OK - 324572/524212 bytes] Verifying checksum... VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV... Flash verification successful. Length = 1204637, checksum = 0x95D9 router(boot)# config term Enter configuration commands, one per line. Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z config-reg 0x2102 ^Z
The configuration register setting 0x2102 tells the system to boot from ROM if netboot fails, disable break, and check for a default netboot filename.
router(boot)# reload ...
The system is now ready to be configured to boot from the new image you copied to Flash memory. For more information on the copy tftp flash command, and other related commands, refer to the router products configuration publication.
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