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Cisco 4000 Series Virtual Configuration Register
This appendix describes the Cisco 4000 series virtual configuration register, the factory default settings, and the procedures for changing those settings.
The Cisco 4000 series has a 16-bit virtual register, which is written into the nonvolatile random access memory (NVRAM). Use the processor configuration register information contained in this appendix to do the following:
Table B-1 lists the meaning of each of the virtual configuration memory bits.
Table B-1 Virtual Configuration Bit Meanings
| Bit No. | Hex | Meaning |
|---|---|---|
| 00--03 | 0x0000--0x000F | Boot field (see Table B-2) |
| 06 | 0x0040 | Causes system software to ignore nonvolatile memory contents |
| 07 | 0x0080 | OEM bit enabled |
| 08 | 0x0100 | Break disabled |
| 10 | 0x0400 | IP broadcast with all zeros |
| 11--12 | 0x0800--0x1000 | Console line speed |
| 13 | 0x2000 | Boots default ROM software if network boot fails |
| 14 | 0x4000 | IP broadcasts do not have net numbers |
| 15 | 0x8000 | Enables diagnostic messages and ignores NVRAM contents |
Some common reasons to modify the value of the virtual configuration register follow:
To change the configuration register while running the IOS software, follow these steps:
router> enable Password: router#
router# configure term Enter configuration commands, one per line. Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z
config-register 0x value
Configuration register is 0x142 (will be 0x102 at next reload)
The lowest four bits of the processor configuration register (bits 3, 2, 1, and 0) form the boot field. (See Table B-2.)
Table B-2 Explanation of Boot Field (Configuration Register Bits 00--03)
| Boot Field | Meaning |
|---|---|
| 00 | Stays at the system bootstrap prompt (ROM monitor) on a reload or power cycle |
| 01 | Boots the boot helper image as a system image |
| 02-F | Specifies a default netboot filename Enables default booting from system Flash memory Enables boot system commands that override default netboot filename(1) |
The boot field specifies a number in binary. If you set the boot field value to 0, you must have console port access to boot the operating system manually. Boot the operating system by entering the b command at the bootstrap prompt as follows:
> b [tftp] flash filename
Definitions of the various command options follow:
For more information about the b [tftp] flash filename command, see the appropriate software publications.
If you set the boot field value to a value of 2 through F, and there is a valid system boot command stored in the configuration file, the router boots the system software as directed by that value. (See Table B-3.) 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.
If there are no boot commands in the configuration file, the router attempts to boot the first file in system Flash memory. If no file is found in system Flash memory, the router attempts to netboot a default file whose name is derived from the value of the boot field (for example: cisco2-4500). If the netboot attempt fails, the boot helper image in boot Flash will boot up.
If boot commands are in the configuration file, the router software processes each boot command in sequence until the process is successful or the end of the list is reached. If the end of the list is reached without a file being successfully booted, the router will retry the netboot commands up to six times unless the boot default ROM software if netboot fails bit (bit 13 of the virtual configuration register) is set. If bit 13 is set, the system boots the boot helper image found in boot Flash memory without any retries.
In the following example, the virtual configuration register is set to boot the router automatically 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 Ctrl-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.
Table B-3 Default Boot Filenames
| Action/File Name | Bit 3 | Bit 2 | Bit 1 | Bit 0 |
|---|---|---|---|---|
| bootstrap mode | 0 | 0 | 0 | 0 |
| ROM software | 0 | 0 | 0 | 1 |
| cisco2-4000or cisco2-4500 | 0 | 0 | 1 | 0 |
| cisco3-4000or cisco3-4500 | 0 | 0 | 1 | 1 |
| cisco4-4000or cisco4-4500 | 0 | 1 | 0 | 0 |
| cisco5-4000or cisco5-4500 | 0 | 1 | 0 | 1 |
| cisco6-4000or cisco6-4500 | 0 | 1 | 1 | 0 |
| cisco7-4000or cisco7-4500 | 0 | 1 | 1 | 1 |
| cisco10-4000or cisco104500 | 1 | 0 | 0 | 0 |
| cisco11-4000or cisco11-4500 | 1 | 0 | 0 | 1 |
| cisco12-4000or cisco12-4500 | 1 | 0 | 1 | 0 |
| cisco13-4000or cisco13-4500 | 1 | 0 | 1 | 1 |
| cisco14-4000or cisco14-4500 | 1 | 1 | 0 | 0 |
| cisco15-4000or cisco15-4500 | 1 | 1 | 0 | 1 |
| cisco16-4000or cisco16-4500 | 1 | 1 | 1 | 0 |
| cisco17-4000or cisco17-4500 | 1 | 1 | 1 | 1 |
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 Break as a command to force the system into the bootstrap monitor, 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> |
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 indefinitely to attempt loading a boot file from the network. By factory default, bit 13 is cleared to 0.
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 factoryset 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 |
To enable booting from Flash memory, set bits 3, 2, 1, and 0 to a value between 2 through 15. To specify a filename to boot, enter the system software configuration command boot system flash filename in the configuration file.
To enter the configuration mode while in the system software image, enter the configure command at the enable prompt as in the example following:
Gateway# configure Configuring from terminal, memory, or network [terminal]? term 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 a value as follows:
config-reg 0x102 CTRL/Z
If you set the configuration register value to 0x102, as in the example, it is not necessary to enter the boot system flash command unless there is more than one image in Flash.
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