Changing the Software Configuration Register Settings

Overview of the Software Configuration Register

The Catalyst 8500 campus switch router uses a 16-bit software configuration register, with which you can set specific system parameters. Settings for the software configuration register are written into nonvolatile random access memory (NVRAM).

Some reasons for changing the software configuration register settings are as follows:

• To set and display the configuration register value
  
  
• To force the system into the ROM monitor or boot ROM
  
  
• To select a boot source and default boot filename
  
  
• To enable or disable the Break function
  
  
• To control broadcast addresses
  
  
• To set the console terminal baud rate
  
  
• To load operating software from Flash memory
  
  
• To enable booting from a Trivial File Transfer Protocol (TFTP) server
  
  
• To recover a lost password
  
  
• To manually boot the system using the boot command at the bootstrap program prompt.
  
  
• To force the switch router to boot automatically from the system bootstrap software (boot image) or from its default system image in onboard Flash memory, and read any boot system commands that are stored in the configuration file in NVRAM
  
  

Boot Field

The lowest four bits of the configuration register (bits 3, 2, 1, and 0) form the boot field. The order in which the switch router looks for system bootstrap information depends on the boot field setting in the configuration register.

Bits 0 through 3 of the software configuration register form the boot field, specified as a binary number. The factory default configuration register setting for systems is 0x102; for switch processor spares the default is 0x101.

When the boot field is set to either 0 or 1 (0-0-0-0 or 0-0-0-1), the system ignores any boot instructions in the system configuration file and the following occurs:

• When the boot field is set to 1 (the factory default), the system boots the first image in the onboard bootflash single in-line memory module (SIMM).
  
  
• When the boot field is set to 0, you must boot the operating system manually by giving a boot command to the system bootstrap program, or ROM monitor.
  
  

Boot Command

You can enter the boot command only, or include additional boot instructions with the command, such as the name of a file stored in Flash memory or a file that you specify for booting from a network server. For details, see Table C-2, "Configuration Register Boot Field."

If you use the boot command without specifying a file or any other boot instructions, the system boots from the default Flash image (the first image in onboard Flash memory). Otherwise, you can instruct the system to boot from a specific Flash image (using the boot system flash filename command), or boot from a network server by sending broadcast TFTP requests (using the boot system filename command), or by sending a direct TFTP request to a specific server (using the boot system filename address command).

You can also use the boot command to boot images stored in the Personal Computer Memory Card International Association (PCMCIA) Flash memory cards in the route processor. If you set the boot field to any bit pattern other than 0 or 1, the system uses the resulting number to form a filename for booting over the network.

The system uses the filename to invoke the system image by booting over the net. However, if the configuration file contains any boot instructions, the system uses those boot instructions instead of the filename it computed from the configuration register settings.

You must set the boot field for the boot functions you require. For more detailed information on the software configuration register features, refer to the following sections.

Changing the Software Configuration Register

Settings for the software configuration register are written into NVRAM. The factory default value for the register is 0x0101.

You can change the default configuration register setting with the enabled config-mode command config-register. Use a hexadecimal number as the argument to this command (see Table C-1, "Software Configuration Register" for a list of values). For example, the command

Router(config)# config-register 0x10F

configures the switch router to examine the startup file in NVRAM for boot system options.

A Summary of Boot Tasks and Their Register Values

To change the boot field and leave all the other bits set to their default values, follow these guidelines:

• To enter the ROM monitor (primarily a programmer's environment), set the configuration register value to 0x100. This value sets the boot field bits to 0-0-0-0.
  
  

  From the ROM monitor, boot the operating system manually by issuing the boot command at the ROM monitor prompt.

• To boot the system automatically from ROM, set the configuration register to 0x101. This value sets the boot field bits to 0-0-0-1.
  
  
• To configure the system to use the boot system commands in NVRAM, set the configuration register to any value from 0x102 to 0x10F. These values set the boot field bits to 0-0-1-0 through 1-1-1-1.
  
  

Table C-1 lists the meaning of each of the software configuration memory bits, and Table C-2 defines the boot field.

To check the boot field setting and verify the results of the config-register command, use the show version command.

Caution To avoid confusion and possibly halting the switch router, remember that valid configuration register settings might be combinations of settings and not just the individual settings listed in Table C-1. For example, the factory default value of 0x0101 is a combination of settings: bit 8 = 0x0100 and bits 00 through 03 = 0x0001.

Understanding the Boot Field Commands

The lowest four bits of the software configuration register (bits 3, 2, 1, and 0) form the boot field (see Table C-3). The boot field specifies a number in binary form. If you set the boot field value to 0, you must boot the operating system manually by entering the boot command at the bootstrap prompt ( > ), as follows:

> boot [tftp] flash filename

Table C-3 displays the boot command options and their functions:

For more information about the boot [tftp | flash] [filename] command, refer to the set of configuration fundamentals publications listed in the section "More Configuration Information" in the chapter "Configuring the Catalyst 8500 Software."

If you set the boot field value to 0x2 through 0xF and there is a valid boot system command stored in the configuration file, the switch router boots the system software as directed by that value. If there is no boot system command, the switch router forms a default boot filename for booting from a network server. (See Table C-3 for the format of these default filenames.)

In the following example, the software configuration register is set to boot the switch router from onboard Flash memory and to ignore Break at the next restart of the switch router:

8500# config terminal

8500(config)# config-register 0x102

8500(config)# boot system flash [filename]
8500(config)# Crtl+z

Table C-4 lists the default boot filenames or actions for the switch 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. Regardless of the setting of the break enable bit, a break will cause a return to the ROM monitor during the first few seconds (approximately five seconds) of booting.

Bit 9 is unused.

Bit 10 controls the host portion of the IP 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 C-5 shows the combined effect of bits 14 and 10.

Bit 11 and Bit 12 in the configuration register determine the data transmission rate of the console terminal. Table C-6 shows the bit settings for the four available rates. The factory-set default data transmission rate is 9600.

Bit 13 determines the server response to a bootload failure. Setting bit 13 causes the server to load operating software from Flash memory 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 cleared to 0.

Changing Register Settings While Running System Software

To change the configuration register while running the system software, follow these steps:

Step 1   At the privileged EXEC prompt (8500#), enter the configure terminal command to enter global configuration mode.

Step 2   Set the contents of the configuration register by entering the config-register value configuration command, where value is a hexadecimal number preceded by 0x (see Table C-1, "Software Configuration Register"), as in the following example:

Step 3   Press Ctrl-Z to exit configuration mode.

Step 4   Display the current configuration register value, which will be used at the next system reload, by entering the show version command.

The value is displayed on the last line of the screen display, as in the following example:

Step 5   Restart the Catalyst 8500 campus switch router.

Changes to the configuration register take effect only when the system reloads.