The lowest four bits of the 16-bit configuration register (bits 3, 2, 1, and 0) form the boot field. The following boot field values determine if the router loads an operating system and where it obtains the system image:

• When the entire boot field equals 0-0-0-0 (0x0), the router does not load a system image. Instead, it enters ROM monitor or "maintenance" mode, from which you can enter ROM monitor commands to manually load a system image.
• When the entire boot field equals 0-0-0-1 (0x1), the router loads the boot helper or rxboot image.
• When the entire boot field equals a value between 0-0-1-0 (0x2) and 1-1-1-1 (0xF), the router loads the system image specified by boot system commands in the startup configuration file.
• When the startup configuration file does not contain boot system commands, the router first tries to load a valid image from the default Flash device, and then if one does not exist it tries to load a default system image stored on a network server.
• When loading a default system image from a network server, the router uses the configuration register settings to determine the default system image filename for booting from a network server. The router forms the default boot filename by starting with the word cisco and then appending the octal equivalent of the boot field number in the configuration register, followed by a hyphen (-) and the processor type name (cisconn-cpu). See the appropriate hardware installation guide for details on the configuration register and the default filename.

The following flow chart shows, in more detail, how the configuration register value and initial state of the router dictate the manner in which the Cisco device will boot. For more information on rebooting a Cisco router, see Rebooting a Router (Note: document is in .pdf format).

The table below explains each of the bits that make up the configuration register and the functions that each of these bits control. Note that the functions corresponding to the bits shown are listed for a Cisco 7500 Series Router. However, many of the functions are identical for other series routers. The factory default value for the software configuration register of a Cisco 7500 Series Router is 0x0101. This value is a combination of the following: binary bit 8 = 0x0100 and bits 00 through 03 = 0x0001. This default value may also be different for different series routers.

 

Bit No.	Hex Value	Meaning/Function
00 to 03	0x0000 to 0x000F	Defines the source of a default Cisco IOS Software image required to run the router: 00—Stays at the system bootstrap prompt. 01—Boots the first system image in onboard Flash memory. 02 to 0F—Specifies a default netboot filename. Enables boot system commands that override the default netboot filename.
06	0x0040	Causes system software to ignore NVRAM contents.
07	0x0080	Enables the original equipment manufacturer (OEM) bit.
08	0x0100	Disables the Break function.
09	0x0200	Uses secondary bootstrap.
10	0x0400	Broadcasts Internet Protocol (IP) with all zeros.
11 and 12	0x0800 to 0x1000	Defines the console baud rate (the default setting is 9600 baud).
13	0x2000	Boots default Flash software if network boot fails.
14	0x4000	Causes IP broadcasts to leave out network numbers.
15	0x8000	Enables diagnostic messages and ignores the contents of NVRAM.

After the router has completed the initialization process, it begins operations. At this point, you can add new configuration parameters or alter an existing configuration. In either case, you interact with the Cisco IOS command-line interface (CLI) to enter configuration commands. If there is a configuration file stored in NVRAM, it is called the startup configuration. The configuration the router is actually running, which may be modified while in operation, is called the running configuration.

To learn more about the configuration register hexadecimal values and how to set them, view the following netbit on configuration registers by clicking on the icon to the right.