When a router starts up it needs to know which software to load and which configuration file to use. The way it determines these two things is by looking at its configuration register setting and its startup configuration in nonvolatile RAM (NVRAM). This lesson will cover how a router initializes. Then it will discuss what the configuration register settings mean and how they are used.

When a router is booting up, it searches its configuration register to determine where to find the Cisco IOS® Software. After the Cisco IOS Software has been loaded, the router attempts to load the configuration file, if one exists. This file contains all the user-specified configuration information for the router.

If your router does not find a valid system image, or if you interrupt the boot sequence, the system enters ROM monitor mode. From ROM monitor mode, you can boot the device or perform diagnostic tests. You can configure the router to automatically initiate ROM monitor mode every time the router starts up.

You can also configure the router to boot the Cisco IOS image file from ROM or to look in NVRAM for user-defined instructions on where to locate the image file. The configuration file is stored in NVRAM; however, you can configure the router to load the configuration file from a Trivial File Transfer Protocol (TFTP) server.

If  no configuration file exists, the router reverts to setup mode, an interactive dialog that allows you to create a basic configuration for the router. If the router is configured to load the software from a TFTP server and the server cannot be found, the router uses the configuration file in NVRAM. If the TFTP server is available, the router loads the alternate configuration file stored there.

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:

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.

 Next, go to Lab: Config-Register Settings .

All contents are Copyright © 2000-2002 Cisco Systems, Inc. All rights reserved. Important Notices and Privacy Statement.