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Uses of Configuration Register in Cisco Router

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Computers and other devices are connected to the Internet using routers. A router serves as a dispatcher, determining the most efficient path for your data to take. It links your company to the outside world, safeguards data from security risks, and even determines which computers receive priority over others. A router enables you to connect numerous devices to the Internet and each other. Additionally, setting up of device local networks can also be done using routers. If you wish to transfer files between devices or allow staff to share software tools, these local networks can be helpful.

Configuration Register:

A router has to know which software to load and which configuration file to utilize when it first powers up. It makes these two determinations by examining the settings in its configuration register and its startup configuration in nonvolatile RAM (NVRAM).

Initialization of Configuration Register:

During boot-up, a router looks in its configuration register to find the location of the Cisco IOS Software. The router tries to load the configuration file if it is present after the Cisco IOS Software has been loaded. All the router configuration data that has been specified by the user is contained in this file. The system goes into ROM monitor mode if your router cannot locate a legitimate system image or if you interrupt the boot process. Booting the device or running diagnostic tests are both possible from ROM monitor mode. When the router first powers on, you can set it up to automatically enter ROM monitor mode. Additionally, you can set up the router to search for user-defined instructions to locate the image file in NVRAM or to boot the Cisco IOS image file from ROM. The configuration file is kept in NVRAM, but you can set the router to download it from a TFTP (Trivial File Transfer Protocol) server instead of directly from storage. In the absence of a configuration file, setup mode an interactive dialog which enables you to set up the router’s fundamental configuration returns to the router. When a TFTP server that the router is set up to utilize for software loading cannot be located, the router falls back on the configuration file stored in NVRAM. When the TFTP server is accessible, the router loads the backup configuration file that is kept there. The boot field is made up of the lowest four bits of the 16-bit configuration register, which are bits 3, 2, 1, and 0. The router’s ability to load an operating system and where it gets the system image depends on the values in the boot field, which are as follows:

  • The router won’t load a system image if the full boot field is equal to 0-0-0-0 (0×0). Instead, it switches to “maintenance” mode, which allows you to manually load a system image by entering ROM monitor commands.
  • The router loads the boot helper or rxboot image when the full boot field equals 0-0-0-1 (0×1).
  • The system image given by boot system commands in the startup configuration file is loaded by the router when the full boot field equals a value between 0-0-1-0 (0×2) and 1-1-1-1 (0xF).
  • In the absence of boot system commands in the startup configuration file, the router first tries to load a valid image from the default Flash device, and if that fails, it tries to load a default system image that is stored on a network server. The router uses the configuration register settings to determine the default system image filename when loading a default system image from a network server. The router starts with the word “Cisco” and adds the octal equivalent of the boot field number in the configuration register, followed by a hyphen (-), and the processor type name to create the default boot filename (cisconn-cpu).

Each bit that makes up the configuration register is described in the table below, along with the functions that each of these bits controls. Keep in mind that a Cisco 7500 Series Router’s functions listed below correlate to the bits indicated. Many of the features, nevertheless, are the same for other series routers. A Cisco 7500 Series Router’s software configuration register has the factory default value of 0×0101. Binary bit 8 = 0×0100 and bits 00 through 03 = 0×0001 combine to form this value. Additionally, this default value could differ for routers in various series.

Bit No. Hex ValueMeaning/Function
00 to 030x0000 to 0x000F00   Remains at the system boot-up prompt.
01 Starts up the initial system image stored in onboard Flash memory.
Sets the default netboot filename from 02 to 0F. enables overriding the default netboot filename with boot system commands.
060x0040This causes the NVRAM contents to be ignored by system software.
070x0080Activates the OEM bit, which is the original equipment manufacturer.
080x0100Removes the ability to use Break.
090x0200Employs secondary bootstrap.
100x0400Transmits Internet Protocol (IP) data using only zeros.
11 and 120x0800 to 0x1000Establishes the console baud rate (the default setting is 9600 baud).
130x2000If the network boot fails, start the built-in Flash program.
140x4000Causes network numbers to be omitted from IP transmissions.
150x8000Enables diagnostic messages while ignoring NVRAM data.

The initialization phase of the router’s operation is finished when it starts up. Now, you can change an existing configuration or add new configuration settings. To enter configuration commands, you use the Cisco IOS command-line interface (CLI) in any situation. It is referred to as the startup configuration if a configuration file is kept in NVRAM. Running configuration refers to the configuration that the router is using at any given time, which is subject to change.

Uses of Configuration Register:

The configuration register can be used in a number of ways to modify router behavior, including:

  • The router’s boot process (into ROMmon, NetBoot)
  • Options of Boot (ignore configuration, disable boot messages)
  • Console speed (baud rate for a terminal emulation session)

With the config-register command, the configuration register can be changed while in configuration mode. Run the confreg command from ROMmon. 

Configuration Register Values:

 Configreg:

You can understand the relevance of your configuration register’s value if you are aware of its value. The output of the show version or show tech-support commands can be collected and entered into the Cisco CLI Analyzer (registered customers only) tool to obtain details about the configuration register, potential problems, and remedies.

Some typical parameters that are applicable to most platforms are listed in this table.

Configuration Register Set AtRouter Behavior
0x102
  • Ignores break
  • 9600 console baud rate
0x1202
  • 1200 baud rates
0x2101
  • Boots into bootstrap
  • Ignores break
  • Boots into ROM if initial boot fails
  • 9600 console baud rates
0x2102
  • Ignores break
  • Boots into ROM if initial boot fails
  • 9600 console baud rate default value for most platforms
0x2120
  • Boots into ROMmon
  • 19200 console speed
0x2122
  • Ignores break
  • Boots into ROM if initial boot fails
  • 19200 console baud rates
0x2124
  • NetBoot
  • Ignores break
  • Boots into ROM if initial boot fails
  • 19200 console speed
0x2142
  • Ignores break
  • Boots into ROM if initial boot fails
  • 9600 console baud rates
  • Ignores the contents of Non-Volatile RAM (NVRAM) (ignores configuration)
0x2902
  • Ignores break
  • Boots into ROM if initial boot fails
  • 4800 console baud rates
0x2922
  • Ignores break
  • Boots into ROM if initial boot fails
  • 38400 console baud rates
0x3122
  • Ignores break
  • Boots into ROM if initial boot fails
  • 57600 console baud rates
0x3902
  • Ignores break
  • Boots into ROM if initial boot fails
  • 2400 console baud rates
0x3922
  • Ignores break
  • Boots into ROM if initial boot fails
  • 115200 console baud rates

Find out which bits were set to compute the value if the value you have for the configuration register is not in the table:

Bit numberHexSignificance
00-030x0000-0x000F

Field Specifications for Boots:

  • Remains at the system bootstrap prompt with the value 0x0000.
  • The first system image in the onboard Flash memory is booted at 0x0001 (EPROM).
  • A default netboot filename is defined by the value 0x0002-0x000F. allows for boot system instructions to be used instead of the netboot filename that is normally used.
060x0040
  • Ignore NVRAM contents
070x0080
  • Disable boot messages
080x0100
  • Break disabled
090x0200
  • This causes the system to select the secondary bootstrap. This is typically not used (set to 0).
100x0400
  • IP broadcast with all zeros

5

11

12

0x0020, 0x0800, 0x1000
  • Console line speed
130x2000
  • If the network boot fails, start the ROM software by default.
140x4000
  • IP broadcasts lack network numbers.
150x8000
  • Makes diagnostic messages possible
  • Ignores the contents of NVRAM


Last Updated : 13 Nov, 2022
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