Here is what you need to know with regards to the Configuration Register in Cisco IOS.

All Cisco routers have a 16-bit software register in NVRAM. The default is for Cisco routers to load Cisco IOS from FLASH MEMORY and to look for and load the STARTUP-CONFIG file from NVRAM.

Some Reference Stuff

Here is what the Register looks like:

 Reg |    First #    |   Second #    |    Third #    |    Fourth #
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Bit# | 15| 14| 13| 12| 11| 10| 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
     | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

The default setting is 0x2102:

 Reg |       2       |       1       |       0       |       2       |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Bit# | 15| 14| 13| 12| 11| 10| 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
     | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

The meaning of the Bit #'s above are defined as follows:

Bit Hexadecimal Description
0-3 0x0000-0x00F Boot File Field (see below)
6 0x0040 Ignore NVRAM contents
7 0x0080 OEM Bit Enabled
8 0x0101 Break disabled
10 0x0400 IP Broadcast with all zeros
11-12 0x0800-0x1000 Console Line Speed
13 0x2000 Boot default ROM software if network boot fails
14 0x4000 IP Broadcasts do not have net numbers
15 0x8000 Enable diagnostic messages and ignore NVM contents



Boot Field Meaning Usage
00 ROM Monitor Mode Boot in ROMMON mode
01 Boot image from ROM Router will show 'router(boot)>' prompt
02-F Specifies which file to boot Use values 2102-210F

Some Examples:

To change the configuration register settings, use the config-register command in global configuration mode.

config-register value

This command applies only to platforms that use a software configuration register.

config-register 0x2102       - this boots from start-config in NVRAM

config-register 0x2142       - this boots ignoring NVRAM contents

Finally, some things to remember:

  • If you set the configuration register boot field value to 0x0, you must boot the operating system manually with the boot command.
  • If you set the configuration register boot field value to 0x1, the router boots using the default ROM software.
  • If you set the configuration register boot field to any value from 0x2 to 0xF, the router uses the boot field value to form a default boot filename for booting from a network server.
We hope this helps as a quick reference to the Configuration Register.  You can find more details here.

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