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.
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