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C-2
Cisco 3600 Series Routers Hardware Installation Guide
OL-2056-05
Appendix C Configuration Register
Changing Configuration Register Settings
Changing Configuration Register Settings
You might want to modify the value in the configuration register to perform the following tasks:
• Recover a lost password
• Change the console data rate
• Enable or disable the Break function
• Manually boot the operating system using the b command at the ROM monitor prompt
• Force the router to automatically boot its system image in Flash memory, or boot in accordance with
any boot system commands stored in the router’s configuration file in NVRAM
You can change the configuration register from either the ROM monitor or the operating system
software. To change the configuration register from the ROM monitor, see the “Configuration Register
Commands” section on page B-7. To change the configuration register from the system software, do the
following:
Step 1 Connect a console terminal to the console port of the router as described in the “Connecting to the
Console Port” section on page 3-44, using the blue RJ-45 to DB-9 console adapter cable.
Note If you have a terminal with a DB-25 port, use an RJ-45 rollover cable and DB-25 adapter. The
RJ-45-to-DB-25 adapter (Cisco part number 29-0810-01) can be purchased from Cisco.
For information about cable pinouts, refer to the online document Cisco Modular Access Router Cable
Specifications. This document is available online and on the Cisco Documentation CD-ROM.
Table C-1 Configuration Register Bit Meanings
Bit
Number Hexadecimal Meaning
00–03 0x0000-0x000F Boot field. (See Table C-2.)
06 0x0040 Causes the system software to ignore the contents of
NVRAM.
07 0x0080 OEM bit enabled.
08 0x0100 Break disabled.
09 0x0200 Causes the system to use the secondary bootstrap.
This is typically not used (set to 0).
10 0x0400 IP broadcast with all zeros.
5, 11, 12 0x0020,
0x0800, 0x1000
Console line speed.
13 0x2000 Boots default ROM software if the network boot fails.
14 0x4000 IP broadcasts do not have net numbers.
15 0x8000 Enables diagnostic messages and ignores the contents
of NVRAM.