The configuration register can be used to change router behavior in several ways, such as:
- how the router boots (into ROMmon, NetBoot)
- options while booting (ignore configuration, disable boot messages)
- console speed (baud rate for a terminal emulation session)
To view the current setting of the configuration register, use the show version command:
Ciscozine#sh ver Cisco Internetwork Operating System Software IOS (tm) C2600 Software (C2600-I-M), Version 12.3(26), RELEASE SOFTWARE (fc2) Technical Support: http://www.cisco.com/techsupport Copyright (c) 1986-2008 by cisco Systems, Inc. Compiled Mon 17-Mar-08 15:23 by dchih ROM: ROMMON Emulation Microcode ROM: C2600 Software (C2600-I-M), Version 12.3(26), RELEASE SOFTWARE (fc2) Ciscozine uptime is 3 minutes System returned to ROM by unknown reload cause - suspect boot_data[BOOT_COUNT] 0x0, BOOT_COUNT 0, BOOTDATA 19 System image file is "tftp://255.255.255.255/unknown" cisco 2621 (MPC860) processor (revision 0x202) with 56320K/9216K bytes of memory. Processor board ID 00000000000 (1880125456) M860 processor: part number 0, mask 0 Bridging software. X.25 software, Version 3.0.0. 2 FastEthernet/IEEE 802.3 interface(s) 128K bytes of non-volatile configuration memory. 8192K bytes of processor board System flash (Read/Write) Configuration register is 0x2142 Ciscozine#
The factory-default setting for the configuration register is 0x2102. This indicates that the router should attempt to load a Cisco IOS software image from Flash memory and load the startup configuration with a console speed of 9600 baud.
The configuration register can be set from configuration mode using the config-register command. From ROMmon, use the confreg command. To apply the new setting, you must reload the router.
There are many configuration register combinations; a good tool to view these combinations is Config Register Calculator. You can find it here http://www.google.com/search?q=Config+Register+Calculator
Some examples of configuration register:
| Configuration Register Setting | Router Behavior |
| 0x102 | Ignores break 9600 console baud |
| 0x1202 | 1200 baud rate |
| 0x2101 | Boots into bootstrap Ignores break Boots into ROM if initial boot fails 9600 console baud rate |
| 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 rate |
| 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 rate Ignores the contents of Non-Volatile RAM (NVRAM) (ignores configuration) |
| 0x2902 | Ignores break Boots into ROM if initial boot fails 4800 console baud rate |
| 0x2922 | Ignores break Boots into ROM if initial boot fails 38400 console baud rate |
| 0x3122 | Ignores break Boots into ROM if initial boot fails 57600 console baud rate |
| 0x3902 | Ignores break Boots into ROM if initial boot fails 2400 console baud rate |
| 0x3922 | Ignores break Boots into ROM if initial boot fails 115200 console baud rate |
You may find additional information on configuration register here.
