Sep
23
2008

The configuration register

An article by Fabio Semperboni

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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 0×2102. 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
0×102	Ignores break 9600 console baud
0×1202	1200 baud rate
0×2101	Boots into bootstrap Ignores break Boots into ROM if initial boot fails 9600 console baud rate
0×2102	Ignores break Boots into ROM if initial boot fails 9600 console baud rate default value for most platforms
0×2120	Boots into ROMmon 19200 console speed
0×2122	Ignores break Boots into ROM if initial boot fails 19200 console baud rate
0×2124	NetBoot Ignores break Boots into ROM if initial boot fails 19200 console speed
0×2142	Ignores break Boots into ROM if initial boot fails 9600 console baud rate Ignores the contents of Non-Volatile RAM (NVRAM) (ignores configuration)
0×2902	Ignores break Boots into ROM if initial boot fails 4800 console baud rate
0×2922	Ignores break Boots into ROM if initial boot fails 38400 console baud rate
0×3122	Ignores break Boots into ROM if initial boot fails 57600 console baud rate
0×3902	Ignores break Boots into ROM if initial boot fails 2400 console baud rate
0×3922	Ignores break Boots into ROM if initial boot fails 115200 console baud rate