Sep
23
2008

The configuration register

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.

Summary
The configuration register
Article Name
The configuration register
Description
The configuration register can be used to change router behavior in several ways.
Author