Changing the Configuration Register
You can change the configuration register value to modify how the router boots and runs.
These are the main reasons you would want to change the configuration register:
To force the system into the ROM monitor mode
To select a boot source and default boot filename
To enable or disable the
Break
function
To control broadcast addresses
To set the console terminal baud rate
To load operating software from ROM
To enable booting from a TFTP server
Configuration Register
Command Meaning
config-register Directs the router where to load the IOS and configuration files from
show flash Displays a list of directories and files in flash memory
Here is an example of how to change the configuration register and then I’ll show you how
to verify:
Router(config)#
config-register 0x2101
Router(config)#
^Z
Router#
sh ver
[output cut]
Configuration register is 0x2102 (will be 0x2101 at next
reload)
Here is the router after setting the configuration register to 0x2101 and reloading:
Router(boot)#
sh ver
Cisco IOS Software, 2800 Software (C2800NM-ADVSECURITYK9-M), Version 12.4(12),
RELEASE SOFTWARE (fc1)
[output cut]
ROM: System Bootstrap, Version 12.4(13r)T, RELEASE SOFTWARE (fc1)
Router uptime is 3 minutes
System returned to ROM by power-on
System image file is "flash:c2800nm-advsecurityk9-mz.124-12.bin"
[output cut]
Configuration register is 0x2101
At this point, if you typed
show flash
, you’d still see the IOS in flash memory ready to go.
But in the previous code you told the router to load from ROM, which is why the hostname
shows up with
(boot)
here:
Router(boot)#
sh flash
-#- --length-- -----date/time------ path
1 21710744 Jan 2 2007 22:41:14 +00:00 c2800nm-advsecurityk9-mz.124-12.bin
2 1823 Dec 5 2006 14:46:26 +00:00 sdmconfig-2811.cfg
3 4734464 Dec 5 2006 14:47:12 +00:00 sdm.tar
4 833024 Dec 5 2006 14:47:38 +00:00 es.tar
5 1052160 Dec 5 2006 14:48:10 +00:00 common.tar
6 1038 Dec 5 2006 14:48:32 +00:00 home.shtml
7 102400 Dec 5 2006 14:48:54 +00:00 home.tar
8 491213 Dec 5 2006 14:49:22 +00:00 128MB.sdf
9 1684577 Dec 5 2006 14:50:04 +00:00 securedesktop-ios-3.1.1.27-k9.pkg
10 398305 Dec 5 2006 14:50:34 +00:00 sslclient-win-1.1.0.154.pkg
32989184 bytes available (31027200 bytes used)
If you want to set the configuration register back to the default, just type this:
Router(boot)#
config t
Router(boot)(config)#
config-register 0x2102
Router(boot)(config)#
^Z
Router(boot)#
reload