Bottom line: To earn your Cisco CCNA & CCENT certifications, you have got to understand the configuration register ® on Cisco routers &
switches. There is more. This is also a great practical skill to have, because one day you’ll have to change the config register, most likely to perform a password
recovery.
I will just give this warning one time: If you change the register ® to an incorrect value & then reload the router, you can cripple the router & even
Cisco can not bring it back.
No pressure, right? 
You really just have to be very careful & get the right value for what you are trying to do before you
change the config register. Another key is to change the register ® back to the original value once you are done with your work.
To see the current config
register value, run the always-helpful command show version. The config register ® value is at the very bottom of that output, but while we are here, let us take a look at
all of this information.
Router1#show version
Cisco Internetwork Operating System Software
IOS ™ 2500 Software (C2500-I-L), Version
12.1(20), RELEASE SOFTWARE (fc2)
Copyright (c) 1986-2003 by cisco Systems, Inc.
Compiled Thu 29-May-03 22:00 by kellythw
Image
text-base: 0×03041F34, data-base: 0×00001000
ROM: System Bootstrap, Version 11.0(10c)XB1, PLATFORM SPECIFIC RELEASE
SOFTWARE
(fc1)
BOOTLDR: 3000 Bootstrap Software (IGS-BOOT-R), Version 11.0(10c)XB1, PLATFORM SP
ECIFIC RELEASE
SOFTWARE (fc1)
Router1 uptime is 4 days, 10 hours, 43 minutes
System returned to ROM by reload
System image file is
“flash:/c2500-i-l.121-20.bin”
cisco 2521 (68030) processor (revision L) with 14336K/2048K bytes of memory.
Processor board ID 22066955, with
hardware revision 00000003
Bridging software.
X.25 software, Version 3.0.0.
Basic Rate ISDN software, Version 1.1.
1 Token
Ring/IEEE 802.5 interface(s)
2 Serial network interface(s)
2 Low-speed serial(sync/async) network interface(s)
1 ISDN Basic Rate
interface(s)
32K bytes of non-volatile configuration memory.
16384K bytes of processor board System flash (Read ONLY)
Configuration
register is 0×2102
The first bolded field tells you what IOS software & version this router is running.
The second bolded field shows you how long
the router’s been up, why the router went down (”reload”), & the IOS file contained in flash.
Below that, but not bolded, is a list of interfaces on the router
and the capacity of NVRAM & Flash.
The third bolded field shows you the amount of NVRAM your router has. Since the startup config file is kept there,
that’s as large as the startup config file can get!
Finally, the all-important config register ® value. You will learn about manipulating the config register ® during your
CCNA studies, but you should know that the value shown, 0×2102, is the factory default. This value forces the router to look in its own Flash memory for a valid IOS
on startup.
The config-register value does require a reload for a changed value to take effect. I will change this value to 0×2142 & run show version again,
cropping out all information except the config-register. The register ® setting 0×2142 forces the router to bypass the startup configuration file kept in
NVRAM.
Router1(config)#config-register 0×2142
Router1#show version
Configuration register ® is 0×2102 (will be 0×2142 at next
reload)
Like debugs, the config-register command should be used with caution.
Another common value used with config-register is 0×2100, which
boots the router into ROM Monitor mode.
To review these common configuration register ® settings:
0×2102: The default. Router looks for a startup
configuration file in NVRAM & for a valid IOS image in Flash.
0×2142: NVRAM contents are bypassed, startup configuration is ignored.
0×2100:
Router boots into ROM Monitor mode.
A real-world reminder: When you change the configuration register ® value to perform password recovery, do not forget
to change it back & then reload the router!
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free
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can also visit his blog, which is updated many times daily with new Cisco certification articles, free tutorials, & daily CCNA / CCNP exam questions!
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