Cisco CCNA 640-802 CCENT Certification Exam Training: The Configuration Register

Bottom line: To earn your Cisco CCNA and CCENT certifications, you've got to understand the configuration register on Cisco routers and switches. This is also a beneficial practical skill to have, because one day you will have to convert the config register, most possible to perform a password recovery.

I testament just give this warning one time: If you change the register to an incorrect value and then reload the router, you can cripple the router and still Cisco can't bring it back.

No pressure, right? :)

You really just have to be careful and entertain the right value for what you're trying to achieve before you spending money the config register. Another basic is to change the register back to the original value once you're done with your work.

To see the now config register value, break the always-helpful command show version. The config register fee is at the very bottom of that output, but while we're here, let's takings a look at all of this information.

Router1#show version

Cisco Internetwork Operating System Software

IOS (tm) 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: 0x03041F34, data-base: 0x00001000


ROM: System Bootstrap, Version 11.0(10c)XB1, PLATFORM Particular RELEASE SOFTWARE

(fc1)

BOOTLDR: 3000 Bootstrap Software (IGS-BOOT-R), Chronicle 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 Degree interface(s)

32K bytes of non-volatile configuration memory.

16384K bytes of processor board System flash (Read ONLY)


Configuration register is 0x2102


The beginning bolded world tells you what IOS software and version this router is running.

The second bolded field shows you how long the router's been up, why the router went down ("reload"), and the IOS file contained in flash.

Below that, but not bolded, is a list of interfaces on the router and the energy of NVRAM and Flash.

The third bolded field shows you the amount of NVRAM your router has. Since the startup config folder is kept there, that's as large as the startup config file can get!

Finally, the all-important config register value. You'll attain approximately manipulating the config register during your CCNA studies, but you should notice that the value shown, 0x2102, is the factory default. This bill forces the router to look in its own Flash memory for a authentic IOS on startup.

The config-register value does require a reload for a changed value to take effect. I'll change this value to 0x2142 and run show version again, cropping out all information except the config-register. The register setting 0x2142 forces the router to bypass the startup configuration list kept in NVRAM.

Router1(config)#config-register 0x2142


Router1#show version


Configuration register is 0x2102 (will be 0x2142 at consequent reload)


Allying debugs, the config-register command should be used with caution.

Another common value used with config-register is 0x2100, which boots the router into ROM Monitor mode.

To another look these common configuration register settings:

0x2102: The default. Router looks for a startup configuration file in NVRAM and for a valid IOS image in Flash.

0x2142: NVRAM text are bypassed, startup configuration is ignored.

0x2100: Router boots into ROM Monitor mode.

A real-world reminder: When you replace the configuration register value to perform password recovery, don't forget to change it back and then reload the router!

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