The Configuration Register

By: Chris Bryant

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 great practical skill to have, because one day you will 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 and then reload the router, you can cripple the router and even Cisco can't bring it back.

No pressure, right?? :)

You really just have to be careful and get the right value for what you're trying to do before you change the config register.? Another key is to change the register back to the original value once you're 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're here, let's take 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 SPECIFIC RELEASE SOFTWARE

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 0x2102

The first bolded field 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 capacity of NVRAM and 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'll learn about manipulating the config register during your CCNA studies, but you should know that the value shown, 0x2102, 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'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 file kept in NVRAM.

Router1(config)#config-register 0x2142

Router1#show version

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

Like 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 review 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 contents are bypassed, startup configuration is ignored.

0x2100:? Router boots into ROM Monitor mode.

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

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