The Configuration Register


router#show version

Last line of output tells you what the configuration register is set to

router#config t

 

router(config)#config-register 0x2142

Changes the configuration register to 2142


The Configuration Register: A Visual Representation

The configuration register is a 16-bit eld stored in NVRAM. The bits are numbered from 15 to 0 looking at the bit stream from left to right. Bits are split up into groups of 4, and each group is represented by a hexadecimal digit.


15 14 13 12    11 10 9 8    7  6  5  4    3  2  1  0

Bit places


0   0   1   0      0   0  0  1    0  1  0  0    0  0  1  0

Register bits


      2                     1                4                2

Bits represented in hex


The Configuration RegisterWhat the Bits Mean

Bit Number

Hexadecimal

Meaning

0003

0x0000 0x000F

Boot Field

06

0x0040

Ignore NVRAM contents

07

0x0080

OEM bit enabled

08

0x0100

Break disabled

09

0x0200

Cause system to use secondary bootstrap (typically not used)

10

0x0400

IP broadcast with all zeros

5, 11, 12

0x0020, 0x0800, 0x1000

Console line speed

13

0x2000

Boots default ROM software if network boot fails

14

0x4000

IP broadcasts do not have net numbers

15

0x8000

Enables diagnostic messages and ignores NVRAM contents


The Boot Field

Note:

Even though there are 16 possible combinations in the boot field, there are only three that are used.


Boot Field

Meaning

00

Stays at the ROM Monitor on a reload or power cycle

01

Boots the first image in Flash memory as a system image

02 F

Enables default booting from Flash memory

Enables boot system commands that override default booting from Flash memory


Tip:

Because the default boot field has 14 different ways to represent it, a configuration register setting of 0x2102 is the same as 0x2109, or 210F. The boot system command is described in Chapter 16, "Backing Up and Restoring Cisco IOS Software and Configuration."


Console Terminal Baud Rate Settings

Baud

Bit 5

Bit 12

Bit 11

115200

1

1

1

57600

1

1

0

38400

1

0

1

19200

1

0

0

9600

0

0

0

4800

0

0

1

2400

0

1

1

1200

0

1

0


Changing the Console Line SpeedCLI

router#config t

 

router(config)#line console 0

Enters console line mode

router(config-line)#speed 19200

Changes speed to 19200 baud


Tip:

Cisco IOS software does not allow you to change the console speed bits directly with the config-register command.


Changing the Console Line SpeedROM Monitor Mode1700/2600 Series

rommon1>confreg

Shows configuration summary. Step through the questions, answering with the defaults until you can change the console baud rate

  Configuration Summary enabled are: load rom after netboot fails console baud: 9600 boot: image specified by the boot system commands      or default to: x (name of system image) do you wish to change the configuration? y/n [n]: y enable    "diagonstic mode"? y/n [n]: n enable    "use net in IP bcast address"? y/n [n]: n disable   "load rom after netboot fails"? y/n [n]: n enable    "use all zero broadcast"? y/n [n]: n enable    "break/abort has effect"? y/n [n]: n enable    "ignore system config info"? y/n [n]: n change console baud rate? y/n [n]: y enter rate: 0=9600, 1=4800, 2=1200, 3=2400                4=19200, 5=38400, 6=57600, 7=115200 [0]: 7 

 

     Configuration Summary enabled are: load rom after netboot fails console baud: 115200 boot: image specified by the boot system commands          or default to: x (name of system image) 

 

change the boot characteristics? y/n [n]: n

After the summary is shown again, choose n to not change the configuration and go to the rommon> prompt again

rommon2>

 


Tip:

Make sure that after you change the console baud rate, you change your terminal program to match the same rate!





CCNA Self-Study(c) CCNA Portable Command Guide
CCNA Portable Command Guide
ISBN: 1587201585
EAN: 2147483647
Year: 2006
Pages: 261
Authors: Scott Empson

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