Configuration Registers

7/28/2019 Configuration Registers

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Bit # Hex Meaning

00-03 0x0000-0x000F Boot Field

06 0x0040 Causes the system software to ignore nonvolatile memory (NVRAM) contents

07 0x0080 OEM bit enabled

08 0x0100 Break disabled

10 0x0400 IP broadcast with all zeros

11-12 0x0800-0x1000 Console line speed

13 0x2000 Boots default ROM software is initial boot fails

14 0x4000 IP broadcasts do not have net numbers

15 0x8000 Enables diagnostic messages and ignores NVRAM contents

Bits 0-3

The lowest four bits of the processor configuration register (bits 3, 2, 1, and 0) form the boot field.

Table 2 - Bits 0-3 Settings

BootField

Meaning

0 Stays at the system bootstrap prompt (ROM monitor) on a reload or power cycle1 Boots the boot helper image as a system image

2 Full boot process, which loads the Cisco IOS image into Flash memory

2-F

Specifies a default filename for booting over the network from a TFTP server.

Enables boot system commands that override the default filename for booting over the networkfrom a TFTP server.

The boot field specifies a number in binary. If you set the boot field value to 0, you must have console portaccess to boot the operating system manually. Boot the operating system by entering the b command at thebootstrap prompt as follows:

>b [tftp] flash filename

Definitions of the various command options follow:b--Boots the default system software from ROMb flash--Boots the first file in Flash memoryb filename [host] --Boots over the network using TFTPb flash filename-- Boots the file ( filename ) from Flash memory



If you set the boot field value to a value of 2 through F, and there is a valid system boot command stored inthe configuration file, the router boots the system software as directed by that value. (See Table 3.) If you setthe boot field to any other bit pattern, the router uses the resulting number to form a default boot filename fornetbooting.

If there are no boot commands in the configuration file, the router attempts to boot the first file in systemFlash memory. If no file is found in system Flash memory, the router attempts to netboot a default file whosename is derived from the value of the boot field (for example: cisco2-4500). If the netboot attempt fails, theboot helper image in boot Flash will boot up.

If boot commands are in the configuration file, the router software processes each boot command insequence until the process is successful or the end of the list is reached. If the end of the list is reachedwithout a file being successfully booted, the router will retry the netboot commands up to six times unlessthe boot default ROM software if netboot fails bit (bit 13 of the virtual configuration register) is set. If bit 13is set, the system boots the boot helper image found in boot Flash memory without any retries.

The server creates a default boot filename as part of the automatic configuration processes. To form the bootfilename, the server starts with cisco and links the octal equivalent of the boot field number, a dash, and theprocessor-type name. Table 3 lists the default boot filenames or actions.

Note- A boot system configuration command in the router configuration in NVRAM overrides the defaultnetboot filename.

Table 3 - Default Boot Filenames

Action/File Name Bit 3 Bit 2 Bit 1 Bit 0

bootstrap mode 0 0 0 0

ROM software 0 0 0 1

Flash software 0 0 1 0

cisco3-<processor_name>1 0 0 1 1

cisco4-<processor_name> 0 1 0 0

cisco5-<processor_name> 0 1 0 1cisco6-<processor_name> 0 1 1 0







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Bit 6

Bit 6 causes the system software to ignore nonvolatile memory (NVRAM) contents.

Bit 7

Bit 7 enables the OEM bit. It disables the boot strap messages at start up.

Bit 8

Bit 8 controls the console Break key. Setting bit 8 (the factory default) causes the processor to ignore theconsole Break key. Clearing bit 8 causes the processor to interpret Break as a command to force the systeminto the bootstrap monitor, halting normal operation. A Break can be sent in the first sixty seconds while thesystem reboots, regardless of the configuration settings.

Bit 10 and 14

Bit 10 controls the host portion of the Internet broadcast address. Setting bit 10 causes the processor to useall zeros; clearing bit 10 (the factory default) causes the processor to use all ones. Bit 10 interacts with bit14, which controls the network and subnet portions of the broadcast address. See Table 4 which shows thecombined effect of bits 10 and 14.

Table 4 - Bits 10 and 14 Settings

Bit 14 Bit 10 Address (<net><host>)

Off Off <ones><ones>

Off On <zeros><zeroes>

On On <net><zeros>

On Off <net><ones>

Bits 11 and 12

Bits 11 and 12 in the configuration register determine the baud rate of the console terminal. See Table 5 which shows the bit settings for the four available baud rates. (The factory set default baud rate is 9600.)

Table 5 - Bits 12 and 11 Settings

Baud Bit 12 Bit 11

9600 0 0

4800 0 1











2400 1 1

1200 1 0

Bit 13

Bit 13 determines the server response to a bootload failure. Setting bit 13 causes the server to load operatingsoftware from ROM after five unsuccessful attempts to load a boot file from the network. Clearing bit 13causes the server to continue indefinitely to attempt loading a boot file from the network. By factory default,bit 13 is cleared to 0.

Bit 15

Bit 15 enables diagnostic messages and ignores NVRAM contents.

Displaying the Configuration Register

- while running IOS - while running ROM Monitor

While running IOS

The configuration-register can be viewed by doing a ̀show version' or ̀ show hardware'. Sample output:

Rout er#sh ver si onCi sco I nt er net wor k Oper at i ng Syst em Sof t war eI OS ( t m) 4500 Sof t ware ( C4500- J - M) , Versi on 11. 1( 4) , RELEASE SOFTWARE( f c1)

- - - out put t r uncat ed - - - -

1 Et her net / I EEE 802. 3 i nt er f ace.2 Token Ri ng/ I EEE 802. 5 i nt er f aces.4 Ser i al net wor k i nt er f aces.

128K byt es of non- vol at i l e conf i gur at i on memor y.4096K bytes of pr ocessor boar d Syst em f l ash ( Read/ Wr i t e)4096K byt es of pr ocessor boar d Boot f l ash ( Read/ Wr i t e)

Conf i gur at i on r egi st er i s 0x2102

Rout er#

While running ROM Monitor

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1. If the prompt is ̀ >', the command ̀ o' displays the virtual configuration register currently in effect. Itincludes a description of the bits. Sample output:

>oConf i gur at i on r egi st er = 0x2100 at l ast boot

Bi t # Conf i gur at i on r egi st er opt i on set t i ngs:15 Di agnost i c mode di sabl ed14 I P br oadcast s do not have net wor k number s13 Boot def aul t ROM sof t war e i f net wor k boot f ai l s12- 11 Consol e speed i s 9600 baud10 I P br oadcast s wi t h ones09 Do not use secondary boot st r ap08 Br eak di sabl ed07 OEM di sabl ed06 I gnor e conf i gur at i on di sabl ed04 Fast boot di sabl ed

03- 00 Boot t o ROM moni t or>

2. If the prompt is ̀ rommon1', the command ̀ confreg' displays the virtual configuration register currently ineffect. It includes a description of the bits. Sample output:

r ommon 1 > conf r eg

Conf i gur at i on Summar yenabl ed are:l oad r om af t er net boot f ai l s

consol e baud: 9600boot : t he ROM Moni t ordo you wi sh to change the conf i gur at i on? y/ n [ n] :

Setting the Configuration Register

- while running IOS - while running ROM Monitor

While running IOS

The configuration-register can be set by in the configuration mode with the command ̀conf i g-r egi st er 0x<val ue>'. Sample output:

Rout er#conf i g tEnt er conf i gur at i on commands, one per l i ne. End wi t h CNTL/ Z.Rout er ( conf i g) #conf i g- r egi st er 0x2142Rout er ( conf i g) #end

Rout er#%SYS- 5- CONFI G_I : Conf i gur ed f r om consol e by consol e

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While running ROM Monitor

1. If the prompt is ̀ >', the command ̀ o/r 0x<value>' sets the configuration register. Sample output:

>o/ r 0x2102 >

2. If the prompt is ̀ rommon1', the command ̀ confreg' sets the configuration register. It will prompt the userabout each bit. Sample output:

r ommon 1 > conf r eg


consol e baud: 9600boot : t he ROM Moni t or

do you wi sh t o change t he conf i gur at i on? y/ n [ n] : yenabl e "di agnost i c mode"? y/ n [ n] : nenabl e "use net i n I P bcast addr ess"? y/ n [ n] : ndi sabl e " l oad r om af t er net boot f ai l s"? y/ n [ n] : nenabl e "use al l zer o br oadcast "? y/ n [ n] : nenabl e "br eak/ abor t has ef f ect "? y/ n [ n] : nenabl e "i gnor e syst em conf i g i nf o"? y/ n [ n] : nchange consol e baud rat e? y/ n [ n] : n

change t he boot char act er i st i cs? y/ n [ n] : y

ent er t o boot :0 = ROM Moni t or1 = t he boot hel per i mage2- 15 = boot syst em[0] : 2


consol e baud: 9600boot : i mage speci f i ed by t he boot syst em commandsor def aul t t o: ci sco2- C4500

do you wi sh t o change t he conf i gur at i on? y/ n [ n] : n You must r eset or power cycl e f or new conf i g t o t ake ef f ectr ommon 2 >

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Configuration Registers