Configuration Register

Last Updated on Fri, 09 Dec 2022 | Routing and Switchin

The configuration register is a 16-bit virtual register that specifies boot sequence and break parameters and sets the console baud rate. The register is usually represented in hexadecimal. Figure 3-1 shows one of the common values for the configuration register, 0x2102. Another common value is 0x0101. The significance of each bit in the configuration register is described in this section.

Figure 3-1 Configuration Register

Bit:	5	4	3	1 2	1 1	1 0	9	8	7	6	5	4	3	2	1	0
0x2102	0	0	1	0	0	0	0	1	0	0	0	0	0	0	1	0

You can change the configuration register by using the global configuration command config-register. The following changes the configuration register to 0x10e:

Router(config)#config-register 0x10e

Boot Sequence

The last four bits (bits 3 to 0) of the configuration register specify the location of the boot file that the router must use when booting up:

• 0x0000 specifies to go to ROM monitor mode.

• 0x0001 specifies to boot from ROM.

• 0x0002 to 0x000F specify to examine the configuration file in NVRAM for boot system commands.

If no boot system commands are in the configuration file, the router attempts to boot the first file in system flash memory. If no file is found in system flash memory, the router attempts to boot a default file from the network whose name is derived from the value of the boot field (e.g., cisco2-4500) by using Trivial File Transfer Protocol (TFTP). If the attempt to boot from a network server fails, the boot helper image in boot Flash boots up.

The default filename is constructed by the word cisco, the value of the boot bits, and the router model or processor name. The format is ciscon-processor_name, where n is a value between 2 and 15 and equal to the value of the boot field. If the boot field is set to 3 (0011 binary) on a Cisco 4500, the default boot name is cisco3-4500.

If several boot system commands are in the configuration, they are attempted in the order that they appear. Example 3-7 shows the configuration for a router where it first attempts to boot an image from flash, and if that fails, to boot from a TFTP server, and if that fails, to boot from ROM. The IP address of the TFTP server is specified in the boot system tftp command.

Example 3-7 The Order of boot system Commands Determines Which IOS Is Loaded

boot

system

flash mc381

3-a2isv5-mz.120-7.XK1.bin

boot

system

tftp mc3810

-a2isv5-mz.120-7.XK1.bin 1.1.1.1

boot

system

rom

This section covers a sample boot sequence for a MC3810 router. For the test, you must have a high level of familiarity with the boot sequence of a router. Following are the four stages:

• System bootstrap

• Bootloader

• Booting of the system IOS image

• Initialization of interfaces/System Restart

The first stage of the boot sequence is the loading of bootstrap software. The bootstrap software initializes the CPU and launches the bootloader:

System Bootstrap, Version 11.3(1)MA1, MAINTENANCE INTERIM SOFTWARE Copyright (c) 1998 by cisco Systems, Inc. Compiled Sat 24-Jan-98 14:55 by krunyan PPC860 PowerQUICC, partnum 0x0000, version A03(0x0013) MC3810 platform with 32768 Kbytes of main memory

The second stage of the boot sequence is the bootloader. The bootloader is not a full router image. It contains minimal functionality for reading the configuration and accessing the flash file system (highlighted). The router at this point can act as an end host with no routing functionality. If the bootloader is not present, the router attempts to load the first file present in the flash file system. The system identifies which messages are from the bootloader:

program load complete, entry point: 0x23000, size: 0x11437c

Self decompressing the image : ################################################# ############################################################### [OK] Slot 3 OK. Configured as T1 TEB CSU Serial #08076360 Version 4.70

Initialize Flash file system

total size = 8388608

flashfs[4]: 1 files, 1 directories flashfs[4]: 1 orphaned files, 1 orphaned directories flashfs[4]: Total bytes: 8128000

flashfs[4]: Bytes used: 6757888

flashfs[4]: Bytes available: 1370112flashfs[4]: flashfs fsck took 18 seconds.

flashfs[4]: Initialization complete.

Readfile

(flash:mc3810-a2isv5-mz.120-7.XK1.bin) into ram (0x41E0E0) ...

flashfs[4]: dostat, unable to lookup filemap for fileid 0!!!!!!!!!!!!!!!!!!!!!!!

%SYS-6-BOOT_MESSAGES: Messages above this line are from the boot loader.

The third stage of the boot sequence is the booting of the system IOS image. The IOS image can be contained in the flash file system or booted from a TFTP server. After the image is loaded, the router lists the IOS version and the recognized interfaces:

UART re-init OK (disable AUX port)

program load complete, entry point: 0x23000, size: 0x663250

Self decompressing the image : ################################################# ################################################################################

################################################################################ ################################################################################ ################################################################################

###################################################### [OK]

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Use, duplication, or disclosure by the Government is subject to restrictions as set forth in subparagraph (c) of the Commercial Computer Software - Restricted Rights clause at FAR sec. 52.227-19 and subparagraph (c) (1) (ii) of the Rights in Technical Data and Computer Software clause at DFARS sec. 252.227-7013.

cisco Systems, Inc.

170 West Tasman Drive

San Jose, California 95134-1706

Cisco Internetwork Operating System Software

IOS (tm) MC3810 Software (MC3810-A2ISV5-M), Version 12.0(7)XK1, EARLY DEPLOYMENT

Slot 3 OK. Configured as T1 TEB CSU Serial #08076360 Version 4.70

Cisco MC3810 (MPC860) processor (revision 04.06) with 28672K/4096K bytes of memory

Processor board ID 07548638

PPC860 PowerQUICC, partnum 0x0000, version A03(0x0013)

Channelized E1, Version 1.0.

Bridging software.

X.25 software, Version 3.0.0.

Primary Rate ISDN software, Version 1.1.

MC3810 SCB board (v04.K0)

1 Multiflex T1(slot 3) RJ45 interface(v01.K0) 1 Six-Slot Analog Voice Module (v07.B0) 1 Analog FXS voice interface (v05.A0) port 1/1 1 Analog FXS voice interface (v05.A0) port 1/2 1 6-DSP(slot2) Voice Compression Module(v01.K0) 1 Ethernet/IEEE 802.3 interface(s)

1 Serial network interface(s)

2 Serial(sync/async) network interface(s) 1 Channelized E1/PRI port(s)

1 Channelized T1/PRI port(s)

256K bytes of non-volatile configuration memory.

8192K bytes of processor board System flash (INTEL28F016)

Press RETURN to get started!

The fourth and final stage of the boot sequence is the initialization of the interfaces. The %SYS-5-RESTART: System restarted message is shown by the router in this stage. Several UPDOWN interface and port messages are shown in this stage:

AVM PIM present

Initialize Flash file system

total size = 8388608

../src-m860-mc3810/mc3810_avm.c, 2105, Serial # 201205538 Version # 8.1

avm system bus fpga init pass: :FPGA program OK. avm fpga programming successful :avm qslac fpga init pass:

1/1 circuit type voice 1/2 circuit type voice

%ATM-5-UPDOWN: Changing VC 0/16 VC-state to PVC activated. %ATM-5-UPDOWN: Changing VC 0/16 VC-state to PVC created. %SYS-5-CONFIG_I: Configured from memory by console

00	00	03
00	00	03
00	00	16
00	00	16
00	00	18
00	00	18
00	00	18
00	00	18
00	00	18
down
00	00	18
00	00	18

%LINK-3-UPDOWN %LINK-3-UPDOWN %LINK-3-UPDOWN

Interface Ethernet0, changed state to up Interface Serial1, changed state to down Interface Serial0, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface ATM0, changed state to

%LINK-5-CHANGED: Interface ATM0, changed state to reset %SYS-5-RESTART: System restarted -Cisco Internetwork Operating System Software

IOS (tm) MC3810 Software (MC3810-A2ISV5-M), Version 12.0(7)XK1, EARLY DEPLOYMENT

00:00:19: %LINK-5-CHANGED: Interface FR-ATM20, changed state to administratively down 00:00:20:

to up 00:00:20: to down flashfs[9] flashfs[9] flashfs[9] flashfs[9] flashfs[9] flashfs[9] flashfs[9]

%LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0, changed state %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state %LINEPROTO-5-UPDOWN: Line protocol on Interface FR-ATM20, changed state 1 files, 1 directories

0 orphaned files, 0 orphaned directories

Total bytes: 8128000

Bytes used: 6699008

Bytes available: 1428992

flashfs fsck took 20 seconds.

Initialization complete.

39:	%Voice-port 1/1	is up.
39:	%Voice-port 1/2	is up.
39:	%LINK-3-UPDOWN:	Interface	FXS	1/2,	changed	state	to	up
39:	%LINK-3-UPDOWN:	Interface	FXS	1/1,	changed	state	to	up

Configuration Register Bit Meanings

Table 3-1 shows a description of each bit in the configuration register. Bits 0 to 3 are the boot field, which select the boot characteristics for the router. You use bit 6 to perform password recovery. When set, the configuration register has the number 4, as in 0x2142. Bit 8 is commonly set to permit a user to halt the router while operating. Bit 10 changes the broadcast type, but it is not commonly used. Bits 11 and 12 change the console line speed; by default, these bits are set to 00 for 9600 speed. Bit 13 is set to use the image on ROM if a network boot fails. Bit 15 enables diagnostic messages.

Table 3-1 describes the meaning of each bit of the virtual configuration register.

Table 3-1 Configuration Register Bit Meanings

Bit Number Hex Value Description

0-3 (boot field) 0x0000 to 0x000f Selects boot characteristics.

Boot field:

0000—Stay at bootstrap prompt

0001—Boot image on EPROM

0002 to 1111 —Use the boot system commands in the configuration; selects default network boot filenames.

4	-	Unused.
5	-	Unused.
6	0x0040	Causes system to ignore the configuration stored in NVRAM.
7	0x0080	OEM bit enabled.
8	0x0100	When set, the Break key is disabled when system is operating. If
		not set, the system enters the bootstrap monitor, halting normal
		operation.
9	-	Unused.
10	0x0400	IP broadcast with all zeros.
11-12	0x0800 to 0x1800	Console line speed:
		[12/11] Hex Baud
		00 0x0000 9600
		01 0x0800 4800
		10 0x1000 1200
		11 0x1800 2400
13	0x2000	Boots default ROM software if network boot fails.
14	0x4000	IP broadcasts do have network numbers.
15	0x8000	Enables diagnostics messages and ignores NVRAM contents.

The most common configuration register settings are 0x102 and 0x2102. When the configuration register is set to 0x0102, it has the Break key disabled when the router is operating, and it looks into the configuration to determine the boot sequence. With 0x2102, the settings are the same as in 0x0102, but the system also boots the default ROM software if the system attempts to boot from the network and fails.

Another common configuration register setting is 0x2142, which is for password recovery. It ignores the configuration in NVRAM when booting and prompts the user with the Initial Configuration Dialog.

Continue reading here: Initial Configuration Dialog

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Licia
What is configuration register?

1 year ago
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Configuration register is a special 16-bit register used in Cisco devices. It is used to control or boot the IOS of the device which gives control over the startup and operation of the device. It also helps to identify the URL of the IOS image which is to be loaded into the device.

Configuration Register

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