Cisco IOS is the kernel of Cisco routers and most switches. IOS was created to deliver network services an enable networked application. Almost ass Cisco routers run the same IOS but only about half of the switches currently run the Cisco IOS like Catalyst 1900 switch. Cisco IOS and configuration file reside in various locations in the Cisco device and it is very important to understand where these files are places and how they work. In this lesson, you will learn the commands which are used to load Cisco IOS on to the router and how to maintain backup files. In order to configure the Cisco internetwork, you need to understand some of the major components of router. The major components include ROM, RAM. Flash memory, NVRAM and config-register. The configuration register value tells the router to load IOS from the flash memory and it also tells the router to load the configuration from NVRAM. In this lesson, you will learn some of the commands used to verify and change the configuration register. Changing the configuration register value helps you to change the boot sequence and to recover the password. Here in this lesson you will find the steps used to recover the password.
Configuration register values
All Cisco routers have 16-bit software register which is written into NVRAM. The configuration register is set to load Cisco IOS from flash memory and to load startup-config file from NVRAM.
The router checks the system bootstrap information depending on the boot field setting in the configuration register. The default configuration register setting can be changed with the help of global configuration-mode command config-register.
In the below example the configuration register is set so that the router checks the startup file in NVRAM for boot system options. The configuration register is 16-bit register in NVRAM and the lowest 4 bits of the configuration register (bits 3, 2, 1 and 0) form the boot field. The default configuration setting on Cisco router is 0x2102. The 0x tells you that it is in hex and rest of the numbers are the actual settings. The easiest way to understand the configuration register is to convert the hex value into binary. Hexadecimal is base 16, which means that one of the 16 values can be represented with hexadecimal digit. The digits range from 0 through F, with 0 representing the decimal 0 and F representing decimal 15. To convert hex to binary, take each hex digit, convert it into decimal and then convert that into binary, as each hex digit represents four binary digits. The table below shows the conversion from hex to decimal to binary.
Conversions from Hex to decimal to Binary
| HEX | DECIMAL | BINARY |
| 0 | 0 | 0000 |
| 1 | 1 | 0001 |
| 2 | 2 | 0010 |
| 3 | 3 | 0011 |
| 4 | 4 | 0100 |
| 5 | 5 | 0101 |
| 6 | 6 | 0110 |
| 7 | 7 | 0111 |
| 8 | 8 | 1000 |
| 9 | 9 | 1001 |
| A | 10 | 1010 |
| B | 11 | 1011 |
| C | 12 | 1100 |
| D | 13 | 1101 |
| E | 14 | 1110 |
| F | 15 | 1111 |
So, if you want to change the default configuration register value 0x2102, first split the numbers into four individual digits and then convert each digit into binary and then put in correct order again. After converting, you can see that bit 13, 8 and 1 are on. In the table below, you can see that each set of 4 bits is read in binary with value 8, 4, 2 and 1.
Configuration Register Bit Numbers
( If the value you have for the configuration register is not in the table, then determine which bits are set in order to compute the value )
Configuration Register Bit Numbers
| Configuration Register Setting | Router Behavior |
|---|---|
| 0x102 |
|
| 0x1202 |
|
| 0x2101 |
|
| 0x2102 |
|
| 0x2120 |
|
| 0x2122 |
|
| 0x2124 |
|
| 0x2142 |
|
| 0x2902 |
|
| 0x2922 |
|
| 0x3122 |
|
| 0x3902 |
|
| 0x3922 |
|
( If the value you have for the configuration register is not in the table, then determine which bits are set in order to compute the value )
The table below lists the software configuration bit meanings. You can see that the default value sets the router to boot to ROM if network boot fails (bit 13), disables the break function in bit 8 and sets the router to boot normally (bit 0-3).
Software configuration meanings
| Bit Number | Hex | Meaning |
|---|---|---|
| 00-03 | 0x0000-0x000F | Boots Field Parameters:
|
| 06 | 0x0040 |
|
| 07 | 0x0080 |
|
| 08 | 0x0100 |
|
| 10 | 0x0400 |
|
| 5,11,12 | 0x0020, 0x0800, 0x1000 |
|
| 13 | 0x2000 |
|
| 14 | 0x4000 |
|
| 15 | 0x8000 |
|
Note: Remember bit 4, 5 and 9 are reserved.
The boot field consists of bits 0-3 in the configuration register, controls the router boot sequence. Table describes the boot field bits.
The Boot system command
| Value of boot field | Meaning | Description |
| 0x---0 | ROM Monitor mode | To boot to ROM monitor mode, set the configuration register value to 2100. You have to manually boot the router by using “b” command. The prompt will rommon > |
| 0x---1 | Boot image from ROM | To boot an IOS image stored in ROM, set the configuration register to 2011. The router shows the prompt router(boot)> |
| 0x---2 to 0x---F | Specifies the default boot file name | Anyvalue from 2101 to 210F indicaes that the router use the boot commands speciies in NVRAM |
Checking the current Configuration Register Value
The current value of the configuration register can be viewed by using show version command (sh version or sh ver). The show version command also displays the hardware configuration information, software version, names and sources of configuration file and boot image on a router. For example;
Router#show version
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (C2500-JS-L), Version 12.1(5), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2000 by cisco Systems, Inc.
Compiled Wed 25-Oct-00 05:18 by cmong
Image text-base: 0x03071DB0, data-base: 0x00001000
ROM: System Bootstrap, Version 5.2(8a), RELEASE SOFTWARE
BOOTFLASH: 3000 Bootstrap Software (IGS-RXBOOT), Version 10.2(8a), RELEASE SOFTWARE (fc1)
Router uptime is 7 minutes
System returned to ROM by reload
System image file is "flash:c2500-js-l_121-5.bin"
cisco 2500 (68030) processor (revision D) with 16384K/2048K bytes of memory.
Processor board ID 03867477, with hardware revision 00000000
Bridging software.
X.25 software, Version 3.0.0.
SuperLAT software (copyright 1990 by Meridian Technology Corp).
TN3270 Emulation software.
1 Token Ring/IEEE 802.5 interface(s)
2 Serial network interface(s)
32K bytes of non-volatile configuration memory.
16384K bytes of processor board System flash (Read ONLY)
Configuration register is 0x2142
In this example, the configuration register value is 0x2102 and it is default setting. It indicates the router to look for the boot sequence in NVRAM. The show version command also gives the IOS version and in this example it is 12.1(5).
Changing the Configuration Register
Configuration register value can be changed to modify how router boots and runs. Configuration register value can be changed by using config-register command. The commands in the example below tell the router to boot a small IOS from ROM monitor mode and it also shows the current configuration register value.
The new configuration register setting becomes active once the router reloads.Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#config
Router(config)#config-register 0x2102
Router(config)#end
Router#show version
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (C2500-JS-L), Version 12.1(5), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2000 by cisco Systems, Inc.
Compiled Wed 25-Oct-00 05:18 by cmong
Image text-base: 0x03071DB0, data-base: 0x00001000
ROM: System Bootstrap, Version 5.2(8a), RELEASE SOFTWARE
BOOTFLASH: 3000 Bootstrap Software (IGS-RXBOOT), Version 10.2(8a), RELEASE SOFTWARE (fc1)
Router uptime is 11 minutes
System returned to ROM by reload
System image file is "flash:c2500-js-l_121-5.bin"
cisco 2500 (68030) processor (revision D) with 16384K/2048K bytes of memory.
Processor board ID 03867477, with hardware revision 00000000
Bridging software.
X.25 software, Version 3.0.0.
SuperLAT software (copyright 1990 by Meridian Technology Corp).
TN3270 Emulation software.
1 Token Ring/IEEE 802.5 interface(s)
2 Serial network interface(s)
32K bytes of non-volatile configuration memory.
16384K bytes of processor board System flash (Read ONLY)
Configuration register is 0x2142 (will be 0x2102 at next reload)
Router#reload
System configuration has been modified. Save? [yes/no]: n
Proceed with reload? [confirm]
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