Abstract of CCNA study guide-16 -Managing a Cisco Internetwork 1

Managing a Cisco Internetwork

The Internal Components of a Cisco Router

Table below describes the major Cisco router components.

Cisco Router Components

Component	Description
Bootstrap	Stored in the microcode of the ROM. It will boot the router and then load the IOS.
POST (power-on self-test)	Stored in the microcode of the ROM, the POST is used to check the functionality of the router and which interfaces are present.
ROM monitor	Stored in the microcode of the ROM, used for manufacturing, testing, and troubleshooting.
Mini-IOS	Called the RXBOOT or bootloader by Cisco, it is a small IOS in ROM that can be used to bring up an interface and load a Cisco IOS into flash memory.
RAM	hold packet buffers, ARP cache and  routing tables. Running-config is stored in RAM.
ROM	Used to start and maintain the router. Holds the POST , bootstrap and mini-IOS.
Flash memory	Stores the Cisco IOS by default.
NVRAM (nonvolatile RAM)	Used to hold the router and switch configuration. store The configuration register .
Configuration register	Used to control how the router boots up. This value can be found as the last line of the show version command output and by default is set to 0x2102, which tells the router to load the IOS from flash memory as well as to load the configuration from NVRAM.

The Router Boot Sequence

The Router boot sequence consists of the following steps:

1. The router performs a POST. The POST tests the hardware to verify that all components of the device are operational and present.

2. The bootstrap then looks for and loads the Cisco IOS software.default, the IOS software is loaded from flash memory in all Cisco routers.

3. The IOS software looks for a valid configuration file stored in NVRAM. This file is called startup-config

4. If a startup-config file is in NVRAM, the router will copy this file and place it in RAM  and call the file running-config.

If a startup-config file is not in NVRAM, the router will broadcast out any interface that detects carrier detect (CD) for a TFTP host looking for a configuration, and when that fails , it will start the setup mode configuration process.

Managing Configuration Register

All Cisco routers have a 16-bit software register that’s written into NVRAM. By default, the configuration register is set to load the Cisco IOS from flash memory and to look for and load the startup-config file from NVRAM.

Understanding the Configuration Register Bits

The 16 bits (2 bytes) of the configuration register are read from 15 to 0, from left to right. The default configuration setting on Cisco routers is 0x2102. This means that bits 13, 8, and 1 are on, as shown in  Table below . Notice that each set of 4 bits (called a nibble) is read in binary with a value of 8, 4, 2, 1.

Checking the Current Configuration Register Value

You can see the current value of the configuration register by using the show version command :

Router# sh version

Cisco IOS Software, 2800 Software (C2800NM-ADVSECURITYK9-M), Version

12.4(12), RELEASE SOFTWARE (fc1)

[output cut]

Configuration register is 0x2102

The last information is the value of the configuration register. The configuration register setting of 0x2102 tells the router to look in NVRAM for the boot sequence.

Changing the Configuration Register

These are the main reasons you would want to change the configuration register:

- To force the system into the ROM monitor mode

- To select a boot source and default boot filename

- To enable or disable the Break function

- To control broadcast addresses

- To set the console terminal baud rate

- To load operating software from ROM

- To enable booting from TFTP server

You can change the configuration register by using the config-register command.

The following commands tell the router to boot a small IOS from ROM:

Router(config)# config-register 0x2101

Router(config)#^Z

Router# sh ver

[output cut]

Configuration register is 0x2102 (will be 0x2101 at next reload)

Notice that the show version command displays the current configuration register value and also what that value will be when the router reboots. Any change to the configuration register won’t take effect until the router is reloaded. The 0x2101 will load the IOS from ROM the next time the router is rebooted.

Here is our router after setting the configuration register to 0x2101 and reloading:

Router(boot)#sh ver

 [output cut]

ROM: System Bootstrap, Version 12.4(13r)T, RELEASE SOFTWARE (fc1)

Router uptime is 3 minutes

System returned to ROM by power-on

System image file is “flash:c2800nm-advsecurityk9-mz.124-12.bin”

[output cut]

Configuration register is 0x2101

At this point, if you typed show flash , you’d still see the IOS in flash memory ready to go. But we told our router to load from ROM, which is why the hostname shows up with (boot).

Router(boot)# sh flash

-#- --length-- -----date/time------ path

1 21710744 Jan 2 2007 22:41:14 +00:00 c2800nm-advsecurityk9-mz.124-12.bin

2 1823 Dec 5 2006 14:46:26 +00:00 sdmconfig-2811.cfg

3 4734464 Dec 5 2006 14:47:12 +00:00 sdm.tar

4 833024 Dec 5 2006 14:47:38 +00:00 es.tar

5 1052160 Dec 5 2006 14:48:10 +00:00 common.tar

6 1038 Dec 5 2006 14:48:32 +00:00 home.shtml

7 102400 Dec 5 2006 14:48:54 +00:00 home.tar

8 491213 Dec 5 2006 14:49:22 +00:00 128MB.sdf

9 1684577 Dec 5 2006 14:50:04 +00:00 securedesktop-ios-3.1.1.27-k9.pkg

10 398305 Dec 5 2006 14:50:34 +00:00 sslclient-win-1.1.0.154.pkg

32989184 bytes available (31027200 bytes used)

Recovering Passwords

If you’re locked out of a router because you forgot the password, you can change the configuration

register to recover it. The bit 6 in the configuration register is used to tell the router whether to use the contents of NVRAM to load a router configuration.

The default configuration register value is 0x2102, meaning that bit 6 is off. With the default setting, the router will look for and load a router configuration stored in NVRAM (startup-config).

To recover a password, you need to turn on bit 6. Doing this will tell the router to ignore the NVRAM contents. The configuration register value to turn on bit 6 is 0x2142.

Here are the main steps to password recovery:

1. Boot the router and interrupt the boot sequence (break), which will take into ROM monitor mode.

2. Change the configuration register to turn on bit 6 (with the value 0x2142).

3. Reload the router.

4. Enter privileged mode.

5. Copy the startup-config file to running-config.

6. Change the password.

7. Reset the configuration register to the default value.

8. Save the router configuration.

9. Reload the router (optional).

Interrupting the Router Boot Sequence

perform a break by pressing the Ctrl+Break key while the router first reboots. After you’ve performed a break, you should see some lines then the prompt:

rommon 1 >

At this point, you will be at the rommon 1> prompt, which is called ROM monitor mode.

Changing the Configuration Register

you can change the configuration register by using the config-register command. To turn on bit 6, use the configuration register value 0x2142.

Cisco ISR/2600 Series Commands

To change the bit value on a Cisco ISR/2600 series router, you just enter the command at the

rommon 1> prompt:

rommon 1 >confreg 0x2142

You must reset or power cycle for new config to take effect

rommon 2 >reset

Cisco 2500 Series Commands

To change the configuration register on a 2500 series router, type o after creating a break sequence on the router. This brings up a menu of configuration register option settings. To change the configuration register, enter the command o/r, followed by the new register value.

>o

Configuration register = 0x2102 at last boot

Bit#     Configuration register option settings:

15        Diagnostic mode disabled

14        IP broadcasts do not have network numbers

13        Boot default ROM software if network boot fails

12-11   Console speed is 9600 baud

10        IP broadcasts with ones

08        Break disabled

07        OEM disabled

06        Ignore configuration disabled

03-00   Boot file is cisco2-2500 (or ‘boot system’ command)

>o/r 0x2142

Reloading the Router and Entering Privileged Mode

At this point, you need to reset the router like this:

- From the ISR/2600 series router, type I (for initialize) or reset.

- From the 2500 series router, type I.

The router will reload and ask if you want to use setup mode. Answer no to entering setup mode, press Enter to go into user mode, and then type enable to go into privileged mode.

Viewing and Changing the Configuration

Copy the startup-config file to the running-config file:

copy startup-config running-config

Or use the shortcut:

copy start run

The configuration is now running in random access memory (RAM), and you’re in privileged mode, meaning that you can now view and change the configuration. But you can’t view the enable secret setting for the password since it is encrypted. To change the password, do this:

config t

enable secret todd

Resetting the Configuration Register and Reloading the Router

After you’re finished changing passwords, set the configuration register back to the default value with the config-register command:

config t

config-register 0x2102

Finally, save the new configuration with a copy running-config startup-config and reload the router.

Boot System Commands

You can configure router to boot another IOS if the flash is corrupted.

There are some boot commands you can play with that will help you manage the way your router boots the Cisco IOS—but remember, we’re talking about the router’s IOS here, not the router’s configuration!

Router>en

Router#config t

Router(config)#boot ?

bootstrap          Bootstrap image file

config              Configuration file

host                  Router-specific config file

network           Network-wide config file

system                         System image file

the boot system command will allow you to tell the router which file to boot from flash memory. Remember that the router, by default, boots the first file found in flash. You can change that with the following commands:

Router(config)#boot system ?

WORD             TFTP filename or URL

flash                Boot from flash memory

ftp                    Boot from a server via ftp

mop                 Boot from a Decnet MOP server

rcp                   Boot from a server via rcp

rom                  Boot from rom

tftp                   Boot from a tftp server

Router(config)#boot system flash c2800nm-advsecurityk9-mz.124-12.bin

The next command  makes your Router boot from a TFTP host:

Router(config)#boot system tftp ?

WORD             System image filename

Router(config)#boot system tftp c2800nm-advsecurityk9-mz.124-12.bin ?

Hostname or A.B.C.D Address from which to download the file

Router(config)#boot system tftp c2800nm-advsecurityk9-mz.124-12.bin 1.1.1.2

Router(config)#

As your last recommended : if the IOS in flash doesn’t load and the TFTP host does not produce the IOS, load the mini-IOS from ROM like this:

Router(config)#boot system rom