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Understanding the Internal components of a Cisco Router

To configure and troubleshoot a Cisco internetwork, you need to recognize the major components that Cisco routers and also understand what every one does. Table 2-1 defines the significant Cisco router components.

Table 2-1: Cisco Router Components

BootstrapStored in the microcode that the ROM, the bootstrap bring a router up throughout initialization. It will boot the router and then fill the IOS.
Power-on self-test (POST)Stored in the microcode of the ROM, the write-up is used to inspect the an easy functionality that the router hardware and determines i m sorry interfaces are present.
ROM monitorStored in the microcode of the ROM, the ROM monitor is offered for manufacturing, testing, and troubleshooting.
Mini-IOSCalled the RXBOOT or bootloader through Cisco, the mini-IOS is a little IOS in ROM that can be used to open an interface and load a Cisco IOS into flash memory. The mini-IOS can additionally perform a couple of other maintenance operations.
Random-access memory (RAM)Used to organize packet buffers, ARP cache, routing tables, and the software and also data frameworks that enable the router to function. The running-config paper is save on computer in RAM, and most routers increase the IOS from flash right into RAM upon boot.
Read-only memory (ROM)Used come start and maintain the router. Holds the POST and also the bootstrap program, and also the mini-IOS.
Flash memoryUsed to save the Cisco IOS through default. Flash storage is no erased once the router is reloaded. It is electronically erasable programmable read-only memory (EEPROM) created by Intel.
Nonvolatile lamb (NVRAM)Used to organize the router and also switch configuration. NVRAM is no erased once the router or move is reloaded. Does not save an IOS. The configuration-register document is stored in NVRAM.
Configuration registerUsed to manage how the router boots up. This value can be uncovered as the last line of the display version command output, and also by default that is set to 0x2102, which speak the router to fill the IOS from speed memory and also to pack the configuration from NVRAM.
Managing the construction Register

All Cisco routers have actually a 16-bit software register that’s written into NVRAM. By default, the configuration it is registered is collection to load the Cisco IOS from speed memory and also to look for and load the startup-config record from NVRAM.

Checking the existing Configuration register Value

You deserve to see the present value of the configuration register by utilizing the present version command (sh version or display ver because that short), together demonstrated in Table 2-2.

Table 2-2: The show version Command

show versionDirects the router where to load the IOS and also configuration papers from

The only way to view the configuration register is through the display version command:

Router#sh versionCisco IOS Software, 2800 software (C2800NM-ADVSECURITYK9-M),Version 15.1(4)M6, RELEASE software application (fc1)Configuration register is 0x2102The critical information provided from this command is the value of the configuration register. In this example, the worth is 0x2102, i beg your pardon is the default setting. The configuration register setup of 0x2102 tells the router to look in NVRAM for the boot sequence.

Changing the configuration Register

You can adjust the construction register value to modify just how the router boots and runs. These room the key reasons you would want to change the construction register:

To force the system right into the ROM monitor modeTo choose a boot source and default boot filenameTo enable or disable the rest functionTo regulate broadcast addressesTo set the console terminal baud rateTo load operating software program from ROMTo allow booting indigenous a TFTP server

Table 2-3 reflects the two regulates we’ll use to edit the configuration register:

Table 2-3: construction Register

config-registerDirects the router whereby to fill the IOS and also configuration records from
show flashDisplays a perform of directories and files in speed memory

Here is an instance of how to change the configuration register and then I’ll present you just how to verify:

Router(config)#config-register 0x2101Router(config)#^ZRouter#sh verConfiguration register is 0x2102 (will it is in 0x2101 at following reload)Here is the router after setup the configuration it is registered to 0x2101 and reloading:

Router(boot)#sh verCisco IOS Software, 2800 software program (C2800NM-ADVSECURITYK9-M), variation 15.1(4)M6, RELEASE software (fc1) ROM: mechanism Bootstrap, variation 15.1(4)M6, RELEASE software program (fc1) Router uptime is 3 minutesSystem returned to ROM through power-onSystem image paper is "flash:c2800nm-advsecurityk9-mz.151-4.M6.bin" Configuration it is registered is 0x2101At this point, if you typed present flash, you will do still check out the IOS in speed memory prepared to go. However in the previous code you told the router to pack from ROM, i beg your pardon is why the hostname mirrors up through (boot) here:

Router#sh flash-#- --length-- -----date/time------ path1 45392400 Apr 14 2013 05:31:44 +00:00 c2800nm-advsecurityk9-mz.151-4.M6.bin18620416 bytes easily accessible (45395968 bytes used)If you desire to set the configuration register ago to the default, just form this:

Router(boot)#config tRouter(boot)(config)#config-register 0x2102Router(boot)(config)#^ZRouter(boot)#reload

Recovering Passwords

If you locked the end of a router since you forgot the password, you can readjust the configuration it is registered to aid you get earlier on your feet.

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To recoup a password, you need to turn on little 6. Law this will certainly tell the router to neglect the NVRAM contents. The configuration register worth to rotate on little 6 is 0x2142.