Router Components:

1. POST(Power On Self Test) : POST is a program which is hard coded in the ROM and run from ROM. Basically POST is a diagnostic utility which is used to test the router’s hardware components.

2. Bootstrap : bootstrap is program which is hard coded in the ROM. It is  responsible for finding where IOS program is located and then loading the IOS image.

3. ROM:  ROM is the Read Only Memory which is available on the motherboard of the device. ROM stores POST, bootstrap program and basic operating system software.

4. RAM: RAM is a volatile memory, which means information stored in RAM will be lost when device is reloaded. RAM  do the below following functions:

  • IOS is copied into RAM during the boot up.
  • Used to hold packet buffers, ARP cache, routing tables, and also the software and data structures that allow the router to function.
  • Running configuation file is stored in RAM.

5. NVRAM: NVRAM is a Non volatile memory, which means  information stored in NVRAM will not lost when device is reloaded. NVRAM stores startup-configuration file.

6. Flash: Flash memory is an Erasable Programmable Read only memory (EPROM) which is used to  store IOS image and if we want then can store multiple images.

7. Configuration File: we have two sets of configuration file where one is know as startup-config and other one is known as running-config file.

When we make any change on a CISCO device, the change applies to the running-configuration and will affect on device immediately. However, if the device is reloaded for whatever reason, all your changes will be lost as they are not in the startup-config. In order for you to save our configuration, the contents of the running-config needs to be copied to the startup-config. Running-config is stored in RAM and startup-config is stored in NVRAM.

8. Configuration register Value: Configuration register is a 16-bit number and represented in hexadecimal format. Configuration register value instructs the bootstrap program that where to find the IOS. Value can be changed using “config-register command”.

  • 0x2102 : Default configuration value which tells the device to load the IOS from flash memory as well as telling the router to load the Configuration file from NVRAM.
  • 0x2100: will always boot router in ROMMON mode (ROMMON>).
  • 0x2101 : means always boot device in RxBOOT mode (boot>).

9. Boot system Command:– Used to specify the system image that router loads at startup. this command is useful when we have multiple copies of IOS and we want router to loads with specific image then we can use boot system command in top down manner means command will be tried in Top down manner.

Router Boot Process:

  1. Perform POST
  2. Load and execute Bootstrap Program which is responsible to locate the IOS and load it.
  3. Locate IOS: Once the bootstrap loaded, Bootstrap looks into the NVRAM for the configuration register value and read it, based on that value router will decide that how the router boots up. If the configuration register value instructs to look for a startup-config file (which is the default), then the startup-config file will be read and based on boot parameter in startup-config file , router  will load the IOS. If IOS is not found in flash then it try to locate IOS from TFTP and then ROM.
  4. Once IOS is loaded, IOS locate the startup-config file and load it.
  5. If there is no startup-config while device will prompt setup mode.


How many ways to configure Router?
Before going to start that we should know something about line.
Lines identify ports that allow us to connect into cisco device, and then configure to the Cisco devices.

The common examples of lines include:
• Console ports : Console port is an asynchronous serial port which is used for initial configuration, password recovery and local administration of the device.On any router, it appears in the router configuration as line con 0 and in the output of the show line command as cty.

• Auxiliary ports : Auxiliary Port is an asynchornous port which is used to remote administration. Remember we don’t have aux port on cisco switches.On any router, it appears in the router configuration as “”line aux 0” and in the output of the show line command as aux.

• TTY: The TTY lines are asynchronous lines used for inbound or outbound modem and terminal connections and can be seen in a router or access server configuration as “line x”. The specific line numbers are a function of the hardware built into or installed on the router or access server.

• VTY : The VTY lines are the Virtual Terminal lines of the router which used to control inbound Telnet/ssh connections and appear in the configuration as line “vty 0 4 or vty 5 15”.

How can we configure different lines?

Needs to be updated……..

Understand the different plane Control Plane, Data Plane, Management Plane






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