Router Basics

1.   Router Introduction:

·     It is a layer 3 device.

·     It works at Network layer of OSI model.

·     Router performs routing which helps to make communication possible between two or more different networks.

2.   Router can be of two types based on physical architecture:

·     Modular devices: which can be modified as per our need.

·     Demodular device: which can’t be modified as per our need.

3.   Types of router on the basis of services:

·     Integrated Service Router: ISR (Integrated Service Router) is those types of router that comes with lots of features where we don’t need to integrate extra module/license in it. Thus it cost higher than normal router.

·     Non-Integrated Service Router: In non ISR router sometimes for a small addition of services we need to buy extra license/module for it.

4.   Router Physical architecture:

·     Power cable & socket: For power supply.

·     Console port: To connect with console cable for initial device configuration.

·     Auxiliary port: To connect with modem. It can also be used to perform initial device configuration.

·     Flash slot: To insert flash memory.

·     Interfaces: To connect to UTP/STP cable.

5.   Router memories:

·     Flash memory: It is used to store IOS (Internetwork Operating System) of router.

·     RAM: It stands for Random Access Memory. It is used to store current/running-config. It is temporary/volatile memory.

·     NV-RAM: Stands for Nonvolatile-Random Access Memory. It is used to save configurations in the form of startup-configuration. It is a permanent memory.

·     ROM: It stands for Read Only Memory. It is used to store bootstrap program, for recovery purpose.

6.   Booting sequence of a router:

·     Router will perform POST (Power on Self-Test). Initial hardware requirement will be checked.

·     Bootstrap will look for IOS in flash memory. If IOS is not present in flash memory, then it will go into ROMMON (ROM Monitor) recovery mode. If IOS is present, it will load a copy of IOS in RAM.

·     Then it will check/verify configuration register value. If the value is 0x2102, it will further look for saved/startup-config in NV-RAM. If startup-config is present, it will load the configuration in RAM and go into user mode. If the value is other than 0x2102, then router will go into ICD (Initial Configuration Dialogue) mode.

·     At ICD (Initial Configuration Dialogue Mode) or System Configuration Dialogue mode, you will have two choice, YES/NO. If you enter “Yes”, router will force you to do step by step configuration. If you enter “NO”, you will go into User mode. To exit from ICD mode, press ctrl+c.

·     Finally router will go into user mode.

7.   Modes of router:

·     Initial Configuration Dialogue mode:

·     User mode or user exec mode

·     Privilege mode or Enable mode

·     Global mode or global configuration mode.