THERE ARE 7 MAJOR INTERNAL COMPONENTS OF A ROUTER:
CISCO ROUTERS (AND SWITCHES) GENERALLY CONTAIN FOUR TYPES OF MEMORY:
ROM Contains A Bootstrap Program Called ROM Monitor (Or ROMMON). When A Router Is Powered On, The Bootstrap Runs A Hardware Diagnostic Called POST (Power-On Self Test).
ROUTER COMPONENTS AND THEIR FUNCTIONS:
CPU - Executes Operating System Instructions. The CPU Performs Functions Just As It Does In A Normal PC. It Executes Commands Given By The IOS Using Other Hardware Components. High-End Routers May Contain Multiple Processors Or Extra Slots To Add More Cpus Later
RANDOM ACCESS MEMORY (RAM) - Contains The Running Copy Of Configuration File. Stores Routing Table. RAM Contents Lost When Power Is Off. RAM Is Used To Store Operational Information Such As Routing Tables, Router's Running Configuration File. RAM Also Provides Caching And Packet Buffering Capabilities. Its Contents Are Lost When You Switch Off Or Restart The Router.
READ-ONLY MEMORY (ROM) - Holds Diagnostic Software Used When Router Is Powered Up. Stores The Router’s Bootstrap Program.
NON-VOLATILE RAM (NVRAM) - Stores Startup Configuration. This May Include IP Addresses (Routing Protocol, Hostname Of Router). NVRAM Does Not Lose Its Information When Power Is Turned Off. This Is In Contrast To The Most Common Forms Of RAM, Such As DRAM, That Requires Continual Power To Maintain Its Information. NVRAM Is Used By The Cisco IOS As Permanent Storage For The Startup Configuration File (Startup-Config).
All Configuration Changes Are Stored In The Running-Config File In RAM, And With Few Exceptions, Are Implemented Immediately By The IOS. To Save Those Changes In Case The Router Is Restarted Or Loses Power, The Running-Config Must Be Copied To NVRAM, Where It Is Stored As The Startup-Config File. NVRAM Retains Its Contents Even When The Router Reloads Or Is Powered Off.
FLASH MEMORY - Contains The Operating System (Cisco IOS). Flash Memory Is Nonvolatile Computer Memory That Can Be Electrically Stored And Erased. Flash Is Used As Permanent Storage For The Operating System, Cisco IOS. In Most Models Of Cisco Routers, The IOS Is Permanently Stored In Flash Memory And Copied Into RAM During The Bootup Process, Where It Is Then Executed By The CPU. Some Older Models Of Cisco Routers Run The IOS Directly From Flash.
Flash Consists Of Simms Or PCMCIA Cards, Which Can Be Upgraded To Increase The Amount Of Flash Memory.Flash Memory Does Not Lose Its Contents When The Router Loses Power Or Is Restarted.
INTERFACES - There Exist Multiple Physical Interfaces That Are Used To Connect Network.The Interfaces Provide Connectivity To LAN, WAN, And Console/Aux. They Can Be RJ-45 Jacks Soldered Onto The Motherboard, Transceiver Modules, Or Card Modules. Cisco Routers, Especially The Higher-End Models, Can Be Configured In Many Different Ways. They Can Use A Combination Of Transceivers, Card Modules And Onboard Interfaces.
EXAMPLES OF INTERFACE TYPES:
Interfaces Are Identified By Both The Type Of Interface, And The Interface Number (Which Always Begins At “0”). Thus, The First Ethernet Interface On A Router Would Be Identified As Ethernet0.
Certain Router Families (Such As The 3600 Series) Are Modular, And Have Multiple “Slots” For Interfaces. Thus, Interfaces On These Routers Are Identified By Both The Module Number And The Interface Number, Formatted As: Module/Interface. Thus, The Third Fast Ethernet Interface On The First Modular Slot Would Be Identified As Fastethernet0/2.
Lines Identify Ports That Allow Us To Connect Into, And Then Configure, Cisco Devices. The Most Common Examples Of Lines Include:
Just Like Interfaces, Lines Are Identified By Both The Type Of Line, And The Line Number (Again, Always Begins At “0”). Thus, The First Console Port On A Router Would Be Identified As Console0.
A Cisco 2600 series router is a cut above the routers populating the 2500 series because it has a faster processor and can handle many more interfaces. Figure as below shows a diagram of a Cisco 2600 modular router.
SUMMARY OF BOOT SEQUENCE :
1. On Power On Cisco Router First Will Perform The POST( Power On Self Test). The POST Tests The Hardware To Verify That All Components Of The Device Are Operational And Present. For Example, The POST Checks For The Different Interfaces On The Router. The POST Is Stored In And Run From ROM (Read-Only Memory).
2. The Bootstrap Looks For And Loads The Cisco IOS Software. The Bootstrap Is A Program In ROM That Is Used To Execute Programs. The Bootstrap Program Is Responsible For Finding Where Each IOS Program Is Located And Then Loading The File. By 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. Which Is Called As Startup-Config.
4. If A Startup-Config File Is In NVRAM, The Router Will Load And Run This File. The Router Is Now Operational. If A Startup-Config File Is Not In NVRAM, The Router Will Start The Setup-Mode Configuration Upon Bootup.
5. Any Further Modification On Running Router Will Be Stored On RAM, Where You Need To Manually Execute Command Copy Running-Config Startup-Config To Make Your Current Configuration As A Startup-Config, Every Time You Boot Your Router.
