BEFORE WE GO BOOT SEQUENCE FIRST KNOW WHAT CISCO IOS IS:
CISCO IOS:
The Cisco Internetwork Operating System (IOS) Is The Kernel Of Cisco Routers And Switches. Not All Cisco Devices Run The Same IOS. Some Use A Graphical Interface, Some Use A Set/Clear Command−Line Interface, And Some Use A Cisco Command Line Interface (CLI).
Cisco Has Acquired More Devices Than They Have Designed And Built Themselves. Therefore, Cisco Has Adapted The Operating Systems Designed For Each Device They Have Acquired To Use The Protocols And Standards Of The Company. Almost All Cisco Routers Run The Same IOS, But Only About Half Of The Switches Currently Run The Cisco CLI IOS.
WHAT IS CLI (COMMAND LINE INTERFACE):
The CLI Everyone Who Has Worked With Cisco Equipment Is Familiar With The Command Line Interface (CLI). The WAN Interface Is Very Different From The Interfaces In Other Cisco Equipment. To Gain Access To The CLI, You Will Have To Use The Serial Port On The Control Module, The Ethernet Connection, Or A Virtual Terminal).
Knowing What Configuration Mode You Are In And How To Enter Each Configuration Mode On The Cisco CLI Is Important. Recognizing What Each Mode Configures Will Aid You In Using The Proper Configuration Mode.
BEFORE WE GO BOOT SEQUENCE ALSO KNOW WHAT IS CISCO ROUTERS:
A Router Is A Layer 3 Network Device That Moves Data Between Different Network Segments And Can Look Into A Packet Header To Determine The Best Path For The Packet To Travel. Routers Can Connect Network Segments That Use Different Protocols. They Also Allow All Users In A Network To Share A Single Connection To The Internet Or A WAN. Routers Examine A Packet’s Destination IP Address And Determine The Best Path By Using A Routing Table.
CONNECTING TO THE SWITCHES / ROUTERS:
You Can Connect To A Cisco Switches/Routers To Configure The Switch/Router, Verify The Configuration, Or Check Statistics. Although There Are Different Ways Of Connecting To A Cisco Switch, Typically You Would Connect To Its Console Port.
In Lower−End Cisco Switches/Routers, The Console Port Is Usually An RJ−45 Connection On The Back Of The Switch. On A Higher−End Switch, You May Find Console Ports On The Line Cards Such As A Supervisor Engine. By Default There Is No Password Set On The Console Port.
Another Way To Connect To A Cisco Switch Or Router Is Through An Auxiliary Port. This Is Basically The Same As Connecting Through A Console Port, But It Allows You To Connect Remotely By Using A Modem. This Means You Can Dial Up A Remote Switch And Perform Configuration Changes, Verify The Configuration, Or Check Statistics.
A Third Way To Connect To A Cisco Switch Is Through The Program Telnet. Telnet Is A Program That Emulates A Dumb Terminal. You Can Use Telnet To Connect To Any Active Port On The Switch, Such As An Ethernet Or Serial Port.
Cisco Also Allows You To Configure The Switch By Using Switch Manager, Which Is A Way Of Configuring Your Switch Through A Web Browser Using HTTP. This Method Creates A Graphical Interface For Configuring Your Switch. The Switch Manager Allows You To Perform Most Of The Same Configurations As You Can With The CLI.
CONSOLE CABLE FOR CONNECTING TO CISCO DEVICES
When You First Received Your Cisco Devices Out Of The Box, You're Going To Need Console Cable To Start Configuring Your Devices For The First Time.
Console Cable Is A Cable Which Has An RJ45 Connector At One End (To Connect To Your Cisco Devices) And Serial Connector At The Other End (To Connect To Your PC).
Console Cable Made With The Rollover Cable Wiring Scheme, You Can See At The Image Above How You Can Make Your Own Rollover Cable. It's Really Simple Actually, You Just Swap The First Cable At One End To The Last Cable At The Other End, Second Cable To The Seventh Cable, And So On.
Should Have RJ45 Cable That You Should Plug At Your Cisco Device Console Port And The Other End Has Serial Connector That You Connect To Your PC's Serial Port.
The Images On The Left Side Are The Images Of The Console Cable And Serial Connectors (DB25 On The Left And DB9 On The Right) Provided By Cisco.
BELOW IS THE IMAGE OF CONSOLE PORT THAT YOU CAN FIND AT CISCO DEVICES:
CISCO ALSO HAS A CONSOLE CABLE THAT ALREADY HAS AN RJ45 CONNECTOR AT ONE END AND PRE-INSTALLED SERIAL CONNECTOR AT THE OTHER END LIKE THIS:
There's A Reason Why You Should Use Console Cable To Configure Your Cisco Devices For The First Time.
The Devices Come With A Minimum Configuration From The Factory, They Don't Have Any IP Addresses So You Can't Telnet Or SSH To Them - Telnet Or SSH Are The Most Common Ways To Configure Your Devices.
The Best Option Is To Use Serial Connection To Directly Configure The Devices, This Gives You A Freedom To Config Them As You Wish.
IT IS USED TO IMPROVE NETWORK PERFORMANCE BY:
CISCO ROUTER BOOT SEQUENCE:
FIRST OF ALL Any Router To Start Working Will Require Operating System, In Case Of CISCO Router We Call Its Operating System As IOS, Which Is Stored In Flash Drive Bydefaults Comes With Any Router.
SECOND THING It Requires Is Startup-Config Which Is Stored In NVRAM, Built-In In Cisco Router, Which Indicates Router What Function It Has To Run,
AND THE THIRD ONE IS Running-Config Which It Require In Real Time Basis Stored In RAM Memory Of Router, Which Is Same As Our Personal Computer's RAM. Now Keeping All This Fact In Mind.
WHEN A ROUTER BOOTS UP, It Performs A Series Of Steps, The Sequence Of Steps Called As Boot Sequence Occurs Which Helps To The Hardware And Load The Required Software.Before A Router Starts The Booting Process, It Has To Initialize Its Interfaces And Then Load The Operating System And Configuration File. To Control This Process The User Has To Find The Type Of Image File And Configuration File Used By Router.
THE BOOT SEQUENCE CONSISTS OF THE FOLLOWING STEPS.
STEP 1: POWER ON SELF TEST: The Router Performs A POST Is A Series Of Hardware Test That Verify Whether All Components Of The Device Are Functional Such As The POST.
While Performing This Test The Router Also Determines What Hardware Is Present. The POST Is Stored In And Executes From MICROCODE IN ROM.
STEP 2: LOAD AND RUN BOOTSTRAP CODE: The Bootstrap Code Finds And Loads Cisco IOS Software. It Is Stored In ROM That Is Used To Execute Programs. The Bootstrap Program Is Responsible For Finding The Location Of IOS Software And Then Loading The File.
NOTE: Power On Router - > Router Does POST - > Bootstrap Starts IOS Load - > Check Configuration Register To See What Mode The Router Should Boot Up In (Usually 0x102 To 0x10f To Look In NVRAM) - > Check The Startup-Config File In NVRAM For Boot-System Commands (Normally There Aren't Any) - > Load IOS From Flash.
POWERING UP THE SWITCHES / ROUTERS:
When You First Power Up A Cisco Switch, It Runs The Power On Self Test (POST), Which Runs Diagnostics On The Internal Workings Of The Switch. If The Switch Passes This Test, It Will Look For And Load The Cisco IOS From Flash Memory If A File Is Present. Flash Memory Is Read−Only Memory Kept On An EEPROM (A Silicon Chip Inside Of The Switch). The IOS Then Loads The Configuration Contained In Nonvolatile RAM (NVRAM).
NVRAM Is Similar To Random Access Memory (RAM) But Is Not Lost When The Power Is Cycled On The Switch. This Loads The Configuration Of The Cisco IOS And The Cisco User Interface Becomes Available.
The Cisco IOS User Interface Is Divided Into Several Different Modes. The Commands Available To You In Each Mode Determine The Mode You Are In. When You Start A Session On The Switch, You Begin In User EXEC Mode, Often Called EXEC Mode. Only A Limited Subset Of The Commands Is Available In EXEC Mode. In Order To Have Access To All Commands, You Must Enter Privileged EXEC Mode.
From Privileged EXEC Mode, You Can Enter Any EXEC Command Or Enter Global Configuration Mode, Which Offers Even More Command Options. From Global Configuration Mode You Can Also Enter Into Any Interface Configuration Mode To Configure An Interface (Port) Or A Subinterface.
CISCO ROUTER BOOT PROCESS:
STEP 1. The Router Is Powered On.
STEP 2. The Bootstrap Program (Rommon) Is Loaded From ROM.
STEP 3. The Bootstrap Runs POST.
STEP 4. The Bootstrap Attempts To Load The IOS From Flash.
5. The IOS Attempts To Load The Startup-Config File From NVRAM
6. The Startup-Config Becomes The Running-Config In RAM.
If POST Completes Successfully, The Bootstrap Then Attempts To Locate And Load The Cisco IOS (Internetwork Operating System) Stored In Flash Memory. Flash Memory Can Be Erased Or Overwritten, Thus Making The Cisco IOS Upgradeable. The Cisco IOS Is Covered In Great Detail In Other Guides. If The Bootstrap Cannot Find The IOS In Flash, A Stripped-Down Version Of The IOS That Will Be Loaded From ROM Instead. The Contents Of ROM Cannot Be Altered Or Erased, The Entire ROM Chip Must Be Replaced If An Upgrade/Repair Is Necessary.
If The Bootstrap Does Find The IOS In Flash, It Is Loaded Into RAM And Attempts To Find A Startup Configuration (Startup-Config) File In NVRAM. NVRAM Is Non-Volatile, Thus Its Contents Will Survive A Power-Cycle. If The IOS Cannot Find A Startup-Config File In NVRAM, It Will Attempt To Load A Configuration File From A TFTP Server (This Request Is Broadcasted To 255.255.255.255). If No TFTP Server Responds, The IOS Will Enter Initial Configuration Mode, A Series Of Interactive Questions Intended For Quick Configuration Of The Router.
If The IOS Does Find A Startup-Config File In NVRAM, This File Is Loaded Into RAM, And Becomes The Running Configuration (Running-Config). RAM Is A Volatile Memory, And Thus Its Contents Will Be Lost If The Router Is Powercycled.
ROUTER INSIDE COMPONENTS:
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.
CONCLUSION:
The Goal Of This Article Is To Give An Easy Way To Understand The Router Boot Sequence For Every Beginours, Who Are Going To Study CISCO Networking.Thank You!
This Article Written Author By: Premakumar Thevathasan. CCNA, CCNP, CCIP, MCSE, MCSA, MCSA - MSG, CIW Security Analyst, CompTIA Certified A+.
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