Booting Process of Cisco Router

 

Booting Component of Cisco Router

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1. ROM 
2. POST 
3. Bootstrap 
4. ROMMON 
5. Mini IOS 
6. Flash 
7. NVRAM 
8. RAM
9. Configuration register value  

ROM (Read Only Memory)

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$ Contains the power-on self-test and the bootstrap program for the router.  

$ ROM (Read only memory) stores four components.  

1. POST 
2. Bootstrap program 
3. ROMMON Mode   
4. Mini IOS. 

POST (Power On Self-Test)

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1. POST ( Power on self test ) is a diagnostic utility that performs various tests on hardware components.  
2. It verifies that all necessary components are present and operational.  
3. Modular slots are checked in this process for any hardware change like installing new interface or removing existing interfaces.

Bootstrap Loader

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1. Bootstrap is the second utility in booting sequence.  
2. It controls the search and load process of IOS.  
3. Bootstrap program is responsible for bringing up the router, finding IOS on all possible locations and loading it in RAM. 

ROMMON Mode

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1. ROMMON is a portable IOS program that allows us to perform various diagnostic tests.  
2. This program is also used for password recovery procedure.  
3. It has its own mode known as ROMMON mode.  
4. Boot sequence follows a conditional rule for this mode.  
5. If bootstrap successes in finding and loading operation of IOS, then boot sequence will not enter in this mode. Boot sequence will enter in this mode automatically, if it fails to load IOS in RAM from all possible locations. 

Note:- You can manually enter in this mode for diagnostic purpose. Run reload command from privileged mode to reboot the router. Press + C key combination ( CTRL Key with C Key ) in first 60 seconds of boot sequence. 

MINI IOS

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1. Mini-IOS is a fallback utility that contains a stripped down version of IOS.  
2. This is used in critical situations where IOS image in flash is not found. Mini-IOS contains only IP code that allows you to load IOS from other resources such as TFTP Server.  
3. Cisco IOS mode used by this stripped down IOS utility known as RXBOOT mode.

Flash Memory

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1. Flash is a nonvolatile memory.  
2. Data store in flash is not lost when you turn the router off.  
3. You could assume flash as hard disk of router. Like data stored in hard disk remain safe, same as data stored in flash remain safe.  
4. Router uses flash to store IOS image. During the boot process router load IOS file from flash to ram.

NVRAM(Non-Volatile Random Access Memory)

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1. NVRAM is another permanent memory.  
2. Data stored in NVRAM is also remain safe.  
3. Router use NVRAM to store configuration files. NVRAM uses a battery to maintain the data during the power off stage.

Random Access Memory (RAM)

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1. RAM is a temporary memory. Information stored in RAM does not remain in power off stage. Everything in RAM is erased, when you turn off the router. RAM is the fastest memory among these memories. In a powered on router, RAM contains all the information required to function the device. 
2. During the boot process IOS is copied in RAM from Flash. 
3. Same as ISO, running configuration is also copied in RAM from NVRAM. 
4. RAM stores CDP neighbor, routing tables. 

Configuration Register Value

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Configuration register value is a special register that is used to control the booting process. You can check current configuration register value with "show version" command from exec mode. 

1. It is a four characters hexadecimal value, that can be changed from privileged mode. 
2. Default value is 0x2102. 
3. "0x" indicates that the characters that follow are in hexadecimal. 
4. The  third character in the configuration  register can modify  how the router loads the configuration  file. 
5. The  fourth character in the configuration  register is known  as the boot field. Changing  this value, will change  booting  process. 
6. Setting of 0x2100 will always boot router in ROMMON  mode. 
7. Setting of 0x2101 will always boot router in RXBOOT  mode. 
8. Setting of 0x2102 to 0x210F will always load  first valid IOS from flash. 
9. values of 2 through F for the fourth  character specify other IOS image  files in flash. 
10. Router keeps password in startup-config  file that is stored in NVRAM. 
11. The setting of 0x2142 causes the router to ignore the startup-config file in NVRAM and proceed without a configuration.  Thus, the setting of 0x2142 is used for password recovery process of router.