Cisco Router Boot Sequence Explained

Router follows a boot sequence to bring it up. Knowing this sequence will help you in managing this device easily.

the main components of a Cisco router and how the boot process takes place.

Types of memory

Generally Cisco routers (and switches) contain four types of memory:

Read-Only Memory (ROM): ROM stores the router’s bootstrap startup program, operating system software, and power-on diagnostic test programs (POST).

Flash Memory: Generally referred to simply as “flash”, the IOS images are held here. Flash is erasable and reprogrammable ROM. Flash memory content is retained by the router on reload.

Random-Access Memory (RAM): Stores operational information such as routing tables and the running configuration file. RAM contents are lost when the router is powered down or reloaded.

Non-volatile RAM (NVRAM): NVRAM holds the router’s startup configuration file. NVRAM contents are not lost when the router is powered down or reloaded.

Some comparisons to help you remember easier:

•  RAM is a volatile memory so contents are lost on reload, where NVRAM and Flash contents are not.
•  NVRAM holds the startup configuration file, where RAM holds the running configuration file.
•  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).

The boot sequence consists of the following steps:

• Router is powered on.
• Power on self test (POST) is performed to check hardware components including memory and interfaces.
• Bootstrap program is loaded and executed.
• Bootstrap reads configuration register value to determine how the router will boot up.
• Depending on the value of configuration register, bootstrap program finds and loads the IOS image.
• If bootstrap fails to load IOS from all possible locations it will drop boot sequence in ROMMON mode for troubleshooting.
• If IOS is loaded, the IOS will try to find and load the configuration.
• If configuration is not presented, system configuration dialog would be launched.
• If configuration is loaded, you would be presented in CLI interface.

Configuration register value

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 .

• It is a four characters hexadecimal value, that can be changed from privileged mode.
• Default value is 0x2102.
• "0x" indicates that the characters that follow are in hexadecimal.
• The third character in the configuration register can modify how the router loads the configuration file.
• The fourth character in the configuration register is known as the boot field. Changing this value, will change booting process.
• Setting of 0x2100 will always boot router in ROMMON mode.
• Setting of 0x2101 will always boot router in RXBOOT mode.
• Setting of 0x2102 to 0x210F will always load first valid IOS from flash.
• values of 2 through F for the fourth character specify other IOS image files in flash.
• Router keeps password in startup-config file that is stored in NVRAM.
• 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.