Router Boot Process Overview:
A router goes through three steps when booting up:
POST (Power-on Self Test)
Locate and Load IOS
Locate and Run Device Configuration File
Before we explore this procedure in more detail, let’s familiarize ourselves with the types of memory used by the router. Usually, a router uses four types of memory:
ROM
FLASH
NVRAM
RAM
ROM
ROM (Read-Only Memory) stores four components: POST, Bootstrap program, ROMMON mode, and Mini IOS.
POST:POST (Power-on Self Test) is a low-level diagnostic utility that performs various tests on hardware components. It verifies that all necessary components are present and operational. Modular slots are checked during this process for any hardware changes, such as installing or removing interfaces.
Bootstrap:Bootstrap is the second utility in the booting sequence. It controls the search and load process of the IOS. The Bootstrap program is responsible for bringing up the router, finding the IOS in all possible locations, and loading it into RAM.
ROMMON:ROMMON is a portable IOS program that allows us to perform various diagnostic tests. This program is also used for the password recovery procedure. It has its own mode (Check our previous article to know more about Cisco IOS modes) known as ROMMON mode. The boot sequence follows a conditional rule for this mode:
If the bootstrap succeeds in finding and loading the IOS, the boot sequence will not enter this mode.
If it fails to load the IOS from all possible locations, the boot sequence will enter ROMMON mode automatically.
You can manually enter ROMMON mode for diagnostic purposes by running the reload command from privileged mode and pressing Ctrl + C during the first 60 seconds of the boot sequence.
Mini-IOS:Mini-IOS is a fallback utility that contains a stripped-down version of IOS. It is used in critical situations where the IOS image in Flash is not found. Mini-IOS contains only the IP code needed to load the IOS from other resources such as a TFTP server. The Cisco IOS mode used by this stripped-down IOS utility is known as RXBOOT mode.
FLASH
Flash is a non-volatile memory. Data stored in Flash is not lost when the router is powered off. You can think of Flash as the router’s hard disk—data stored here remains safe. The router uses Flash to store the IOS image, and during the boot process, the router loads the IOS file from Flash into RAM.
NVRAM
NVRAM is another permanent memory. Data stored in NVRAM is also retained during a power-off stage. The router uses NVRAM to store configuration files. NVRAM uses a battery to maintain the data during power outages.
RAM
RAM is a temporary memory. Information stored in RAM does not persist during a power-off stage; everything in RAM is erased when the router is turned off. RAM is the fastest memory among these and contains all the information needed to operate the device when the router is powered on.
During the boot process, the IOS is copied and decompressed in RAM from Flash.
Similarly, the running configuration is also copied into RAM from NVRAM.
RAM stores ARP, CDP neighbor, routing, and other tables.
Interface input and output buffers are also stored in RAM.
Configuration Register Value
The configuration register value is a special register that controls the booting process. You can check the current configuration register value with the show version command from exec mode.
It is a four-character hexadecimal value, and it can be changed from privileged mode.
The default value is 0x2102.
"0x" indicates that the following characters are in hexadecimal.
The third character in the configuration register affects how the router loads the configuration file.
The fourth character in the configuration register is known as the boot field. Changing this value alters the booting process.
Setting 0x2100 will always boot the router in ROMMON mode.
Setting 0x2101 will always boot the router in RXBOOT mode.
Settings from 0x2102 to 0x210F will load the first valid IOS from Flash.
Values of 2 through F for the fourth character specify other IOS image files in Flash.
The router keeps the password in the startup-config file, which is stored in NVRAM.
Setting 0x2142 causes the router to ignore the startup-config file in NVRAM and proceed without a configuration. This setting is used during the password recovery process.
Router Booting Process
A router goes through the following steps during the boot process:
The router is powered on.
The Power-on Self Test (POST) is performed to check hardware components, including memory and interfaces.
The Bootstrap program is loaded and executed.
The Bootstrap reads the configuration register value to determine how the router will boot up.
Depending on the value of the configuration register, the Bootstrap program finds and loads the IOS image.
If Bootstrap fails to load the IOS from all possible locations, it drops the boot sequence into ROMMON mode for troubleshooting.
If the IOS is loaded, it attempts to find and load the configuration.
If no configuration is found, the system configuration dialog is launched.
If the configuration is loaded, the CLI interface is presented.