Describe the Switch Boot Sequence


In this topic, you will learn the sequence of Cisco IOS commands that a switch executes from the off state to displaying the login prompt. After a Cisco switch is turned on, it goes through the following boot sequence:
The switch loads the boot loader software. The boot loader is a small program stored in NVRAM and is run when the switch is first turned on.
The boot loader:
Performs low-level CPU initialization. It initializes the CPU registers, which control where physical memory is mapped, the quantity of memory, and its speed.
Performs power-on self-test (POST) for the CPU subsystem. It tests the CPU DRAM and the portion of the flash device that makes up the flash file system.
Initializes the flash file system on the system board.
Loads a default operating system software image into memory and boots the switch. The boot loader finds the Cisco IOS image on the switch by first looking in a directory that has the same name as the image file (excluding the .bin extension). If it does not find it there, the boot loader software searches each subdirectory before continuing the search in the original directory.
The operating system then initializes the interfaces using the Cisco IOS commands found in the operating system configuration file, config.text, stored in the switch flash memory.
Recovering from a System Crash
The boot loader also provides access into the switch if the operating system cannot be used. The boot loader has a command-line facility that provides access to the files stored on Flash memory before the operating system is loaded. From the boot loader command line you can enter commands to format the flash file system, reinstall the operating system software image, or recover from a lost or forgotten password.

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