POST (Power-On Self-Test) → Boot Loader → CPU (Central Processing Unit) init → Flash init → Load IOS — a power-on hardware check, then the bootloader initializes hardware and loads the operating system.

* The five-step switch boot sequence. *

1. POST: Stored in ROM (Read-Only Memory), checks CPU, DRAM (Dynamic Random-Access Memory), and flash file system
2. Boot Loader: Small program in ROM, runs immediately after POST completes
3. CPU Initialization: Boot loader initializes CPU registers — controls memory mapping, memory quantity, and speed
4. Flash File System Init: Boot loader initializes the flash file system on the system board
5. Load IOS: Boot loader locates and loads the default IOS image into memory, then hands control to IOS

Tip: Think of it like booting a PC — hardware check first (POST), then the BIOS/bootloader initializes hardware, then the OS loads from disk (flash).

Go deeper:

• Booting (Wikipedia) — the universal POST → bootloader → OS-load chain the Cisco sequence mirrors.
• Power-on self-test (Wikipedia) — what POST actually checks (CPU, DRAM, firmware) before the bootloader runs.

Sets the BOOT environment variable that tells the switch which IOS image to load from flash; if it is not set, the switch just loads the first executable file it can find.

S1(config)# boot system flash:/c2960-lanbasek9-mz.150-2.SE/c2960-lanbasek9-mz.150-2.SE.bin

Command breakdown:

If BOOT variable is not set: The switch loads and executes the first executable file it can find in flash.

The startup-config file is called config.text and is stored in flash. Use show boot to verify the current IOS boot file.