Cisco boot sequence

Switch boot sequence

1. Switch loads a power-on self-test (POST) program stored in ROM. POST checks the CPU subsystem. It tests the CPU, DRAM, and the portion of the flash device that makes up the flash file system.
2. Switch loads the boot loader software. The boot loader is a small program stored in ROM and is run immediately after POST successfully completes.
3. 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.
4. Boot loader initializes the flash file system on the system board.
5. Boot loader locates and loads a default IOS operating system software image into memory and hands control of the switch over to the IOS.

The boot loader finds the Cisco IOS image on the switch as follows:

1. Switch attempts to automatically boot by using information in the BOOT environment variable.
2. If this variable is not set, the switch attempts to load and execute the first executable file it can by performing a recursive, depth-first search throughout the flash file system, each encountered subdirectory is completely searched before continuing the search in the original directory. On Catalyst 2960 Series switches, the image file is normally contained in a directory that has the same name as the image file (excluding the .bin file extension).

BOOT environment variable is set using the boot system global configuration mode command. To preview the current IOS boot file is set to use:

show bootvar 
show boot !in older IOS versions

Access to the switch boot loader - recovery

The boot loader provides access into the switch if the operating system cannot be used because of missing or damaged system files.

1. Connect a PC by console cable to the switch console port. Configure terminal emulation software to connect to the switch.
2. Unplug the switch power cord.
3. Reconnect the power cord to the switch and, within 15 seconds, press and hold down the Mode button while the System LED is still flashing green.
4. Continue pressing the Mode button until the System LED turns briefly amber and then solid green; then release the Mode button.
5. The boot loader switch: prompt appears in the terminal emulation software on the PC.

The boot loader command line supports commands to format the flash file system, reinstall the operating system software, and recover from a lost or forgotten password.