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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 25 Modifying the Switch Boot Configuration
Understanding How the Switch Boot Configuration Works
The lowest four bits (bits 3, 2, 1, and 0) of the 16-bit configuration register form the boot field. The
default boot field value is 0x10F. The possible configuration register boot field settings are as follows:
•
When the boot field equals 0000, the switch does not load a system image. Instead, it enters
ROM-monitor mode from which you can enter ROM-monitor commands to load a system image
manually.
•
When the boot field equals 0001, the switch loads the first valid system image found in onboard flash
memory.
•
When the boot field equals a value between 0010 and 1111, the switch loads the system image that
is specified by the
boot system
commands in the NVRAM configuration. It attempts to boot the
image in the order in which you entered the
boot system
commands. If it cannot boot any image in
the BOOT environment variable list, the switch remains in ROM-monitor mode. The exact booting
sequence is defined by the ROM monitor.
The other bits in the configuration register function as follows when set:
•
Bit 5 (0x0020)—Enables CONFIG_FILE recurrence.
•
Bit 6 (0x0040)—Causes system software to clear NVRAM contents.
•
Bit 7 (0x0080)—Enables OEM bit (not used).
•
Bit 8 (0x0100)—Disables break.
•
Bit 9 (0x0200)—Uses secondary bootstrap (not used by the ROM monitor).
•
Bit 10 (0x0400)—Provides IP broadcast with all zeros (not used).
•
Bits 11/12 (0x0800/0x1000)—Provide console line speed: 0/0=9600, 0/1=1200, 1/0=4800,
1/1=2400 (default is 9600).
•
Bit 13 (0x2000)—Boots default flash software if network boot fails (not used).
•
Bit 14 (0x4000)—IP broadcasts do not have network numbers (not used).
•
Bit 15 (0x8000)—Enables diagnostic messages and ignores NVRAM contents (not used).
Understanding the BOOT Environment Variable
The BOOT environment variable specifies a list of image files on the various devices from which the
switch can boot at startup.
You can add several images to the BOOT environment variable to provide a fail-safe boot configuration.
If the first file fails to boot the switch, subsequent images that are specified in the BOOT environment
variable are tried until the switch boots or there are no additional images to attempt to boot. If there is
no valid image to boot, the system enters ROM-monitor mode where you can manually specify an image
to boot.
The system stores and executes images in the order in which you added them to the BOOT environment
variable. If you want to change the order in which the images are tried at startup, you can either prepend
and clear images from the BOOT environment variable to attain the desired order or you can clear the
entire BOOT environment variable and then redefine the list in the desired order.
