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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-21521-01
Chapter 3 Assigning the Switch IP Address and Default Gateway
Modifying the Startup Configuration
Controlling Environment Variables
With a normally operating switch, you enter the boot loader mode only through a switch console
connection configured for 9600 b/s. Unplug the switch power cord, and press the switch Mode button
while reconnecting the power cord. You can release the Mode button a second or two after the LED
above port 1 turns off. Then the boot loader switch: prompt appears.
The switch boot loader software provides support for nonvolatile environment variables, which can be
used to control how the boot loader, or any other software running on the system, behaves. Boot loader
environment variables are similar to environment variables that can be set on UNIX or DOS systems.
Environment variables that have values are stored in flash memory outside of the flash file system.
Each line in these files contains an environment variable name and an equal sign followed by the value
of the variable. A variable has no value if it is not listed in this file; it has a value if it is listed in the file
even if the value is a null string. A variable that is set to a null string (for example, “ ”) is a variable with
a value. Many environment variables are predefined and have default values.
Environment variables store two kinds of data:
• Data that controls code, which does not read the Cisco IOS configuration file. For example, the name
of a boot loader helper file, which extends or patches the functionality of the boot loader can be
stored as an environment variable.
• Data that controls code, which is responsible for reading the Cisco IOS configuration file. For
example, the name of the Cisco IOS configuration file can be stored as an environment variable.
You can change the settings of the environment variables by accessing the boot loader or by using Cisco
IOS commands. Under normal circumstances, it is not necessary to alter the setting of the environment
variables.
Note For complete syntax and usage information for the boot loader commands and environment variables,
see the command reference for this release.
Table 3-4 describes the function of the most common environment variables.
Tab le 3-4 Environment Variables
Variable Boot Loader Command Cisco IOS Global Configuration Command
BOOT set BOOT filesystem:/file-url ...
A semicolon-separated list of executable files
to try to load and execute when automatically
booting. If the BOOT environment variable is
not set, the system attempts to load and execute
the first executable image it can find by using a
recursive, depth-first search through the flash
file system. If the BOOT variable is set but the
specified images cannot be loaded, the system
attempts to boot up the first bootable file that it
can find in the flash file system.
boot system {filesystem:/file-url ...| switch
{number | all}}
Note The switch {number | all} keywords are
supported only on Catalyst 3750-E
switches.
Specifies the Cisco IOS image to load during the
next boot cycle and the stack members on which
the image is loaded. This command changes the
setting of the BOOT environment variable.