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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter4 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 m ode onl y t hro ugh a sw itch c onso le
connection configured for 9600 bps. Unplug the switch power cord and pr ess the swit ch Mo de button
while reconnecting the power cord. You can release the Mode button a second or two after the LED
above port 1X goes off. Then the boot loader switch: pr ompt is displ aye d.
The switch boot loader software provides support for nonvolatile environment variables, wh ich 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 variabl es t hat ca n b e set on U NIX o r DO S s yst ems.
Environment variables that have values are stored in the Flash file system in various files as shown in
Table 4 -4.
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 IOS configuration file. For exam ple, the name of a
boot loader helper file, which extends or patches the funct iona li ty o f th e boo t loa de r ca n be st ored
as an environment variable.
•Data that controls code, which is responsible for reading the IOS configuration file. For example,
the name of the 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 IOS
commands. Under normal circumstances, it is not nece ssary to al ter the set tin g of t he e nvir onm en t
variables.
Note For complete syntax and usage information for the boot loader commands and environm ent variables,
refer to the Catalyst 3550 Multilayer Switch Command Reference for this release.
Table4-4 Environment Variables Storage Location
Environment Variable Location (file system:filename)
BAUD, ENABLE_BREAK, CONFIG_BUFSIZE,
CONFIG_FILE, MANUAL_BOOT, PS1 flash:env_vars
BOOT, BOOTHLPR, HELPER, HELPER_CONFIG_FILE flash:system_env_vars