Controlling Environment Variables - Cisco 3845 - Security Bundle Router Software Manual

Modifying the Startup Configuration
Command
Step 3
end
Step 4
show boot
Step 5 copy running-config startup-config
To return to the default setting, use the no boot system global configuration command.

Controlling Environment Variables

With a normally operating switch, you enter the boot loader mode only through a switch console
connection configured for 9600 bps. Unplug and then reconnect the switch power cord. After the switch
performs POST, the switch begins the autoboot process. The boot loader prompts the user for a break
key character during the boot-up sequence, as shown in this example:
The break key character is different for each operating system.
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•
Note Cisco TAC has tabulated break keys for most common operating systems and provided an alternative
break key sequence for terminal emulators that do not support the break keys. To see that list go to:
http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a0080174a34.shtml
When you enter the break key, 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:
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Cisco ME 3800X and 3600X Switch Software Configuration Guide
3-20
Purpose
Return to privileged EXEC mode.
Verify your entries.
The boot system global command changes the setting of the BOOT
environment variable.
During the next boot cycle, the switch attempts to automatically boot the
system using information in the BOOT environment variable.
(Optional) Save your entries in the configuration file.
***** The system will autoboot in 5 seconds *****
Send a break key to prevent autobooting.
On a SUN work station running UNIX, Ctrl-C is the break key.
On a PC running Windows 2000, Ctrl-Break is the break key.
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.
Chapter 3 Assigning the Switch IP Address and Default Gateway
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