How To Control Environment Variables - Cisco Catalyst 9500 Manual

How to Control Environment Variables

• The IP address and the configuration filename is reserved for the device, but the TFTP server address is
• Only the IP address is reserved for the device and provided in the DHCP reply. The configuration filename
Note
How to Control Environment Variables
With a normally operating device, you enter the boot loader mode only through the console connection
configured for 9600 bps. Unplug the device power cord, and press the Mode button while reconnecting the
power cord. You can release the Mode button after the system LED begins flashing green and remains solid.
The boot loader device prompt then appears.
The device 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, operates. 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 present; it has a value if it is listed 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.
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.
System Management Configuration Guide, Cisco IOS XE Amsterdam 17.2.x (Catalyst 9500 Switches)
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not provided in the DHCP reply (one-file read method).
The device receives its IP address, subnet mask, and the configuration filename from the DHCP server.
The device sends a broadcast message to a TFTP server to retrieve the named configuration file from
the base directory of the server, and upon receipt, it completes its boot-up process.
is not provided (two-file read method).
The device receives its IP address, subnet mask, and the TFTP server address from the DHCP server.
The device sends a unicast message to the TFTP server to retrieve the network-confg or cisconet.cfg
default configuration file. (If the network-confg file cannot be read, the device reads the cisconet.cfg
file.)
The default configuration file contains the hostnames-to-IP-address mapping for the device. The device
fills its host table with the information in the file and obtains its hostname. If the hostname is not found
in the file, the device uses the hostname in the DHCP reply. If the hostname is not specified in the DHCP
reply, the device uses the default Switch as its hostname.
After obtaining its hostname from the default configuration file or the DHCP reply, the device reads the
configuration file that has the same name as its hostname (hostname-confg or hostname.cfg, depending
on whether network-confg or cisconet.cfg was read earlier) from the TFTP server. If the cisconet.cfg file
is read, the filename of the host is truncated to eight characters.
If the device cannot read the network-confg, cisconet.cfg, or the hostname file, it reads the router-confg
file. If the device cannot read the router-confg file, it reads the ciscortr.cfg file.
The device broadcasts TFTP server requests if the TFTP server is not obtained from the DHCP replies, if all
attempts to read the configuration file through unicast transmissions fail, or if the TFTP server name cannot
be resolved to an IP address.
Performing Device Setup Configuration