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The following examples show the use of
multiple boot system commands to specify the fallback sequence for
booting Cisco IOS software. The three examples show boot system
entries which specify that a Cisco IOS software image will load first
from flash memory, then from a network server, and finally from ROM:
- Flash memory – A system image
from flash memory can be loaded. The advantage is that information
stored in flash memory is not vulnerable to network failures that
can occur when loading system images from TFTP servers.
- Network server – In case
flash memory becomes corrupted, a system image can be loaded from a
TFTP server.
- ROM – If flash memory is
corrupted and the network server fails to load the image, booting
from ROM is the final bootstrap option in software. However, the
system image in ROM will likely be a subset of the Cisco IOS that
lacks the protocols, features and configurations of the full Cisco
IOS. Also, if the software has been updated since the router was
purchased, the router may have an older version stored in ROM.
The command
copy running-config startup-config
saves the commands in NVRAM. The router will execute the boot system
commands as needed in the order in which they were originally entered
into configuration mode.
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Lab Activity
Lab Exercise: Using the Boot System Command
In this lab, the students will display information about the
Cisco IOS Image that runs on the router.
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Lab Activity
e-Lab Activity: Preparing for Boot System Commands
In this lab, the students will display information relating to the router boot
procedure, using the appropriate show commands. 
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Web Links