Cisco CCNA 2 Instructor Manual page 54

It is important to realize that these boot system commands must be saved in NVRAM to be
executed at the next start-up. If no boot system commands are saved in NVRAM, the router
will use the default fallback process, flash, TFTP, and finally ROM.
Review the figure in this section to explain the process that is followed to load the IOS. Make
sure students realize that network problems can affect the process when the IOS is loaded
from a TFTP server. Explain that the IOS loaded from ROM is only a subset of the IOS loaded
from flash.
The figure is not complete because ROM is not included.
5.1.3 Using the boot system command
The boot system command can be used to specify where and the sequence in which the
router will look for the IOS. After the boot system command has been saved to the start-up
configuration in NVRAM, it will be used in the next start up to locate the IOS. When the IOS is
loaded from flash memory, it is located locally, which isolates the process from any network
problems that might be associated with TFTP. The IOS may be loaded from a TFTP server if
the flash memory has been corrupted. If the IOS is not loaded from flash or the TFTP server, a
subset of the IOS can be loaded from ROM. Make sure students understand that the IOS
loaded from ROM is only a subset of the Cisco IOS software and might be an older version.
Use the boot system command to specify a fallback sequence and save it to NVRAM.
Restart the router and allow the students to verify the boot system locations during the next
start-up. Explain why it is important to save the boot system commands to NVRAM.
5.1.4 Configuration register
The configuration register is a 16-bit register that contains the boot field setting in the lowest
four bits. This boot field can be changed with the config-register command and is verified
with the show version command. The least significant bits indicate the location from which
the router will be booted. Zero will cause the router to boot in ROM monitor mode, one will
cause the router to boot from ROM, and two to F will cause the router to use the boot
system command in NVRAM.
Configuration Register Setting
0x0
0x1
0x2 to 0xF
More information can be found at the following website:
http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_command_reference_cha
pter09186a00800ca506.html
5.1.5 Troubleshooting IOS boot failure
Students must be familiar with the boot sequence and the configuration register to be able to
troubleshoot boot errors.
53 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 5
Location from which the router will boot.
ROM monitor mode, manual boot
ROM, automatic
NVRAM
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