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CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 5
Copyright
©
2004, Cisco Systems, Inc.
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
Location from which the router will boot.
0x0
ROM monitor mode, manual boot
0x1 ROM,
automatic
0x2 to 0xF
NVRAM
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.