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The default source for Cisco IOS software
depends on the hardware platform, but most commonly the router looks
to the boot system commands saved in NVRAM. Cisco IOS software allows several alternatives to be used. Other sources can be
specified for the software, or the router can use its own fallback
sequence to load the software.

The settings in the configuration register
enable the following alternatives:
- Global configuration mode boot
system commands can be specified to enter fallback sources for the
router to use in sequence. The router will use these commands as
needed, in sequence, when it restarts.
- If NVRAM lacks boot system commands
that the router can use, the system by default uses the Cisco IOS software in flash memory.
- If flash memory is empty, the router
then attempts to use TFTP to load an IOS image from the network. The
router uses the configuration register value to form a filename from
which to boot a default system image stored on a network server.
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Interactive Media Activity
Drag and Drop: How A Cisco Device Locates And Loads IOS
When the student has completed this activity, the student
will be able to identify the process a Cisco device goes through
in order to find IOS during boot-up.

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