How a Cisco device locates and loads IOS

 How a Cisco device locates and loads IOS

5.1.2

The default source for Cisco IOS software depends on the hardware platform. Most routers use 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 a router to use in sequence. The router will use these commands as needed when it restarts.
• If NVRAM lacks boot system commands that a router can use, the system will use the Cisco IOS software in flash memory by default.
• If flash memory is empty, a router will try to use TFTP to load an IOS image from the network. The router will use the configuration register value to form a filename from which to boot a default system image that is stored on a network server.
• If a TFTP server is unavailable, the router will load the limited version Cisco IOS software image stored in ROM.

The Interactive Media Activity will help students become familiar with the process a Cisco device uses to find and load IOS.
The next page will explain how boot system commands are used.