Cisco Systems CCNA 2 manual Using the boot system command, Configuration register

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-registercommand 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.

53 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 5 Copyright ♥ 2004, Cisco Systems, Inc.