Cisco Gigabit Switch Router 12008 Installation And Configuration Manual page 221

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When the boot field is set to 1, the system automatically boots using the first image
found in the onboard Flash SIMM on the RP.
If you set the boot field to any bit pattern other than 0 or 1, the router uses the software
configuration register settings to compute a filename from which to boot a default system
image stored on a network TFTP server.
To form this filename, the system starts with cisco and links the octal equivalent of the boot
field value and the processor type in the following format:
cisco<bootfieldvalue>-<processorname>
This format, for example, would yield the following range of typical filenames:
cisco2-grp
.
.
.
cisco17-grp
or
cisco2-prp
.
.
.
cisco17-prp
The system would use a filename in this range from which to boot a default system image
stored on a network TFTP server.
However, if the configuration file contains boot instructions, the system uses these
instructions to boot the system, rather than using the filename it computed from the
software configuration register settings.
Observing System Startup and Performing a Basic Configuration 4-37
Performing Other Configuration Tasks