Lab
Activity 9.3.11: Booting up the router
Cisco routers boot Cisco IOS software from:
- Flash
- TFTP server
- ROM (not full Cisco IOS)
Multiple source options provide flexibility and fallback
alternatives
Locating the Cisco IOS Software
Default source for Cisco IOS software:
- Flash (sequential)
- TFTP server (netboot)
- ROM (partial IOS)
Note: boot system commands can be used to specify the
primary IOS source and fallback sequences.
Booting up the router and locating the Cisco IOS.
ROM
- POST
- Bootstrap code executed
- Check Configuration Register value (NVRAM) which can be modified
using the config-register command
0 = ROM Monitor mode
1 = ROM IOS
2 - 15 = startup-config in NVRAM
- Startup-config file: Check for boot system commands (NVRAM)
If boot system commands in
startup-config
- Run boot system commands in order they appear in
startup-config to
locate the IOS
- [If boot system commands fail, use default fallback sequence to
locate the IOS (Flash, TFTP, ROM)?]
If no boot system commands in
startup-config use the default
fallback sequence in locating the IOS:
- Flash (sequential)
- TFTP server (netboot)
- ROM (partial IOS) or keep retrying TFTP depending upon router model
- If IOS is loaded, but there is no startup-config file, the
router will use the default fallback sequence for locating the IOS and
then it will enter setup mode or the
setup dialogue.
- If no IOS can be loaded, the router will get the partial IOS
version from ROM
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