Upgrading the Cisco IOS Notes


Types of Cisco Memory

RAM Running-config
Flash Cisco IOS software
ROM bootstrap ROMMON
NVRAM Startup Config

IOS Boot Sequence

  1. POST
  2. Copies bootstrap from ROM to RAM, then runs bootstrap
  3. Bootstrap decides which IOS to load
  4. IOS finds config file to transfer to running config

configuration register – is the value that tells the router which IOS to load among other settings

How a router picks which IOS to load

  • The last hex value in the configuration register (bootfield)
  • any boot system global config cmd in startup
  • last hex value is also the 4th value

Bootfields and their meanings

  • 0 = ROMMON
  • 1 = First IOS file in FLASH
  • 2-f = Try boot system in start-config, then fist IOS in flash memory.

Ways to erase start up

  • write erase
  • erase start-up config
  • erase nvram

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