Cisco Router Booting Process

  1. POST
  2. load image
  3. check config-register
  4. load startup-config or skip


When you turn the router on, it runs through the following boot process.

The Power-On Self Test (POST) checks the router’s hardware. When the POST completes successfully, the System OK LED indicator comes on.
The router checks the configuration register to identify where to load the IOS image from. A setting of 0×2102 means that the router will use information in the startup-config file to locate the IOS image. If the startup-config file is missing or does not specify a location, it will check the following locations for the IOS image:

1. Flash (the default location)
2. TFTP server
3. ROM (used if no other source is found)

The router loads the configuration file into RAM (which configures the router). The router can load a configuration file from:

+ NVRAM (startup-configuration file)
+ TFTP server
If a configuration file is not found, the router starts in setup mode.