When a router boots up, it performs a series of steps,
called the
boot sequence, to test the hardware and load the necessary software. The
boot sequence consists of the following steps:
1. The router performs a POST. The POST tests the hardware to verify that
all components of the device are operational and present. For example,
the POST checks for the different interfaces on the router. The POST is
stored in and run from ROM (read-only memory).
2. The bootstrap looks for and loads the Cisco IOS software. The bootstrap
is a program in ROM that is used to execute programs. The bootstrap program
is responsible for finding where each IOS program is located and then
loading the file. By default, the IOS software is loaded from flash memory
in all Cisco routers.
3. The IOS software looks for a valid configuration file stored in NVRAM.
This file is called startup-config and is only there if an administrator
copies the running-config file into NVRAM.
4. If a startup-config file is in NVRAM, the router will load and run
this file. The router is now operational. If a startup-config file is
not in NVRAM, the router will start the setup-mode configuration upon
bootup.
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