When the router starts, a series of steps, called boot sequence, is performed to test the hardware and load the required software. The startup sequence includes the following steps:
1. The router performs POST (POST). POST checks the hardware to verify that all components of the device are currently operational. POST is stored in the ROM and run from the ROM.
2. bootstrap finds and loads Cisco IOS software. bootstrap is a program located in a ROM that is used to execute programs. The bootstrap program is responsible for finding the location of each IOS program and then loading the file. By default, all Cisco routers load IOS software from flash memory.
3. IOS software finds valid configuration files in NVRAM. The secondary file is called startup-config, which is generated only when the administrator copies the running-config file into NVRAM.
4. If there is a startup-config file in NVRAM, the router will load and run the file. The router is currently operational. If there is no startup-config file in NVRAM, the router will start setup mode to configure the above startup operation.
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