When Cisco Router is powered on, It boot-ups to start processing and work according to the configurations saved, if not saved router is configured.
Types of Memory
- ROM (Read Only Memory)
- NVRAM (Non-Volatile RAM)
- RAM (Random Access Memory)
When the Cisco Router is powered ON, the bootstrap program save din ROM makes a diagnostic test called POST (Power On Self Test). If the POST completes successfully, it loads the Cisco IOS from the configuration register.
If POST is not completed successfully, it attempts to load Cisco IOS from Flash, If succeeded IOS is loaded else it attempts to load it from TFTP server, If loading is done successfully then IOS starts the configurations file in NVRAM else IOS is loaded from ROM, where a copy of IOS is saved. Flash is erasable memory, so it makes router upgradeable.
When the Cisco IOS is loaded , it finds the startup configurations file in NVRAM and starts working according to the configurations saved in the file. If the configurations file is not found, the router comes to the Setup Mode.