The Boot Process Of The Router

After powering on, the router will first perform POST, Power On Self Test.
After the POST is completed, first read the BootStrap program in the ROM for preliminary booting.
After the initial boot is complete, try to locate and read the full ISO image file. Here, the router will first look for the ISO file in FLASH, and if it finds the ISO file, then read the ISO file and boot the router.
If the ISO file is not found in FLASH, the router will enter BOOT mode, where the ISO file on TFTP can be used. OR USE TFTP/X-MODEM TO UPLOAD AN ISO FILE TO THE ROUTER'S FLASH (GENERALLY WE CALL THIS PROCESS ISO). After the transmission is completed, restart the router, and the router can boot into CLI mode normally.
WHEN THE ROUTER INITIALIZES THE ISO FILE, IT WILL START LOOKING FOR THE STARTUP-CONFIG FILE IN NVRAM, WHICH IS CALLED THE STARTUP CONFIGURATION FILE. This file saves all the configurations and modifications we have made to the router. When the router finds this file, the router will load all the configurations in the file, and learn, generate, and maintain routing tables according to the configuration, and load all the configurations into RAM (router memory), enter user mode, and finally complete the boot process.
IF THERE IS NO STARTUP-CONFIG FILE IN NVRAM, THE ROUTER WILL ENTER THE QUESTION-AND-ANSWER CONFIGURATION MODE, COMMONLY KNOWN AS THE QUESTION-AND-ANSWER CONFIGURATION MODE, IN WHICH ALL CONFIGURATIONS ABOUT THE ROUTER CAN BE CONFIGURED IN THE FORM OF QUESTIONS AND ANSWERS. But in general, we basically don't use this model. We usually enter CLI (Comman Line Interface) command line mode and configure the router.