Cisco Device Startup/Boot Sequence

 When you power on the Cisco router or switches, The first thing they discover the device hardware like interface details, CPU details, Memory details etc etc .

Once the hardware check is ok , They will find the IOS image and load . Once the ios will be loaded then the configuration file will be loaded from NVRAM .

The whole process we called as Power on self test .

Normally there are 3 types memory we have in devices, 

1-Flash 

2-NVRAM(Non-Volatile random Access Memory).

3-DRAM(Dynamic Random Access Memory).

Flash and NVRAM both have one thing is common is that they both are nonvolatile memory , which means after being powered off the device the it can keep the information and retain those files after getting powered on . NVRAM and FLASH are slow.

NVRAM is small in size . because it stores the configuration files only .

FLASH is a little large from NVRAM , This is where it stores the CISCO IOS.

DRAM , When the power is off and the contents are lost .

DRAM is very fast .

DRAM size is large .

DRAM memory size is big , because when you make the power ON the cisco device ,It finds the IOS from FLASH and copies and run from DRAM .When you switch off the device the IOS stores in flash but it run from DRAM .And then the device will load the Startup-configuration file from NVRAM this is also copied from NVRAM to run from DRAM and the name would be running-configuration. 

And also all the table which we have in devices like MAC tables, ARP table,VLAN tables ,Routing Tables, these are the tables and databases which displayed from the DRAM .