BootROM is used for permanently the storage of the startup diagnostic code (ROM Monitor). The main task for the BootROM is to perform some hardware diagnostics during bootup on the router, Power On Self Test (POST), and to load the Cisco IOS software from the Flash to the Memory.
Flash memory is a non-volatile memory storage that doesn't loose the information by turning the power off. Flash memories exist in many forms in routers, internal flash, external flash card or even USB flash cards.
Flash is used for permanent storage of a full Cisco IOS software image in compressed form. In Juniper routers Flash stores the JUNOS image and the configuration files.
Juniper router also has an internal hard drive that stores a backup copy of the JUNOS image, log files and any other required files.
RAM is a volatile data storage type of memory in which stored information is lost with power off. RAM stands for random access memory where the word random refers that stored data can be accessed in any order. RAM can be SRAM or DRAM. DRAM is used in more applications for the simplicity of its structure and lower cost.
RAM is used at run time for executable operating system code, and its subsystems, routing tables, caches, running configuration, packets, and so forth.
NVRAM stands for non-volatile random access memory and is used to describe any type of RAM that stored data is not lost by power turned off. NVRAM is used for writable permanent storage of the startup configuration in CISCO routers.
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