Every router has the following components that make up its con.guration: RAM, NVRAM, Flash, ROM, and interfaces.
Random Access Memory (RAM)
RAM serves as a temporary working storage area for the router. RAM contains data such as routing tables, cache, buffers, and I/O queues. RAM also provides storage for temporary memory for the router’s active IOS and configuration .(i.e., the running-con.g). You lose the entire contents of RAM when you power down or restart the router.
Non-Volatile RAM (NVRAM)
Conversely, NVRAM is permanent and retains its contents when you power down or restart the router. NVRAM stores permanent information, such as the router’s backup con.guration .le. The router retrieves the startup-con.g from NVRAM at start-up and loads into RAM.
Flash
Flash stores the Cisco IOS image and associated microcode. Flash is erasable, programmable, read-only memory (EPROM) that retains its contents when you power down or restart the router. You can store several versions of IOS images in Flash memory. Flash allows you to upgrade the router without adding, removing, or replacing microchips on the router.
Read-Only Memory (ROM)
ROM, like Flash, maintains a copy of the IOS but it is an older version of the IOS. ROM also stores the bootstrap program and power-on diagnostic programs. Unlike Flash, you can only upgrade ROM by replacing chips on the motherboard.
Interfaces
Interfaces provide the network connections where packets move in and out of the router. Depending on the router model, the interfaces might exist on the motherboard or on separate, modular interface cards.