Every Cisco router uses at least four types of memory:

• Read only memory (ROM)
• Random access memory (RAM)
• Nonvolatile random access memory (NVRAM)
• Flash memory

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Read only memory is used to store the start-up code that bootstraps the router. This code checks the hardware and loads the main IOS image, which is usually stored in flash memory, into RAM. (Some models of Cisco routers, such as the 2500 series, can run IOS directly from flash memory, which is addressable by the CPU [central processing unit]. This method is called “run from flash.”) When the main IOS image has been loaded and given control, it copies the router configuration from NVRAM to a buffer in RAM and passes it to the parser for processing. The parser then dynamically processes corresponding configuration commands.

In addition to the CPU and memory, Cisco routers also have input/output (I/O) devices, such as network interface controllers (NICs) or asynchronous serial management ports. Network interface controllers, hardware devices inside the router, provide low-level functionality for network attachments, such as LAN (local area network) or WAN (wide area network) interfaces.