Router Memory Types
Cisco routers actually contain different types of memory components that provide the storage and dynamic caching required. The following list provides information about the various memory components found in a Cisco router.
ROM—Contains the power-on self test and the bootstrap program for the router. The ROM chips also contain either a subset or the complete router IOS (for example, the ROM on the 2505 router contains only a subset of the IOS, whereas the 7000 series contains the full IOS). The fact that the IOS is available on the ROM enables you to recover from major disasters, such as the wiping out of your Flash RAM (discussed in the following paragraphs). The ROM chips on Cisco routers are removable and can be upgraded or replaced.
NVRAM (nonvolatile RAM)—Stores the startup configuration file for the router. NVRAM can be erased, and you can copy the running configuration on the router to NVRAM. The great thing about NVRAM is that it retains the information it holds even if the router is powered down (which is extremely useful considering you won't want to have to reconfigure the router every time the power goes down).
Flash RAM—Flash is a special kind of ROM that you can actually erase and reprogram. It is used to store the Cisco IOS that runs on your router. You also can store alternative versions of the Cisco IOS on the Flash.
RAM—Similar to the dynamic memory you use on your PC, RAM provides the temporary storage of information and holds information such as the current routing table. RAM also holds the currently running router configuration (changes that you make to the configuration are kept in RAM until you save them to NVRAM).