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RAMRAM stores the instructions and data needed to be executed by the CPU. RAM is used to store these components:Operating System: The Cisco IOS (Internetwork Operating System) is copied into RAM during bootup.Running Configuration File: This is the configuration file that stores the configuration commands thatthe router IOS is currently using. With few exceptions, all commands configured on the router are stored in the running configuration file, known as running-config.IP Routing Table: This file stores information about directly connected and remote networks. It is used to determine the best path to forward the packet.ARP Cache: This cache contains the IPv4 address to MAC address mappings, similar to the ARP cache on a PC. The ARP cache is used on routers that have LAN interfaces such as Ethernet interfaces.Packet Buffer: Packets are temporarily stored in a buffer when received on an interface or before theyexit an interface.
RAM is volatile memory and loses its content when the router is powered down or restarted. However, the router also contains permanent storage areas, such as ROM, flash and NVRAM. ROMROM is a form of permanent storage. Cisco devices use ROM to store:The bootstrap instructionsBasic diagnostic softwareScaled-down version of IOSROM uses firmware, which is software that is embedded inside the integrated circuit. Firmware includes the software that does not normally need to be modified or upgraded, such as the bootup instructions. Many of these features, including ROM monitor software, will be discussed in a later course. ROM does not lose its contents when the router loses power or is restarted.Flash MemoryFlash memory is nonvolatile computer memory that can be electrically stored and erased. Flash is used as permanent storage for the operating system, Cisco IOS. In most models of Cisco routers, the IOS is permanently stored in flash memory and copied into RAM during the bootup process, where it is then executed by the CPU. Some older models of Cisco routers run the IOS directly from flash. Flash consists of SIMMs or PCMCIA cards, which can be upgraded to increase the amount of flash memory.Flash memory does not lose its contents when the router loses power or is restarted.NVRAMNVRAM (Nonvolatile RAM) does not lose its information when power is turned off. This is in contrast to the most common forms of RAM, such as DRAM, that requires continual power to maintain its information. NVRAM is used by the Cisco IOS as permanent storage for the startup configuration file (startup-config). All configuration changes are stored in the running-config file in RAM, and with few exceptions, are implemented immediately by the IOS. To save those changes in case the router is restarted or loses power, the running-config must be copied to NVRAM, where it is stored as the startup-config file. NVRAM retains its contents even when the router reloads or is powered off.