Most Cisco networking hardware, with the exception of the Cisco PIX firewall series, ships with the Cisco Inter-network Operating System, or IOS. IOS hardware includes network routers, switches, and other similar devices. The basic design of most of this hardware controls the types of memory a device is equipped with, and what functions the various types of Cisco memory serve. At the hardware level, there are four main types of Cisco memory: DRAM, EPROM, NVRAM, and Cisco Flash Memory.
DRAM, or Dynamic Random Access Memory caters to two main device requirements. The first of these is known as Processor Memory which is reserved for exclusive access by the CPU, which it uses when executing software running on the Cisco IOS platform. Processor Memory also stores crucial data that is used constantly, like the configuration settings in current use, and any routing tables. The second is Shared Memory, also known as I/O Memory, or Cisco Packet Memory. The function of Packet Memory is simple and self-explanatory: Data coming in and being sent out is buffered to the Packet Memory portion of the available DRAM before it is transmitted over the network interface.
EPROM, or Erasable Programmable Read-Only Memory is usually referred to as a BootROM. EPROM is generally programmed at some point during the latter stages of manufacture, and cannot generally be changed by consumers. In Cisco devices, EPROM is generally loaded with two crucial firmware components. The first is a boot loader which takes over should the device fail to find a valid bootable image in Flash Memory, and provides alternate boot options. If even this failsafe should fail, the second firmware application installed on Cisco EPROM is used, the ROM Monitor. ROM Monitor has a user interface and includes options for troubleshooting failures of the ROM chips.
In Cisco devices, NVRAM, or Non-Volatile Random Access Memory, stores important configuration information that is used by IOS during boot and by some programs during startup, which is stored in the Startup Configuration File. NVRAM also allows the functionality provided by the Cisco Software Configuration Register, which allows a device to be booted and selection from multiple Cisco IOS images that may be available in Flash Memory. It is sometimes called Shared Memory.
Cisco Flash Memory is the most diverse of each of these types, and it comes in many forms, however, its primary use is to store a bootable Cisco IOS image from which a device can start. Most devices have onboard Flash memory from which the device boots, however, some equipment - particularly higher-end hardware like Cisco GSR routers - also have the capability to boot from an image stored on a Cisco Flash Memory card, which is removable. Regardless of the memory type, it is important to find a memory reseller who is reliable. Things to look for when choosing a memory reseller are that they have memory for every Cisco device in stock, offer a warranty on the memory, and have technical people with whom you can speak if you have any problems.