Router Overview - Router Hardware and Memory

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At first glance, routers seem a lot like a PC. They have a CPU, memory, and, on the back, ports and interfaces to hook up peripherals and various communications media. They sometimes even have a monitor to serve as a system console.

But there’s one defining difference from a PC: routers are diskless. They don’t even have floppy disks. If you think about it, this makes sense. A router exists to do just that: route. They don’t exist to create or display information or to store it, even temporarily. Routers have as their sole mission the task of filtering incoming packets and routing them outbound to their proper destinations.

Another difference is in the kind of add-on modules that can be plugged into routers. Whereas the typical PC contains cards for video, sound, graphics, or other purposes, the modules put into routers are strictly for networking (for obvious reasons). These are called interfacemodules, or just plain interfaces. When people or documents refer to a router interface, they mean an actual, physical printed circuit board that handles a particular networking protocol. EO and E1, for example, probably mean Ethernet interface numbers 1 and 2 inside a router. Interface modules are always layer-2 protocol specific. There is one protocol per interface.

Interfaces are added according to the network environment in which they will work. For example, a router might be configured with interface modules only for Ethernet. A router serving in a mixed-LAN environment, by contrast, would have interfaces for both the Ethernet and Token Ring protocols, and if that router were acting as a LAN-to-WAN juncture, it might also have an ISDN module.

There is one last difference between routers and general-purpose computers—a more subtle one. Computer product lines are almost always based on a common central processor (CPU) architecture, for example, Wintel PCs on the venerable Intel x86 architecture, Apple’s Motorola 68000 variants, Sun’s SPARC, and so on.

In contrast, Cisco routers use a variety of CPUs, each chosen to fit a particular mis- sion. Cisco SOHO 70 Series routers, for example, employ 50 MHz CPUs. Cisco probably made this selection because the 70 Series is designed for small office or home office use, where activity loads are light. The Motorola MPC 855T RISC chip is reliable; capable of handling the job; and, perhaps most important, inexpensive. Moving up the router product line, Cisco uses progressively more powerful general-purpose processors from Motorola, Silicon Graphics, and other chip makers.

Router Memory

Routers use various kinds of memory to operate and manage themselves. Figure 4-9 depicts the layout of a motherboard in a Cisco 4500 router (a good example because it’s one of the most widely used routers in the world today). All Cisco router motherboards use four types of memory, each dedicated to performing specific roles.

Each Cisco router ships with at least a factory default minimum amount of DRAM and flash memory. Memory can be added at the factory or upgraded in the field. As a general rule, the amount of DRAM can be doubled or quadrupled (depending on the spe- cific model), and the amount of flash can be doubled. If traffic loads increase over time, DRAM can be upgraded to increase a router’s throughput capacity.

RAM/DRAM

RAM/DRAM stands for random access memory/dynamic random access memory. Also called working storage, RAM/DRAM is used by the router’s central processor to do its work, much like the memory in your PC. When a router is in operation, its RAM/DRAM contains an image of the Cisco IOS software, the running configuration file, the routing table, other tables (built by the router after it starts up), and the packet buffer.

Don’t be thrown by the two parts in RAM/DRAM. The acronym is a catch-all. Virtually all RAM/DRAM in Cisco routers is DRAM—dynamic random access memory. Nondynamic memory, also called static memory, became obsolete years ago. But the term RAM is still so widely used that it’s included in the literature to avoid confusion on the subject.

Cisco’s smallest router, the 70 Series, ships with a minimum of 16MB of DRAM. At the other end of the spectrum, the 12816-gigabit switch router, one of Cisco’s largest, supports up to 4GB.

NOTE: Shared memory (also called packet memory) is a specialized type ofDRAM. Shared memory DRAM is dedicated to handling the router’s packet buffer. Cisco’s designers separate out shared memory to help assure I/O throughput. Shared memory is even physically nearer to the interface modules to further boost performance.

NVRAM

NVRAM stands for nonvolatile RAM. Nonvolatile means memory that will retain information after losing power. Cisco routers store a copy of the router’s configuration file in NVRAM (configuration files are covered later in this chapter). When the router is intentionally turned off, or if power is lost, NVRAM enables the router to restart in its proper configuration.

Flash Memory

Flash memory is also nonvolatile. It differs from NVRAM in that it can be erased and reprogrammed as needed. Originally developed by Intel, flash memory is in wide use in computers and other devices. In Cisco routers, flash memory is used to store one or more copies of the IOS software. This is an important feature because it enables network managers to stage new versions of IOS on routers throughout an internetwork and then upgrade them all at once to a new version from flash memory.

ROM

ROM stands for read-only memory. It, too, is nonvolatile. Cisco routers use ROM to hold a so-called bootstrapprogram, which is a file that can be used to boot to a minimum configuration state after a catastrophe. ROM is also referred to as ROMMON. In fact, when you boot from ROM, the first thing you’ll see is the rommon>> prompt. ROMMON (for ROM monitor) harks back to the early days of the UNIX operating system, which relied on ROMMON to reboot a computer to the point at which commands could at least be typed into the system console monitor. In smaller Cisco routers, ROM holds a bare-bones subset of the Cisco IOS software. ROM in some high-end Cisco routers holds a full copy of IOS.

This chapter is from Cisco: A Beginner's Guide, by Velte and Velte (McGraw-Hill/Osborne, 2004, ISBN: 0072256354). Check it out at your favorite bookstore today.  Buy this book now.

Next: Router Ports and Modules >>
 

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