Router Components
Before examining the IOS, a review of hardware and hardware terminology is useful. In addition to handling the logic of routing packets, the IOS controls the use of different physical components, which includes memory, processor, and interfaces. This section of the book reviews common hardware details.
All Cisco routers have a console port, and most have an auxiliary port. The console port is intended for local administrative access from an ASCII terminal or a computer using a terminal emulator. The auxiliary port, missing on a few models of Cisco routers, is intended for asynchronous dial access from an ASCII terminal or terminal emulator; the auxiliary port is often used for dial backup.
Each router has different types of memory, as follows:
• RAM—Sometimes called DRAM for dynamic random-access memory, RAM is used by the router just as it is used by any other computer: for working storage.
• ROM—This type of memory (read-only memory) stores a bootable IOS image, which is not typically used for normal operation. ROM contains the code that is used to boot the router until the router knows where to get the full IOS image.
• Flash memory—Either an EEPROM or a PCMCIA card, Flash memory stores fully functional IOS images and is the default where the router gets its IOS at boot time. Flash memory also can be used to store configuration files on Cisco 7500 series platforms.
• NVRAM—Nonvolatile RAM stores the initial or startup configuration file.
All these types of memory are permanent memory except RAM. No hard disk or diskette storage exists on Cisco routers. Figure 2-2 summarizes the use of memory in Cisco routers.
|
RAM (Working Memory) |
Flash (IOS) |
ROM (IOS) |
NVRAM (Config) |
The processors in the routers vary from model to model. Although they are not specifically listed as requirements for the CCNA exam, some reference to terminology is useful. In most routers, only one processor option is available; thus, you would not order a specific processor type or card. The exception to this is the 7200 and 7500 families of routers. For instance, on the 7500 series, you choose either a Route Switch Processor 1 (RSP-1), RSP-2, or RSP-4 processor. In any case, all 7200 and 7500 routers, as well as most of the other Cisco router families, run IOS. This commonality enables Cisco to formulate exams, such as CCNA, that cover the IOS features without having to cover many hardware details.
Interfaces are used by a router for routing packets and bridging frames through a router. The types of interfaces available change over time due to new technology. For example, packet-over-SONET and voice interfaces are relatively recent additions to the product line. However, some confusion exists about what to call the actual cards that house the physical interfaces. Table 2-2 summarizes the terminology that might be referred to on the test.
Table 2-2 Samples of Router Interface Terminology
|
Model Series |
What the IOS Calls Interfaces |
What the Product Catalog Calls the Cards with the Interfaces on Them |
|
2500 |
Interface |
Modules and WAN interface cards |
|
3600 |
Interface |
Network modules and WAN interface cards |
|
4500 |
Interface |
Network processor modules |
|
7200 |
Interface |
Port adapters and service adapters |
|
7500 |
Interface |
Interface processors, and versatile interface processors with port adapters |
Physical interfaces are referred to as interfaces by the IOS commands, as opposed to ports or plugs. IOS commands familiar on one platform will be familiar on another. Some nuances are involved in numbering the interfaces, however. In some smaller routers, the interface number is a single number. However, with some other families of routers, the interface is numbered first with the slot in which the card resides, followed by a slash and then the port number on that card. For example, port 3 on the card in slot 2 would be interface 2/3. Numbering starts with 0 for card slots and 0 for ports on any card. In some cases, the interface is defined by three numbers: first the card slot, then the daughter card (typically called a port adapter), and then a number for the physical interface on the port adapter. The 2600 and 3600 families also use a slot/port numbering scheme.
In this book, the single-digit interface numbers are used simply for consistency and readability.
If you want to dig deeper, you might want to read about processors and interfaces in the Cisco Product Catalog (http://www.cisco.com/univercd/cc/td/doc/pcat/).
Continue reading here: Command Line Interface
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