Roles of the Router in WANs

More often than not, internetworks are quite extensive in terms of the number of routers, transmission facilities, and attached end systems. In an extensive internetwork, such as the Internet or even large private networks, it would be virtually impossible for any given machine to know about every other machine. Therefore, some semblance of hierarchy is needed. Hierarchical organization of internetworked machines creates the need for specialized routing functions.

Routers can specialize in learning about and distributing routing information about end systems within their domain. These routers are called interior gateways. Alternatively, routers can specialize in collecting routing information about machines that lie beyond their domain. These routers are known as exterior gateways.

Networking is often used as a generic, or universal, term. However, networked machines communicate in tremendously different ways. Routers can function in different capacities in an internetwork, for example, as interior, exterior, or border routers.

Note It is not uncommon to find interior routers, exterior routers, and border routers described as interior gateways, exterior gateways, and border gateways, respectively. The term gateway is as old as routing itself. Over time, this term has lost some of its descriptive value. Consequently, both sets of terms are technically correct, except in the presence of technological purists. Then you will have to determine which terminology they consider correct!

These functional specializations are more than merely academic. In fact, many routing protocols were specifically designed for use in one of these three capacities. Part III, "'Routing Protocols," describes some of the most common of the specialized routing protocols. Understanding the differences among them requires examining them in the context of a WAN. Therefore, a logical starting point is an examination of the context! The terms WAN, network, internetwork, and autonomous system are all used interchangeably, yet each has a slightly different meaning:

• WAN---A WAN is a collection of related LANs linked together via routers and serial transmission facilities such as leased lines or Frame-Relay circuits. Implicit in this definition is that the LANs in the WAN may be geographically dispersed, but they still fall under the auspices of a single organization such as a company, school, and so on.

• Network—Network is a more nebulous term that defies specificity. Everything from LANs to WANs can be classified as a network. Consequently, for the purposes of this book, a network identifies a generic collection of related networking mechanisms. Therefore, a network may be a LAN or a WAN, but it must belong to a single organization and feature a consistent addressing architecture.

• Internetwork—Internetwork is only slightly more concrete than network. An internetwork is a collection of loosely related networks that are interconnected. The interconnected networks can belong to different organizations. For example, two companies can use the Internet to interconnect their private WANs. The resulting internetwork consists of one public network and two private

Routers and WANs networks linked together.

• Autonomous system---An autonomous system (AS) is a network (either LAN or WAN) that is relatively self contained. It is administered by a single person (or group of persons), features a single routed protocol, address architecture, and usually just one routing protocol. An autonomous system may support connections to other auto- nomous systems owned and operated by the same organization. Alternatively, an AS may have connections to other networks, such as the Internet, yet it retains autonomy of operation. This term is usually used in conjunction with specific routing protocols, such as OSPF, that enable a network to be carved into numbered subsections.

Given these definitions, it is possible to better define the functional classes of routers. An interior router is one that can be used by end systems in a network to access other end systems within the same network.

The interior router supports no connections to any other network.

Figure 4-2 illustrates a small network and identifies those devices that function as interior routers.

Figure 4-2: Interior routers in a network.

Figure 4-2: Interior routers in a network.

An exterior router is one that lies beyond the boundaries of any given network. In Figure 4-1, the Internet is depicted as a featureless cloud. In reality, this cloud has a specific topology. Figure 4-3, although not pretending to depict the Internet's actual topology, presents a highly simplified Internet topology that is solely intended to demonstrate what an exterior router is.

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