During the early 1980s, many organizations started to build their operations around local area networks, with a few point-to-point wide area links to facilitate LAN interconnection. Wide area bridges were often used to provide packet forwarding, accomplished using either source routing or the Spanning-Tree protocol [1]. With the rise of the Internet as a business resource; the increased use of remote access, the World Wide Web, intranets and extranets; and new multimedia applications, the demands placed on backbones are now huge. In this chapter we investigate a smarter method of optimizing packet forwarding over expensive WAN links, using a device called the router. Routers are the glue that binds together internetworks into a single entity, albeit often a heterogeneous entity. Routers from many different vendors cooperate using sophisticated distributed algorithms in order to create a holistic view of the network in the form of routing tables. Compared to bridges, routing is much more sensitive to topology, bandwidth, performance, and availability, and therefore provides superior traffic engineering. Routers may also have to deal with heterogeneous media, providing scheduling support for differential services and possibly filtering or basic firewalling features. All of these features mean that a router-based network design is more robust and can be far better optimized than a flat network design.

In this chapter we are primarily concerned with the logical topology of the network rather than the physical topology. Layer 3 devices concern themselves only with logical interfaces, which are abstracted from the Physical Layer through path...