your network. I can show you some of the more common corporate and e-commerce network designs out there, and explain why I've chosen to focus on these designs. Corporate Networks Most networks are designed along the lines of the classic three-tier model. The three- tier model has core, distribution, and access levels. These levels are clearly delineated and served by different devices. Traditionally, routing was the slowest, most expen- sive process. For these reasons, routing was done in the core. All the other levels were usually switched. With the advent of inexpensive layer-3 switching, the three-tiered model is now often collapsed for corporate networks. We'll look at the traditional model as it might be used today, as well as a couple of collapsed-core models. Three-tiered architecture The three-tiered architecture that is most commonly seen in textbooks is still widely used in the industry. Physical separation of the three levels usually occurs when there is a physical need to do SO. An excellent example would be a college or business cam- pus: there might be core switches (possibly in a central location), distribution switches in each building, and access switches close to the users in each building. Network Designs The specifics would depend on the physical layout of the campus. Figure 33-9 shows a textbook three-tiered corporate network. Internet Server farm 00000 Users, printers, etc. Figure 33-9. Typical three-tiered corporate network At the bottom of the drawing, users, printers, IP phones, and wireless access points (APs) are connected to the access-layer switches. The access-layer switches are connected to the distribution-layer switches in the middle row. The distribution switches connect to the top two switches, which are the core of the network. The Internet and the server farm are both connected to the core in this example. Collapsed core-no distribution Collapsed-core networks are very common. I work a lot in Manhattan, where skyscrapers are the norm. Office space in skyscrapers is usually all