Figure 123 Global WAN

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In this case, the core routers have some redundancy to each other and to their distribution layer peers. Redundancy could be added from the access layer devices to the distribution layer by adding more routers or even multiple access layer routers for device redundancy in each location.

From the WAN's perspective, each site is at the access layer. From each site's perspective, which might each have a design similar to Figure 12-1, the WAN is at the access layer.

Core, distribution, and access models really become beneficial when used in large networks. If you look at the previous few figures and think, "My network doesn't look anything like this," don't worry. Lots of networks collapse two or more layers because they have no need to keep them separate. What drives the core, distribution, and access model of network design is primarily scalability. This design scales up to the largest networks in the world. It also scales to the smallest by integrating the layers into a smaller number of devices. In addition, in some cases you can use the three-layer core, distribution, and access design for some parts of your network (for your users, for example), but components such as your server farms connect directly to the distribution layer as opposed to going through an access layer. In fact, this is the case in Figure 12-1. Because the number of server farms is small, there is no need to build a separate access layer.

In smaller networks, the most common integration point is the distribution and core layers. Figure 124 shows a midsize network design with a single L3 switch acting as the distribution and core layer for the campus design. These designs are called collapsed designs because you are collapsing the functionality of more than one layer.

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