Typical Campus Design: A Baseline Network
Chapter 14, “Campus Design Models,” takes a detailed look at overall campus design principals. Although there are many options available when laying out a campus network, most are some variation of the network illustrated in Figure 7-1.
Figure 7-1. Typical Design for a Single Building in a Switched Campus Network
Figure 7-1 shows a network that might be contained within several buildings of a large campus environment. The PCs and workstations directly connect on one of the Intermediate Distribution Frame (IDF) switches located on every floor. These IDF switches also link to Main Distribution Frame (MDF) switches that are often located in the building’s basement or ground floor. Because the MDF is carrying traffic for the entire building, it is usually provisioned with a pair of redundant switches for increased bandwidth and reliability. The MDF switches then connect to the rest of the campus backbone where a server farm is generally located.
By looking at this network from the point of view of a single IDF wiring closet, much of the complexity can be eliminated as shown in Figure 7-2.
Figure 7-2. Typical Campus Network Simplified to Show a Single IDF
By reducing the clutter of multiple IDFs and MDFs, Figure 7-2 allows for a much clearer picture of what is happening at the Spanning Tree level and focuses on the fact that most campus designs utilize triangle-shaped STP patterns. (These patterns and their impact on campus design are discussed in detail during Chapters 14, 15, and 17.) Furthermore, because modern traffic patterns tend to be highly centralized, it eliminates the negligible level of IDF-to-IDF traffic present in most networks.
Chapter 14 discusses the differences between two common campus design modules: the campus-wide VLANs model and the multilayer model. In campus-wide VLANs, the network consists of many large and overlapping triangle- and diamond-shaped patterns that can be simplified in the manner shown in Figure 7-2. Because these triangles and diamonds overlap and interconnect, it often leads to significant scaling problems. The multilayer model avoids these issues by using Layer 3 switching (routing) to break the network into many small triangles of Layer 2 connectivity. Because these triangles are isolated by the Layer 3 switching component, the network is invariably more scalable.A detailed discussion of the differences between these two models is beyond the scope of this chapter. Obviously, consult Chapter 14 for more information. However, it is worth noting that this chapter often uses examples from the campus-wide VLANs model because they present more Spanning Tree challenges and opportunities. Nevertheless, you should generally consider using Layer 3 switching for reasons of scalability.
Cat-D in Figure 7-2 is an IDF switch that connects to a pair of MDF switches, Cat-B and Cat-C. The MDF switches connect through the campus backbone to a server farm switch, Cat-A.
Utilize this technique of simplifying the network when trying to understand STP in your network. When doing this, be sure to include paths where unusually heavy traffic loads are common. Having a modular design where a consistent layout is used throughout the network can make this much easier (see Chapters 14 and 15 for more information).