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CISCO INTERNETWORK DESIGN EXAM OBJECTIVES COVERED IN THIS CHAPTER:
The first chapter of this book focused on the many nontechnical facets of network design. This chapter will depart from the nontechnical components and begin to develop the technical components.
The technical components of networking include many different elements. All of these elements require consideration by the network designer in virtually every design. Decisions made in one area can quickly force compromises in another area that may not be fully anticipated. While a full explanation of some of the common issues is beyond the scope of this text (and the exam), the text will take some steps to identify and address these issues.
The network design technologies include the components of the first three OSI model layers. Repeaters, hubs, switches, and routers all work in different ways to integrate within the infrastructure. Designers must understand the differences between these devices and their functions. They must also consider newer technologies and more complex systems. These may include ATM, ATM LANE, FastEtherChannel (FEC), GigEtherChannel (GEC), and VLAN (virtual LAN) trunking. Some vendors are beginning to deploy Layer 5 switching technologya development that may alter design models in future years.
As defined in Chapter 1, most networks are deployed to meet the needs of the business. Businesses that invest in campus LAN projects typically benefit from the collaborative advantages that result from these expenditures. Reduced costs also promote the deployment of a LANimagine if companies bought stand-alone printers for each desktop, for example. The net result would be substantial added expense and a single tactical solution that could not resolve subsequent issues.
At times it seems as if the technology that drives networks is constantly changing. However, it might be simpler to think of the process as more evolutionary. For example, switches are simply an extension from bridges and other technologies to their predecessors.
The importance of understanding the customer needs was presented in Chapter 1, along with a number of high-level criteria for integrating the business needs with the network design. The designer will need to take these criteria and apply technology appropriate to both the current requirements and to a logical growth path that works to preserve the investment.
In the first presentations of network design using switches, vendors advocated the transport of VLANs across the backbone. Recall that VLANs, or virtual LANs, are logical groupings of the broadcast domain. The logic was that workgroups could be physically isolated while retaining the benefits of operating at Layer 2. This design was primarily based on the fact that routers, or any Layer 3 processing, would be slower than switching packets from end to end. Given the evolution of the technologies, vendors now advocate the sole use of Layer 3 processing in the core.
Before dismissing the use of Layer 2 in the core, consider both the positives and negatives of such use. Layer 2 provides a secure environment wherein all traffic is local. Connections between nodes require neither processing by a router nor the conversions that are performed in routing. The number of router interfaces can be lower and the configuration of the network is simplified. All of these benefits gave administrators cause to pursue the design model in the mid-1990s.
However, as the technology advanced and Layer 3 processing moved closer to wire speeds, it became less advantageous from a performance perspective to avoid routers. The benefits of broadcast control and geographic isolation became more attractive to designers, and while it could still cost more to create additional VLANs, integration of Layer 3 into the switching fabric eroded this disadvantage as well.
|Within the context of the current exam, switches are purely Layer 2 devices, and the integration of routing and other technologies is out of scope unless explicitly referenced.|
Designers should also consider business needs when evaluating technologies and the subsequent changes in direction that occur. While vendors profit substantially from the purchase of new equipment, the business may not share in the benefits from the upgrade. The corporation is interested in reliable economic growth, and the network is typically the mechanism by which business is performedit rarely is the business itself. Consider this in a different perspective. Corporation X makes hockey sticks. It doesnt matter whether the network is using EIGRP on FastEthernet with HSRP (Hot Standby Router Protocol). It does matter whether the network operates during the two shifts that manufacture the product and during the end-of-month financial reports. Upgrading to ATM may sound desirable, but if the network is stable on Ethernet and isnt growing, upgrading is unlikely to garner a return on investment.
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