Once the actual requirements are understood, the design work can begin, and it should always start at the top. Earlier in this chapter I described the standard seven-layer OSI protocol model. The top layer in this model is the Application Layer. That is where one has to start when designing a network. The network exists to support applications. The applications exist to fulfill business requirements.
The trick is that the network will almost certainly outlive some of these applications. The organization will implement new applications, and they will likely have new network requirements. They will form new business units, and new departments will replace old ones. A good network design is sufficiently flexible to support these sorts of changes without requiring wholesale redesign. This is why an experienced network designer will generally add certain philosophical requirements to the business requirements that have already been determined.
The network needs to be scalable, manageable, and reliable. Methods for achieving each of these topics will be examined in considerable detail throughout this book. It should be obvious why they are all important, but let me briefly touch on some of the benefits of imposing these as requirements in a network design.
Making a design scalable automatically dismisses design possibilities where switches for different workgroups are either interconnected with a mesh or cascaded one after another in a long string. Scalability will generally lead to hierarchical designs with a Core where all intergroup traffic aggregates.
Manageability implies that you want to see what is going on throughout the network easily. It will also demand simple, rational addressing schemes. Some types of technology are either unmanageable or difficult to manage. You probably wouldn't want to eliminate these outright because they may be cost effective. But you probably don't want to put them in key parts of the network.
Reliability is usually the result of combining a simple, scalable, manageable architecture with the business throughput and traffic-flow requirements. But it also implies that the network designer will study the design carefully to eliminate key single points of failure.
There are other important philosophical principles that may guide a network design. A common one is that, except for specific security exclusions, any user should be able to access any other part of the network. This will help ensure that, when new services are deployed, the network will not need to be redesigned.
Another common design philosophy says that only network devices perform network functions. In other words, never use a server as a bridge or a router. It's often possible to set up a server with multiple interface cards, but this philosophy will steer you away from doing such things. Generally speaking, a server has enough work to do already without having the resources act as some kind of gateway. It will be almost invariably slower and less reliable at these functions than a special-purpose network device.
If your network uses TCP/IP, will you use registered or unregistered IP addresses? This used to be a hotly debated subject, but these days it is becoming clear that there is very little to lose by implementing a network with unregistered addresses, as long as you have some registered addresses available for address-translation purposes.
Perhaps the most important philosophical decisions have to do with what networking standards will be employed. Will they be open standards that will allow easy interoperability among different vendors' equipment? Or will they be proprietary to one vendor, hopefully delivering better performance at a lower price? It is wise to be very careful before implementing any proprietary protocols on your network because it can make it exceedingly difficult to integrate other equipment later. It is always possible that somebody will come along with a new technology that is vastly better than anything currently on the market. If you want to implement this new technology, you may find that the existing proprietary protocols will force a complete redesign of the network.