## Chapter 1. The Key Components for Modeling an Internetwork

Many types of models exist today, from mathematical models and statistical models to the plastic models that you might have built when you were young. Despite their vast differences, they all are models of one type. This book proposes a new kind of model—the internetwork model.

The internetwork model will be defined as a smaller accurate and functional representation of a larger internetwork. Internetwork models, as with all models, are smaller representations of larger networks. The word accurate is used because the model will be built to reflect the precise requirements of "real" networks. For example, not only will you design an Open Shortest Path First (OSPF) network, but you also will focus on the details of the design, such as OSPF handling of specific interface types, what areas they are in, and whether they should be transmitting link states or forming adjacencies. Attention to this type of detail will be important in your studies and thereby warrants the use of the word accurate. Finally, the model must be functional—that is, it will be tested by running actual data and applications across it.

Just as other models serve a purpose to prove a hypothesis of one sort, the goal of the internetwork model will be to prove the functionality or design theory of the larger internetwork. When you are finished, you will be able to transport many kinds of data across many types of networks. You will see and test the functionality of the network by using test hosts and simulated data.

In summary, an internetwork model is a smaller but accurate representation of the larger internetwork. The goal of this book is to walk the network engineer through assembling and configuring all the necessary hardware and software components required to model complex internetworks.

The term internetwork can be defined as a collection of networks, local-area networks (LANs) and wide-area networks (WANs) interconnected by routers, bridges, and switches that function as a singular network. To properly model an internetwork in the lab, you need to simulate different LAN and WAN technologies.

You should go through a logical order when modeling the internetwork. Like mathematics, networking builds on itself. You must first comprehend multiplication and division before you can learn algebra, you must learn algebra before calculus, and so on. Networking follows a similar logical approach:

1. First, build and form all LAN connections.
2. After initial LAN connectivity is established, build and configure all WAN connections.
3. Establish full internetwork connectivity. This is done by laying routing protocols over yosur LANs and WANs.
4. Finally, apply any filters, features, or any other exterior routing protocols, such as Border Gateway Protocol (BGP), that require a fully functional IP network to operate.

## Identifying the Key Components Needed for Modeling Internetworks

You need several key components to fully model complex network designs, including the following:

• Access server
• LANs: switches/hubs and cables
• WANs: routers and cables
• Routers
• Test hosts and applications, preferably Microsoft Windows 95/98/2000 or Windows NT

The preceding list should be viewed more as a list of roles than a list of devices. The specific device type is not relevant; what is relevant is the role that the device plays in the model. There are many ways to simulate a WAN. For example, some network models are more accurate if a Frame Relay switch is deployed in the model, while some models might require only a WAN connection because the protocol needed is irrelevant.

The only component in this list that could be considered optional is the access server. The role of the access server can be a useful one, both in the laboratory environment and in the field. In the field, the access server provides out-of-band management where groups of routers are located. Instead of using multiple modems for dial-in access to routers, an access server can serve as a central point for out-of-band management, requiring only one modem instead of many. The primary role of the access server in the lab will be to provide simple and quick configuration access to a stack of routers. We will go over each one of these components in detail and further discuss their relevance in the lab.