11.3 Background

11.3 Background

Having made technology selections for each area of the network design, we can now focus on mechanisms that connect these technologies to each other within the network and optimize the overall design to achieve our design goals. These mechanisms, termed here interconnection mechanisms, bring the design together.

At this point in the design process, we have a number of technologies that have been chosen for the network. There are also network devices, such as routers, switches, multiplexers, servers, and hubs, where these technologies are applied. What we do not know yet is how these network devices and the technologies applied at each of them are connected together. Figure 11.1 shows multiple technologies and the network devices that implement them. These devices and technologies are not connected.

Figure 11.1: Technology selections and network devices need to be interconnected to complete design.

In understanding, choosing, and applying interconnection mechanisms, we determine how these technologies and network devices are connected (Figure 11.2). Although this seems obvious and (possibly) quite simple, it is actually an important and subtle component of the network design. The selection of interconnection mechanisms affects all architectural components (addressing/routing, network management, performance, and security), as well as the traffic flows determined in the flow analysis process.

Figure 11.2: Technology selections and network devices when connected.

Figure 11.2 represents the interconnection of network devices and technologies. Each arrow is an instance of an interconnection mechanism. Arrows between Technologies A, B, and C represent the interconnection of these technologies, although they would actually be interconnected through their network devices. For example, Technologies A, B, and C would be interconnected via the switches and routers that have their technologies in common.

Interconnection mechanisms include using a shared medium, where all hosts are on the same network or subnetwork and there is no interconnection to other network technologies, various types of bridging (e.g., Ethernet/ATM/frame/IP/wavelength), and routing. Interconnection mechanisms are applied predominantly at the link or network layer but can be found at any layer in the protocol model.

Some interconnection mechanisms are hybrids of switching and routing. They attempt to combine both mechanisms to optimize performance at both the link and the network layer. Part of optimizing performance includes being adaptable to existing technologies and allowing for the creation of virtual networks (e.g., VLANs, virtual private networks [VPNs]). Hybrid mechanisms include LAN emulation (LANE), multiprotocol over ATM (MPOA), the Next-Hop Resolution Protocol (NHRP), multiprotocol label switching (MPLS), and switching based on flow or end-to-end information.

Despite what some vendors may tell their customers, there is no all-encompassing, "right" mechanism. Each mechanism described earlier has its advantages and disadvantages, and each can be successfully applied to network designs. Your job, as a network designer, is to determine which is best for your design based on what you have already done, the analysis, architecture, and technology selection for your network.

Understanding interconnection mechanisms can be somewhat tricky as they evolve and incorporate each other's features. Routing and switching are being integrated through mechanisms such as MPLS to optimize performance, at the trade-offs of complexity and cost, for some networks. The trend in interconnection mechanisms is toward convergence at the link and network layers. The separation of functions and features at each layer is being lost, resulting in a lot of options, and making solutions harder to understand by everyone. Instead of having several network devices that perform a specific function (e.g., routing), network devices now integrate several functions and features into one box.

Figure 11.3 provides the open-systems interconnect (OSI) seven-layer model to reference where the data-link and network layers are located relative to the other layers.

Application

Presentation

Session

Transport

Network

Data Link

Physical

Figure 11.3: OSI seven-layer model.

This integration of link-and network-layer functions is not necessarily good or bad, just different from what we are used to working with. As long as we can maintain some clue about what each mechanism does and what the trade-offs are between them, we can make them useful in the network design. A goal of this chapter is to examine these mechanisms from several perspectives so that you can understand their functions, features, and trade-offs.

As you proceed through this chapter, you will see that there are many reasons for planning routing, switching, bridging, and hybrid mechanisms in a network design. There are also designs in which a shared medium with no interconnections is warranted. We will discuss several factors in evaluating interconnection mechanisms, including supporting scalability of the network, optimizing flow models, providing external connectivity to the network, and applying support service to the design.

We begin with the simplest interconnection mechanism, consisting of no interconnection at all, a shared-medium mechanism.