Chapter 11: Using Intelligent Switches

Overview

The expansion of local area networks (LANs), both in terms of the number of network users supported as well as in the number and type of applications they use, typically results in a considerable increase in the use of network bandwidth. This, in turn , can result in the occurrence of a variety of problems. Those problems can range from sluggish network performance being experienced by workstation users that marginally affects their productivity to the occurrence of session timeouts that terminate network-related work in progress, forcing users to redo previously performed operations or precluding users from performing certain types of network related activity.

Until the early 1990s the primary method employed to overcome the effect of network congestion was segmentation, subdividing a network into two or more entities interconnected by a bridge or router. Today, the network manager and LAN administrator have several options, to include the use of a higher operating rate network such as Fast Ethernet, Gigabit Ethernet, or ATM, or the use of intelligent switches. This chapter focuses attention on the latter, which are also referred to as switching hubs.

To obtain an appreciation for the role of intelligent switches, we first review the operation of conventional hubs, to include the bandwidth constraints associated with their use. Once this is accomplished, we focus our attention on the different operational methods supported by intelligent layer 2 switching hubs, to include the basic switching methods supported by different products that fall into this class of networking device. Using this information as a foundation we then explore the use of both Ethernet and Token Ring intelligent switching hubs, to include obtaining an understanding of the key features built into many products as well as why the presence of some features and the absence of other features can result in degraded performance instead of an expected improvement in performance. Due to this, we examine how the use of certain intelligent layer 2 switch features can result in network problems and how those problems can be alleviated through the use of other device features. Because we will obtain a detailed understanding of the operational effect of a comprehensive set of intelligent switching hub features, the information presented in this chapter can also be used as a guide for the evaluation of switching hubs.

With technological developments extending intelligent switching operations to higher layers in the OSI Reference Model, it is important to note the capabilities of these "upper layer" switches. Although their use does not directly affect performance on individual LANs, their use can significantly affect the flow of traffic between LANs. Due to this, we conclude this chapter by focusing our attention on layer 3 and layer 4 switches.