Why Segment LANs?

Why Segment LANs?

Network designers often face a need to extend the distance of a network, the number of users on the system, or the bandwidth available to users. From a corporate point of view, this is a good thing, because it might indicate growth. From a network administrator's point of view, this is often a bad thing, implying sleepless nights and no weekends. Even so, how does an administrator keep everyone happy while maintaining personal sanity?

A straightforward technology answer might include the deployment of a higher speed network. If users currently attach to a legacy 10 Mbps network, you could deploy a Fast Ethernet network and provide an immediate tenfold improvement in bandwidth. Changing the network infrastructure in this way means replacing workstation adapter cards with ones capable of 100 Mbps. It also means replacing the hubs to which the stations connect. The new hubs must also support the new network bandwidth. Although effective, a wholesale upgrade might be cost prohibitive.

Segmenting LANs is another approach to provide users additional bandwidth without replacing all user equipment. By segmenting LANs, the administrator breaks a network into smaller portions and connects them with some type of internetworking equipment. Figure 2-1 illustrates a before-and-after situation for segmenting networks.

Figure 2-1. A Network Before and After Segmentation

Before segmentation, all 500 users share the network's 10 Mbps bandwidth because the segments interconnect with repeaters. (The next section in this chapter describes how repeaters work and why this is true.) The after network replaces the repeaters with bridges and routers isolating segments and providing more bandwidth for users. Bridges and routers generate bandwidth by creating new collision and broadcast domains as summarized in Table 2-1. (The sections on LAN segmentation with bridges and routers later in this chapter define collision and broadcast domains and describe why this is so.)

Table 2-1. A Comparison of Collision and Broadcast Domain

Device

Collision Domains

Broadcast Domains

Repeater

One

One

Bridge

Many

One

Router

Many

Many

Switch

Many

Configurable

Each segment can further divide with additional bridges, routers, and switches providing even more user bandwidth. By reducing the number of users on each segment, more bandwidth avails itself to users. The extreme case dedicates one user to each segment providing full media bandwidth to each user. This is exactly what switches allow the administrator to build.

The question remains, though, "What should you use to segment the network? Should you use a repeater, bridge, router, or LAN switch?" Repeaters do not really segment a network and do not create more bandwidth. They simply allow you to extend the network distance to some degree. Bridges, routers, and switches are more suitable for LAN segmentation. The sections that follow describe the various options. The repeater is included in the discussion because you might attach a repeater-based network to your segmented network. Therefore, you need to know how repeaters interact with segmentation devices.