Making the Business Case

Switches can enhance the performance, flexibility, and functionality of your network.

The first networks were LANs; they enabled multiple users in a relatively small geographical area to exchange files and messages, and to access shared resources such as printers and disk storage. A huban Open Systems Interconnection (OSI) Layer 1 deviceinterconnected PCs, servers, and so forth as the number of devices on the network grew. However, because all devices connected to a hub are in the same bandwidth (or collision) domainthey all share the same bandwidthusing hubs in anything but a small network is not efficient.

To improve performance, LANs can be divided into multiple smaller LANs, interconnected by a Layer 2 LAN switch. Because each port of the switch is its own collision domain, multiple simultaneous conversations between devices connected through the switch can occur.

By default, all ports of a switch are in the same broadcast domain. Recall (from Appendix B) that a broadcast domain includes all devices that receive each other's broadcasts (and multicasts). A broadcast is data meant for all devices; it uses a special broadcast address to indicate this. A multicast is data destined for a specific group; again, a special address indicates this. Note that Layer 3 broadcast packets are typically encapsulated in Layer 2 broadcast frames, and Layer 3 multicast packets are typically encapsulated in Layer 2 multicast frames (assuming that the packets are going over a data-link technology that supports these types of frames, such as Ethernet).

The implications of this for modern networks are significanta large switched OSI Layer 2 network is one broadcast domain, so any broadcasts or multicasts traverse the entire network. Examples of broadcast traffic include Internet Protocol (IP) Address Resolution Protocol (ARP) packets, and routing protocol traffic such as Routing Information Protocol (RIP) version 1 (RIPv1). Multicast traffic includes packets from more advanced routing protocols such as Open Shortest Path First (OSPF) and applications such as e-learning and videoconferencing. As network use increases, the amount of trafficincluding multicast and broadcast trafficwill also increase.

Today's switches support VLANs so that physically remote devices can appear to be on the same (virtual) LAN. Each VLAN is its own broadcast domain. Traffic within a VLAN can be handled by Layer 2 switches. However, traffic between VLANS, just like traffic between LANs, must be handled by an OSI Layer 3 device. Traditionally, routers have been the Layer 3 device of choice. Today, Layer 3 switches offer the same functionality as routers but at higher speeds and with additional functionality.

The rest of this chapter explains how switchesLayer 2 and Layer 3and the protocols associated with them work, and how they can be incorporated into network designs.