When designing the networks of yesteryear, network engineers had only a limited number of hardware options when purchasing technology for their campus networks. In most cases, hubs were used to connect all network hosts, such as user workstations and network printers, to a single, shared LAN, while routers were used to segment the network as well as to provide connectivity between the LANs.
However, the increasing power of desktop machines and the increased need for more bandwidth has quickly highlighted the fact that shared media or a shared network model of LAN design has both distance limitations and limitations on the number of devices that can be connected to a single LAN. Thus, these networks are incapable of supporting these technological advances adequately.
To address these issues, LAN switches were developed in 1990. These were Layer 2 devices, referred to as bridges, and they were dedicated primarily to solving desktop bandwidth issues. One of the advantages offered by bridges was the ability to segment the LAN. Segmentation is the process by which the LAN is broken down into smaller, more manageable pieces. These segments are then interconnected by internetworking devices that enable communication between LANs while blocking other types of traffic. Segmenting LANs divides users into two or more separate LAN segments, reducing the number of users contending for bandwidth.
The rule of thumb when designing bridged networks was the 80/20 rule. This rule stipulated that while 80% of network traffic remained on the local network segment, up to 20% of network traffic needed to be bridged across segments or routed across the network backbone. Figure 1-13 below illustrates LAN design based on the 80/20 rule:
In the diagram above, local servers and other network devices, such as printers, are present on each LAN segment. Devices such as workstations serve the clients on those respective LAN segments. Because of localized servers and applications, 80% of network traffic is restricted to the local segment. This means that only up to 20% of network traffic will ever need to be bridged, switched, or routed between network segments.
As technology continued to evolve, along with the increasing power of desktop processors, the requirements of client-server and multimedia applications, and the need for greater bandwidth, it was clear that bridges alone were incapable of addressing such needs. This prompted network engineers to replace bridges with LAN switches.
Switches segment LANs in a manner similar to bridges. However, unlike bridges, switches are hardware-based, making them much faster. Switches also go one step further with LAN segmentation by allowing microsegmentation, which further segments the LAN by allowing individual hosts to be connected to individual switch ports. In other words, each individual host device is connected to its own switch port. Each switch port, therefore, provides a dedicated Ethernet segment.
LAN switches allow dedicated communication between devices using full-duplex operations, multiple simultaneous conversations, and media-rate adaption. Additionally, it is important to remember that Multilayer switches are capable of handling protocol issues involved in high-bandwidth applications that have been solved historically by network routers. In modern day networks, LAN switches, not hubs or bridges, are used in the wiring closet primarily because user applications demand greater bandwidth.
The demand for greater bandwidth has stemmed from the exponential growth of the Internet, as well as faster, more processor-intensive applications, which have fueled the implementation of server farms. A server farm, also called a server cluster, is a group of servers that is kept in a centralized location, such as a data center. These servers are networked together, making it possible for them to meet server needs that are difficult or impossible to handle with just one server. With a server farm, the workload is distributed among multiple server components, providing expedited computing processes.
These two factors, the Internet and server farms, have resulted in modern networks being designed based on the 20/80 rule instead. Based on the 20/80 rule, up to 20% of the network traffic is local to the network segment, while 80% of the network traffic is destined to other network segments or traverses the network backbone. This type of LAN design places a greater burden on the network backbone than that imposed by the 80/20 rule.
In addition to this, network engineers should also understand that Layer 3 forwarding (routing) is slower than Layer 2 forwarding (switching), and so greater consideration must be given to the LAN design to avoid bottlenecks within the backbone. To assist in design, Cisco has created a hierarchical model for internetwork design to allow for designing internetworks in layers.