Content Overview Local-area network (LAN) design has developed and changed over time. Network designers until very recently used hubs and bridges to build networks. Now switches and routers are the key components in LAN design, and the capabilities and performance of these devices are continually improving. This module returns to some of the roots of modern Ethernet LANs with a discussion of the evolution of Ethernet/802.3, the most commonly deployed LAN architecture. A look at the historical context of LAN development and various networking devices that can be utilized at Layer 1, Layer 2, and Layer 3 of the OSI model will help provide a solid understanding of the reasons why network devices have evolved as they have. Until recently, most Ethernet networks were built using repeaters. When the performance of these networks began to suffer because too many devices shared the same segment, network engineers added bridges to create multiple collision domains. As networks grew in size and complexity, the bridge evolved into the modern switch, allowing microsegmentation of the network. Today’s networks typically are built using switches and routers, often with the routing and switching function in the same device. Many modern switches are capable of performing varied and complex tasks in the network. This module will provide an introduction to network segmentation and will describe the basics of switch operation. Switches and bridges perform much of the heavy work in a LAN, making nearly instantaneous decisions when frames are received. This module describes in detail how frames are transmitted by switches, how frames are filtered, and how switches learn the physical addresses of all network nodes. As an introduction to the use of bridges and switches in LAN design, the principles of LAN segmentation and collision domains are also covered. Switches are Layer 2 devices that are used to increase available bandwidth and reduce network congestion. A switch can segment a LAN into microsegments, which are segments with only a single host. Microsegmentation creates multiple collision-free domains from one larger domain. As a Layer 2 device, the LAN switch increases the number of collision domains, but all hosts connected to the switch are still part of the same broadcast domain. Students completing this module should be able to:
Content 4.1 Introduction to Ethernet/802.3 LANs 4.1.1 Ethernet/802.3 LAN development The earliest LAN technologies commonly used either thick Ethernet or thin Ethernet infrastructures. It is important to understand some of the limitations of these infrastructures in order to see where LAN switching stands today. Adding hubs or concentrators into the network offered an improvement on thick and thin Ethernet technology. A hub is a Layer 1 device and is sometimes referred to as an Ethernet concentrator or a multi-port repeater. Introducing hubs into the network allowed greater access to the network for more users. Active hubs also allowed for the extension of networks to greater distances. A hub does this by regenerating the data signal. A hub does not make any decisions when receiving data signals. It simply regenerates and amplifies the data signals that it receives to all connected devices. Ethernet is fundamentally a shared technology where all users on a given LAN segment compete for the same available bandwidth. This situation is analogous to a number of cars all trying to access a one-lane road at the same time. Because the road has only one lane, only one car can access it at a time. The introduction of hubs into a network resulted in more users competing for the same bandwidth. Collisions are a by-product of Ethernet networks. If two or more devices try to transmit at the same time a collision occurs. This situation is analogous to two cars merging into a single lane and the resulting collision. Traffic is backed up until the collision can be cleared. When the number of collisions in a network is excessive, sluggish network response times result. This indicates that the network has become too congested or too many users are trying to access the network at the same time. Layer 2 devices are more intelligent than Layer 1 devices. Layer 2 devices make forwarding decisions based on Media Access Control (MAC) addresses contained within the headers of transmitted data frames. A bridge is a Layer 2 device used to divide, or segment, a network. A bridge is capable of collecting and selectively passing data frames between two network segments. Bridges do this by learning the MAC address of all devices on each connected segment. Using this information, the bridge builds a bridging table and forwards or blocks traffic based on that table. This results in smaller collision domains and greater network efficiency. Bridges do not restrict broadcast traffic. However, they do provide greater traffic control within a network. A switch is also a Layer 2 device and may be referred to as a multi-port bridge. A switch has the intelligence to make forwarding decisions based on MAC addresses contained within transmitted data frames. The switch learns the MAC addresses of devices connected to each port and this information is entered into a switching table. Switches create a virtual circuit between two connected devices that want to communicate. When the virtual circuit is created, a dedicated communication path is established between the two devices. The implementation of a switch on the network provides microsegmentation. In theory this creates a collision free environment between the source and destination, which allows maximum utilization of the available bandwidth. A switch is also able to facilitate multiple, simultaneous virtual circuit connections. This is analogous to a highway being divided into multiple lanes with each car having its own dedicated lane. The disadvantage of Layer 2 devices is that they forward broadcast frames to all connected devices on the network. When the number of broadcasts in a network is excessive, sluggish network response times result. A router is a Layer 3 device. The router makes decisions based on groups of network addresses, or classes, as opposed to individual Layer 2 MAC addresses. Routers use routing tables to record the Layer 3 addresses of the networks that are directly connected to the local interfaces and network paths learned from neighboring routers. The purpose of a router is to do all of the following: Routers are not compelled to forward broadcasts. Therefore, routers reduce the