Routers

Routers are devices that operate at Layer 3 of the OSI Model. Routers can be used to connect more than one Ethernet segment with or without bridging. Routers perform the same basic functions as bridges and also forward information and filter broadcasts between multiple segments. Figure 1.2 shows routers segmenting multiple network segments. Using an OSI network Layer 3 solution, routers logically segment traffic into subnets.

Figure 1.2: Routers connecting multiple segments.

Routers were originally introduced to connect dissimilar network media types as well as to provide a means to route traffic, filter broadcasts across multiple segments, and improve overall performance. This approach eliminated broadcasts over multiple segments by filtering broadcasts. However, routers became a bottleneck in some networks and also resulted in a loss of throughput for some types of traffic.

When you are connecting large networks, or when you are connecting networks to a WAN, routers are very important. Routers will perform media conversion, adjusting the data link protocol as necessary. With a router, as well as with some bridges, you can connect an Ethernet network and a Token Ring network.

Routers do have some disadvantages. The cost of routers is very high, so they are an expensive way to segment networks. If protocol routing is necessary, you must pay this cost. Routers are also difficult to configure and maintain, meaning that you will have a difficult time keeping the network up and running. Knowledgeable workers who understand routing can be expensive.

Routers are also somewhat limited in their performance, especially in the areas of latency and forwarding rates. Routers add about 40 percent additional latency from the time packets arrive at the router to the time they exit the router. Higher latency is primarily due to the fact that routing requires more packet assembly and disassembly. These disadvantages force network administrators to look elsewhere when designing many large network installations.

Switches

A new option had to be developed to overcome the problems associated with bridges and routers. These new devices were called switches. The term switching was originally applied to packet−switch technologies, such as Link Access Procedure, Balanced (LAPB); Frame Relay; Switched Multimegabit Data Service (SMDS); and X.25. Today, switching is more commonly associated with LAN switching and refers to a technology that is similar to a bridge in many ways.

Switches allow fast data transfers without introducing the latency typically associated with bridging. They create a one−to−one dedicated network segment for each device on the network and interconnect these segments by using an extremely fast, high−capacity infrastructure that provides optimal transport of data on a LAN; this structure is commonly referred to as a backplane. This setup reduces competition for bandwidth on the network, allows maximum utilization of the network, and increases flexibility for network designers and implementers.

Ethernet switches provide a number of enhancements over shared networks. Among the most important is microsegmentation, which is the ability to divide networks into smaller and faster segments that can operate at the maximum possible speed of the wire (also known as wire−speed).

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