Since both switches are non-blocking, there should be no change in performance. What happens when these same twelve stations are split into two groups of six with six stations connected to one eight-port switch and another six connected to the other eight-port switch? (Figure 1) A single cable connects the two switches together for a net loss of two ports. However, to the user there is no change in performance when twelve stations are each connected, each to a port on the 16-port switch. This must mean that the 16-port switch fabric has higher performance over the eight-port switch fabric in order to accommodate twice as many ports in the same time frame. Is there a difference in performance between the two approaches? Assume that both the 16-port and eight-port switches are non-blocking. What happens if we want to add six more stations to the network? We would either need to replace the eight-port switch with a 16-port switch or we could simply add another eight-port switch using a switch-to-switch connection. If that is the case, the switch is said to be non-blocking. It should be possible for all stations to communicate to one another as if the switch were not there. For example, we have an eight-port switch and six connected stations all operating at 100 Mbps. A switch is called "non-blocking" or "wire- speed" if the switch fabric is fast enough, so that there is no noticeable degradation in throughput with the switch present or absent. Switches have what is called a "switch fabric" that allows for the rapid transfer of data frames from port-to-port within the switch. Traffic can be restricted to certain ports once the switch learns the location of station addresses. Switch ports terminate collision domains allowing for increased distances over what can be achieved using repeating hubs. However, the introduction of switch technology changes everything. ![]() Even when repeating hubs are cascaded, there is no perceivable change in network performance since arbitration rules do not change. ![]() No one station has precedence over another station. This is called shared Ethernet since all stations share the same media including the repeaters residing within the collision domain. With repeating hubs, all stations on the network occupy the same collision domain and obey the rules for arbitrating access to the network. This lesson addresses the concerns involved when cascading switches. When cascading switches, a reduction in performance can occur if the backbone connection is not properly designed.
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