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Switching Operations

June 05, 2015
Alan Thomas


Switches play a vital role in moving data from one device to another. Specifically, switches greatly improve network performance, compared to hubs, by providing dedicated bandwidth to each end device, supporting full-duplex connectivity, utilizing the MAC address table to make forwarding decisions, and utilizing ASICs and CAM tables to increase the rate at which frames can be processed.


A Brief History

A broadcast domain is a collection of devices that can be reached via a broadcast frame. A broadcast frame is delivered to each device. Address Resolution Protocol (ARP) and Dynamic Host Configuration Protocol (DHCP) are two examples of communications protocols that utilize broadcasts.

A broadcast domain is the most basic of computer networks. Terms such as LAN, VLAN, and subnet are all ways of describing a broadcast domain. A LAN is a network, or broadcast domain, that covers a relatively small geographic area. Typically, a LAN covers a room or group rooms, possibly an entire floor, or even an entire building, if the building is small enough.

When LANs were first introduced, they were made of up computers connected to a single coaxial cable that was usually placed around the outer edge of a room, and the computers all tapped into the coaxial cable. The coaxial cable was the physical media or segment (Layer 1) that connected the devices, and the bandwidth it provided was shared, meaning all devices used the same bandwidth.

Ethernet was the Layer 2 protocol used to format data for transmission on the coaxial cable. Ethernet was also the protocol used to control access to the media. In a shared bandwidth environment, Ethernet limits use of the media to one device at a time. This required devices to connect at half-duplex.

If two devices attempted to access the media at the same time, meaning two devices attempted to send data at the same time, a collision occurred. When a collision occurred, both devices would wait a random amount time before trying again to access the media. They would wait different amounts of time in an attempt to prevent further collisions. However, while the two devices that collided both waited, a third device could see the segment as available and begin to send data. This further increased the wait time hosts experienced.

This type of LAN had many limitations. One of the most significant is that they were slow, as devices had to wait relatively long periods of time to transmit data. Also, a failure of one device usually meant that all devices lost their ability to send and receive data.

Eventually, Ethernet was updated to operate over twisted pair copper cabling. This led to the development of Ethernet hubs. Hubs provide a dedicated physical connection for every device, which helps reduce the possibility that a failure of one computer will cause all computers to lose connectivity. However, because a hub is still a shared bandwidth device, connectivity is limited to half-duplex. Additionally, collisions remain an issue as well, so hubs do not help improve the performance of the network.

Today, switches are used extensively-almost exclusively-throughout enterprise networks. Switches are a huge leap forward in efficiency and performance. One reason is that each physical interface on a switch is an independent Ethernet segment. As a result, an end device no longer has to compete to access a segment, which allows devices to use full-duplex connectivity. Additionally, the collision domain is reduced to just two devices, so collisions are virtually eliminated. These advantages result in greatly increased throughput, as the amount of waiting a device must do to send data is significantly reduced.

A second reason switches are efficient is they keep track of the interface on which an end device is connected. With this knowledge, a switch can be much more efficient in its forwarding of data, because data can be forwarded out of a single interface instead of flooded out of all interfaces, which is how a hub always forwards data.

Today's enterprise networks typically consist of multiple LANs (or VLANs/subnets). LANs might group devices together based on physical location or functions within the organization. In either case, switches are the key device for providing access to a LAN, and so a good understanding of how switches operate is crucial for the networking professional.

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