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Switching Your Way To Better Network Performance

By Dan DiNicolo

Any look at network switching concepts will ultimately lead to a discussion about network bridging, and vice versa.

At the end of the day, the roles played by a bridge and a switch are more or less identical - they help to break a network up into a number of smaller collision domains. Recall from earlier in the series that a collision domain is simply a group of computers amongst which data collisions can occur. For example, when connecting systems using a hub, all systems are considered to be part of the same collision domain, since the hub will simply forward data out all ports, with no regard for where the actual destination host is located.

The difference between a bridge and a switch is not very difficult to comprehend. Bridges came on the network scene first, providing a way to break up a single large collision domain into a number of smaller collision domains. The bridging "logic" was usually implemented in software. In contrast, switches are basically an evolution of bridges - they implement their logic in hardware, and typically have a much higher port density than a bridge does. While you may still come across bridges on a network today, they are not nearly as common as they once were, given that switches tend to be much faster and have dropped significantly in price over the last 5 years. For all intents and purposes, beyond the differences that I just mentioned, you can use the words "bridge" and "switch" interchangeably. The concepts relating to their communication processes are basically identical.

Take a look at the diagram below. In it, all computers are connected to a hub, and as such, are part of the same collision domain. Collisions are obviously a bad thing; when they occur, systems must resend their data, which leads to less-than-optimal throughput. Because of the shared nature of Ethernet networks, when systems are plugged into a hub, they can only communicate in half-duplex. As such, only one system can be transmitting or receiving data at any given point in time.

Image 1

In the past, in order to make better use of the network and reduce the number of collisions, companies would implement a network bridge. The purpose of the bridge was to segment the network in a number of smaller collision domains, as shown below. The bridge doesn't have any magical qualities. Its job was simply to help ensure that the "scope" of collisions was made smaller. In the diagram shown below, data from Computer1 and Computer2 is still capable of colliding. However, data sent from Computer1 will never directly collide with data from Conputer3, since the two are in different collision domains. All computers in the diagram below are still in the same broadcast domain, as we'll discuss shortly.

Image 2

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The reason for the reduction is collisions when a bridge is implemented in due to the intelligent nature of a bridge. Unlike a hub, a bridge is capable of making decisions on whether or not to forward a frame based on its destination MAC address. Because it makes decisions based on MAC addressing, a bridge is known as a Data Link layer or Layer 2 device.

Consider the diagram below, in which the bridge breaks the network up into 3 collision domains. The job of the bridge is to learn about which systems are connected on each collision domain, and then store this information in its bridging table. The learning process occurs as the bridge encounters traffic, and uses the source MAC address of frames to determine from which interface a given system can be reached. In this case, as network traffic occurs, the bridge will learn that Computers 1 and 2 are accessible via interface A, Computers 3 and 4 via interface B, and Computers 5 and 6 via interface C. As of this information is gathered by the bridge inspecting the source addresses of the frames that it comes into contact with.

Read the Rest of the Article.

About the Author:
Dan DiNicolo is a technical trainer, consultant, author, and the managing editor of the free IT learning web site When he's not busy traveling the world as an IT volunteer with organizations like Geekcorps, Dan makes his home in the snowy northern backwoods of Canada.

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