Bridges Operate in Both the Physically and the Data Link Layers

By: Jesse Miller

Bridges operate in both the physically and the data link layers. Bridges divide a large network into smaller segments. They can also relay frames between two originally separate segments of one type. Bridges contain logic that allows them to keep the traffic for each segment separate.

Bridges are repeaters that are smart enough to relay a frame only to the side of the segment containing the intended recipient. They filter traffic a fact that makes them useful for controlling congestion and isolating problem links. Bridges can also provide security through this partitioning of traffic. Bridges do not modify the structure or contents of a same packet in any way and can therefore be used only between segments that use the same protocol.

A bridge operates at the data link layer, giving it access to the physical address of all stations connected to it. When a frame enters a bridge, the bridge not only regenerates the signal but checks the address of the destination and forwards the new copy only to the segment to which the address belongs. As a bridge encounters a packet, it reads the address contained in the frame, and compares that address with a table of all the stations on both segments. When it finds a match, it discovers to which segment the station belongs and relays the packet only to that segment.

A bridge must have a look up table that contains the physical address of every station connected to it. The table indicates to which segment each station belongs. There are three types of bridges they are simple, learning and multipoint.

Simple bridges are the most primitive and least expensive type of bridge. A simple bridge links two segments and contains a table that lists the address of all the stations include in each of them. These addresses must be entered manually. Before a simple bridge can be used, an operator must sit down and program the address of every station. Whenever a new station is added, the table must be modified. If a station is removed, the newly invalid address must be deleted. Installation and maintenance of simple bridges are time-consuming and potentially more trouble than the cost savings are worth.

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A learning bridge builds its table of station address on its own. During its first installation a learning bridge is empty. As it encounters each packet, it looks at both the destination and the source addresses.

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