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Bridges: Determining the flow of traffic

Here is an introduction to bridges, and the multiple functions they perform in a network.

In the last issue we had talked about repeaters. We now continue our series on connecting devices with bridges.
Bridges are a step up from repeaters in terms of complexity and capability. Instead of providing a purely physical connection between segments of a network, bridges have some intelligence to perform traffic control. They keep local traffic local and send traffic only to other segments on which it belongs. Bridges have the regenerating capabilities of repeaters, so you don't need to use them in tandem.

A bridge can also divide a network to isolate traffic or problems. For example, if the volume of traffic from one or two computers or a single department is flooding the network with data and slowing down the entire operation, a bridge could isolate those computers or that particular department.
Bridges can be used to:
Expand the distance of a segment.
Provide for an increased number of computers on the network.
Reduce traffic bottlenecks resulting from an excessive number of attached computers.
A bridge can take an overloaded network and split it into two separate networks, reducing the amount of traffic on each segment and making each network more efficient.
Link unlike physical media such as twisted-pair and coaxial Ethernet.
Link unlike network segments such as Ethernet and Token Ring, and forward packets between them.

How bridges work
Bridges work at the Data Link layer of the OSI model. Because they work at this layer, all information contained in the higher levels of the OSI model is unavailable to them. Therefore, they do not distinguish between one protocol and another. Bridges simply pass all protocols along the network. Because all protocols pass across bridges, it is up to the individual computers to determine which protocols they can recognize.

You may remember that the Data Link layer has two sublayers, the Logical Link Control sublayer and the Media Access Control sublayer. Bridges work at the Media Access Control sublayer and are sometimes referred to as Media Access Control layer bridges.

A Media Access Control layer bridge
Listens to all traffic:

Checks the source and destination addresses of each packet.
Builds a routing table as information becomes available.
Forwards packets in the following manner:
If the destination is not listed in the routing table, the bridge forwards the packets to all segments.
If the destination is listed in the routing table, the bridge forwards the packets to that segment (unless it is the same segment as the source).

Types of bridges
There are two main types of bridges, transparent and source-route. A trans-lational bridge is used to connect dissimilar network types.

Transparent bridges
Transparent bridges use hardware network card addresses to determine which data to pass and which to filter. If you remember, each network card has a unique address assigned to it. Bridges use this information to decide which frames are passed and which are not. Computer addresses are stored in a table, one for each port. When data is received, the destination address is checked and compared against this table.

Source-route bridges
While most networks use transparent bridges, Token Ring networks use a different type of bridge called a source-route bridge. Instead of depending on MAC addresses, they use information in the token ring frame to determine whether to pass the data or not.

Translational bridges
Unlike repeaters, a translational bridge allows you to connect dissimilar networks together. Translational bridges have a port for the two different network types. The process that translational bridges use to pass data depends on which two types of networks they are connecting. They handle the conversion of the frames from one type to another and take into account the media access method.

What role do bridges play in your network?
So much for how bridges function in the theoretical world. Now lets take a look at the various possible roles of a bridge on a network.

Traffic control
Bridges keep records of the MAC addresses on each side of the segments they connect, and can compare the source and destination address of a packet with the addresses in the table. If the destination is local, then the bridge filters out the broadcast and leaves it on the local segment. If the destination is on another segment, then the bridge passes the data along to that segment. Either way, the data is only passed to the segment(s) where it belongs, so the network isn't tied up with broadcasts that belong only to one segment.
Security

A bridge's filtering capability also makes it configurable for security purposes. You're probably quite familiar with the idea of limiting access to shared drives, directories, and files with user rights, file permissions, and passwords, but this kind of restriction goes one step further. Using the filtering capabilities of a bridge, you can forbid any physical connection between nodes, making it impossible for an engineer, for example, to access the accounting files from his local machine. You still need the logical security of passwords and user rights and address filtering can give you an added layer of protection.

Connecting disparate networks
As many networks are not carefully planned to produce an integrated system, the capability of some bridges to link more than one kind of network can be useful in making the corporate network more integrated.
So, that's all about bridges. We will catch up with hubs in the next issue. NM

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