Determining the flow of traffic
is an introduction to bridges, and the multiple functions
they perform in a network.
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.
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.
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.
Media Access Control layer bridge
Listens to all traffic:
are two main types of bridges, transparent and source-route.
A trans-lational bridge is used to connect dissimilar
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
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.
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.
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.
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.
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
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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