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Bridging occurs at the
data link layer, which controls data flow, handles transmission errors,
provides physical addressing, and manages access to the physical medium.
Bridges provide these functions by using various link layer protocols that
dictate specific flow control, error handling, addressing, and media access
algorithms. Examples of popular data link layer protocols include Ethernet,
Token Ring, and FDDI.
Upper-layer protocol
transparency is a primary advantage of bridging. Bridges are not required
to examine upper-layer information because they operate at the data link
layer or Layer 2 of the OSI model. Bridges filter network traffic by only
looking at the MAC address, not protocols. It is not uncommon for a bridge
to move protocols and other traffic between two or more network segments.
Because bridges only look at MAC addresses, they can rapidly forward
traffic representing any network-layer protocol. To filter or selectively deliver
network traffic, a bridge builds tables of all MAC addresses located on
their directly connected network segments. 
If data comes along the
network media, a bridge compares the destination MAC address carried by the
data to MAC addresses contained in its tables. If the bridge determines
that the destination MAC address of the data is from the same network
segment as the source, it does not forward the data to other segments of
the network.  -
 If
the bridge determines that the destination MAC address of the data is not
from the same network segment as the source, it forwards the data to the
appropriate segment.  -
 By
doing this, bridges can significantly reduce the amount of traffic between
network segments by eliminating unnecessary traffic. View the Figures -
to
see how bridges handle local network traffic. In contrast, view Figures -
to
see how bridges handle non-local network traffic.
Bridges are
internetworking devices that can be used to reduce large collision domains.
Collision domains are areas where packets are likely to interfere with each
other. They do this by dividing the network into smaller segments and
reducing the amount of traffic that must be passed between the segments.
Bridges operate at Layer 2 or the data link layer of the OSI model, because
they are only concerned with MAC addresses. As data is passed along the
network on its way to a destination, it is picked up and examined by every
device on the network including bridges. Bridges work best where traffic is low from one
segment of a network to other segments. When traffic between network
segments becomes heavy, bridges can become a bottleneck and slow down
communication.
There is another
potential problem with using a bridge. Bridges always spread and multiply a
special kind of data packet. These data packets occur when a device on a
network wants to reach another device on the network, but does not know the
destination address of the device. When this occurs, frequently the source
sends out a broadcast to all devices on a network. Since every device on the network has to pay attention
to such broadcasts, bridges always forward them. If too many
broadcasts are sent out over the network a broadcast storm can result. A
broadcast storm can cause network time-outs, traffic slowdowns, and the
network to operate at less than acceptable performance.
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to
learn the important properties of bridges.
