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A bridge is a device that
connects 2 or more LAN segments together. Unlike a hub,
which blindly forwards all traffic to all network cables plugged into
it, the bridge builds a MAC table after
being powered up, and uses that table to determine where to
send traffic. It is able to see which MAC address
lives on which side of the bridge. The technical name for this
is transparent bridging, to indicate that it happens automatically,
without operator intervention. As the bridge learns which
MAC address is on which side of the bridge, it only forwards
traffic that needs to be forwarded, reducing the amount of unnecessary
traffic flowing from one network to another, thus improving network
performance and security.
Figure 37: Bridge
A bridge operates at the 2nd layer. It only forwards traffic that is destined for MAC addresses on the other side of the bridge or if it doesnt recognize the MAC address and doesnt know what to do with the traffic.
Know that a bridge and a brouter (described below) operate at layer 2(Data link layer) in the OSI model and that they are used to keep local LAN traffic local. (Since a brouter combines the functions of the layer 2 bridges and the layer 3 routers, it actually functions at both layers 2 and 3.)
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