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Table Of Contents  CertiGuide to Network+
 9  Chapter 0011:  Network Hardware
      9  IV  Layer 2 – Data Link Layer

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IV  Layer 2 – Data Link Layer
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Bridge

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

 


Bridge Function

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 doesn’t recognize the MAC address and doesn’t know what to do with the traffic.

Bridge/ Brouter

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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