|  | 7.4 | Layer 2 Devices
| 7.4.4 | Bridge Layer 2 operations

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.