Bridges

Bridges

A bridge is a connectivity device that operates at the OSI Data Link layer. The messaging parameters available at the Data Link layer enable a bridge to pass a frame in the direction of its destination without simultaneously forwarding it to segments for which it was not intended. In other words, a bridge can filter network traffic. This filtering process reduces overall traffic because the bridge segments the network, passing frames only when they can’t be delivered on the local segment and passing frames only to the segment for which they are intended.

Figure 2.11 depicts a simple bridge implementation. In this process, a bridge filters traffic by tracking and checking the Data Link layer’s MAC sublayer addresses of incoming frames. The bridge monitors the source addresses of incoming frames and builds a routing table that shows which nodes are on each of the segments. When a data frame arrives, the bridge checks the frame’s destination address and forwards the frame to the segment that contains the destination node. If the destination node exists on the same segment as the source node, the bridge stops the frame so it doesn’t pass unnecessarily to the rest of the network. If the bridge can’t find the destination address in its routing table, it forwards the frame to all segments except the source segment.

A bridge can perform the same functions a repeater performs, including expanding cabling distance and linking dissimilar cable types. In addition, a bridge can improve performance and reduce network traffic by splitting the network and confining traffic to smaller segments.