Bridges connect two LAN segments of similar or dissimilar types, such as Ethernet and Token Ring. This allows two Ethernet segments to behave like a single Ethernet allowing any pair of computers on the extended Ethernet to communicate. Bridges are transparent therefore computers don’t know whether a bridge separates them.
Bridges map the Ethernet addresses of the nodes residing on each network segment and allow only necessary traffic to pass through the bridge. When a packet is received by the bridge, the bridge determines the destination and source segments. If the segments are the same, the packet is dropped or also referred to as “filtered”; if the segments are different, then the packet is “forwarded” to the correct segment. Additionally, bridges do not forward bad or misaligned packets.
Bridges are also called “store-and-forward” devices because they look at the whole Ethernet packet before making filtering or forwarding decisions. Filtering packets and regenerating forwarded packets enables bridging technology to split a network into separate collision domains. Bridges are able to isolate network problems; if interference occurs on one of two segments, the bridge will receive and discard an invalid frame keeping the problem from affecting the other segment. This allows for greater distances and more repeaters to be used in the total network design.
Dealing with Loops
Most bridges are self-learning task bridges; they determine the user Ethernet addresses on the segment by building a table as packets that are passed through the network. However, this self-learning capability dramatically raises the potential of network loops in networks that have many bridges. A loop presents conflicting information on which segment a specific address is located and forces the device to forward all traffic. The Distributed Spanning Tree (DST) algorithm is a software standard (found in the IEEE 802.1d specification) that describes how switches and bridges can communicate to avoid network loops.