There is confusion in the networking world regarding the purpose of a bridge and its expected behavior. Bridges were created to allow network administrators to segment their networks transparently. This is important because individual stations don’t need to know whether there is a bridge separating them or not. Therefore, it is up to the bridge to make sure that packets get properly forwarded to their destinations. Segmenting a large network with a bridge has numerous benefits including reduced collisions (in an Ethernet network), contained bandwidth utilization, and the ability to filter out unwanted packets.
The are two main types of bridges:
1. Transparent Bridges
A transparent bridge examines a packet’s Destination Data Link Address and looks up the Address in its internal tables to determine to which of its ports, if any, to forward the packet. Promiscuous listening is the key to the bridge’s transparent operation. Since the bridge effectively “hears” all packets that are transmitted, it can decide whether forwarding is necessary without any special behavior from the individual stations.
2. Learning Bridges
A learning bridge can be added to a network, and it will learn the network topology without help from humans. If a station is moved, the bridge will realize it and update its tables appropriately.
Originally, there were some transparent bridges that did not have learning capability, but today, the terms “transparent bridge” and “learning bridge” are used interchangeably.
Beware of bridge loops.
It’s also important to consider bridge loops, which occur in a network and are often unpredictable. In a nutshell, bridging loops cause trouble because of the very feature that makes bridges so desirable: their transparency. Because the bridges don’t realize that packets are being forwarded from another bridge, they continually flood the packets onto their ports, creating more packets, which are flooded, creating more packets, etc.
For more information about bridges, routers, and network connecters we recommend Radia Perlman’s book “Interconnections – Bridges and Routers.”