Bridges are used to divide larger networks into smaller sections. They do this by sitting between two physical network segments and managing the flow of data between the two. By looking at the MAC address of the devices connected to each segment, bridges can elect to forward the data (if they believe that the destination address is on another interface), or block it from crossing (if they can verify that it is on the interface from which it came). Figure 3.4 shows how a bridge can be used to segregate a network.
When bridges were introduced, the MAC addresses of the devices on the connected networks had to be entered manually, a time-consuming process that had plenty of opportunity for error. Today, almost all bridges can build a list of the MAC addresses on an interface by watching the traffic on the network. Such devices are called learning bridges because of this functionality.
Figure 3.4 How a bridge is used to segregate networks.
Bridge Placement and Bridging Loops
There are two issues that you must consider when using bridges. The first is the bridge placement, and the other is the elimination of bridging loops:
Placement—Bridges should be positioned in the network using the 80/20 rule. This rule dictates that 80% of the data should be local and that the other 20% should be destined for devices on the other side of the bridge.
Bridging loops—Bridging loops can occur when more than one bridge is implemented on the network. In this scenario, the bridges can confuse each other by leading one another to believe that a device is located on a certain segment when it is not. To combat the bridging loop problem, the IEEE 802.1d Spanning Tree protocol enables bridge interfaces to be assigned a value that is then used to control the bridge-learning process.
Types of Bridges
Three types of bridges are used in networks:
Transparent bridge—Derives its name from the fact that the devices on the network are unaware of its existence. A transparent bridge does nothing except block or forward data based on the MAC address.
Source route bridge—Used in Token Ring networks. The source route bridge derives its name from the fact that the entire path that the packet is to take through the network is embedded within the packet.
Translational bridge—Used to convert one networking data format to another; for example, from Token Ring to Ethernet and vice versa.
Today, bridges are slowly but surely falling out of favor. Ethernet switches offer similar functionality; they can provide logical divisions, or segments, in the network. In fact, switches are sometimes referred to as multiport bridges because of the way they operate.