Bridges are networking devices that connect networks. Sometimes it is necessary to divide networks into subnets to reduce the amount of traffic on each larger subnet or for security reasons. Once divided, the bridge connects the two subnets and manages the traffic flow between them. Today, network switches have largely replaced bridges.
A bridge functions by blocking or forwarding data, based on the destination MAC address written into each frame of data. If the bridge believes the destination address is on a network other than that from which the data was received, it can forward the data to the other networks to which it is connected. If the address is not on the other side of the bridge, the data is blocked from passing. Bridges "learn" the MAC addresses of devices on connected networks by "listening" to network traffic and recording the network from which the traffic originates. Figure 3.9 shows a representation of a bridge.
Figure 3.9 How a bridge works.
The advantages of bridges are simple and significant. By preventing unnecessary traffic from crossing onto other network segments, a bridge can dramatically reduce the amount of network traffic on a segment. Bridges also make it possible to isolate a busy network from a not-so-busy one, thereby preventing pollution from busy nodes.
Bridge Implementation Considerations
Although implementing bridges can offer huge improvements in performance, you must factor in a number of considerations. The first is bridge placement. Generally, you should follow the 80/20 rule for bridge placement: 80% of the traffic should not cross the bridge, and 20% of the traffic should be on the other side of the bridge. The rule is easy to understand, but accurately determining the correct location for the bridge to accommodate the rule is another matter.
Another, potentially more serious, consideration is bridging loops, which can be created when more than one bridge is used on a network. Multiple bridges can provide fault tolerance or improve performance. Bridging loops occur when multiple bridges become confused about where devices are on the network.
As an example of bridging loops, imagine that you have a network with two bridges, as depicted in Figure 3.10. During the learning process, the north bridge receives a packet from Interface A (step 1 in Figure 3.11) and determines that it is for a system that is not on Network Z; therefore, the bridge forwards the packet to Network X (step 2 in Figure 3.11). Now, the south bridge sees a packet originating on Network X on Interface C (step 3 in Figure 3.11); because it thinks the destination system is not on Network X, it forwards the packet to Network Z (step 4 in Figure 3.11), where the north bridge picks it up (step 5 in Figure 3.11). The north bridge determines that the destination system is not on Network Z, so it forwards the packet to Network X—and the whole process begins again.
Figure 3.10 A network with two bridges.
Figure 3.11 A bridging loop.
You can work around the looping problem by using the Spanning Tree Algorithm (STA). When STA is used, each interface on a bridge is assigned a value. As the bridge forwards the data, the value is attached to the packet. When another bridge sees the data, if the STA value for the interface is higher than that assigned to its interfaces, the bridge doesn’t forward the data, thus eliminating the possibility of a bridging loop. STA eliminates the bridging loop but still provides the fault tolerance of having more than one bridge in place. If the bridge with the higher STA value (sometimes referred to as the primary bridge) fails, the other bridge continues functioning because it becomes the bridge with the higher STA value. All this is achieved by the Spanning Tree Protocol (STP).
Types of Bridges
Three types of bridges are used in networks. You don’t need detailed knowledge of how each bridge works, but you should have an overview:
Transparent bridge—A transparent bridge is invisible to the other devices on the network. Transparent bridges perform only the function of blocking or forwarding data based on the MAC address; the devices on the network are oblivious to these bridges’ existence. Transparent bridges are by far the most popular types of bridges.
Translational bridge—A translational bridge can convert from one networking system to another. As you might have guessed, it translates the data it receives. Translational bridges are useful for connecting two different networks, such as Ethernet and Token Ring networks. Depending on the direction of travel, a translational bridge can add or remove information and fields from the frame as needed.
Source-route bridge—Source-route bridges were designed by IBM for use on Token Ring networks. The source-route bridge derives its name from the fact that the entire route of the frame is embedded within the frame. This allows the bridge to make specific decisions about how the frame should be forwarded through the network. The diminishing popularity of Token Ring makes the chances that you’ll work with a source-route bridge very slim.
As switches become ever cheaper, bridges have been overtaken by switches in terms of both functionality and performance. Expect to be working with switches more often than with bridges.