Bridges

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Bridges

Repeaters ended up being exceptionally good at retransmitting data. So good, in fact, that when using a broadcast protocol like NetBEUI, the same bandwidth considerations discussed earlier became a problem again. Machines had a hard time trying to transmit data because they continued to collide with other machines trying to transmit data. This meant that the functional size of an ethernet network was really only a little over 100 machines or so. Numbers greater than this, and sometimes numbers even less than 100, on any segment or extended segment resulted in so many collisions that it usually hung up the entire network. The result was that you, as the network administrator, had to tell everyone to reboot—but please, not all at once! Repeaters were not going to be the only device necessary to make networking feasible.

Bridges were designed to be smarter than repeaters, transmitting data from one network segment to another only if absolutely necessary. Repeaters cannot serve this function because they simply regurgitate anything they see on one segment onto the other segment. Although useful, this is not always a terribly bright idea, as Figure 5.5 demonstrates. The repeater correctly identifies that broadcasts are important to retransmit. In fact, most networking protocols provide for some way to implement a broadcast on the network, whether its purpose is to identify a server resource or find a physical address to initiate communication between two machines (even TCP/IP).

Even though protocols typically use some form of broadcast to begin communicating with another machine, they are not broadcasting all the time. Besides announcing services, broadcasting is used usually only when you don’t know the physical address of the machine with which you’re trying to communicate, and have to ask the whole network. After both parties know each other’s physical addresses, the machines no longer need broadcasts at this level to communicate.

Designers needed a way to isolate the traffic to only the segments necessary for two machines to communicate after they knew the source and destination addresses. In this way, the previous downside to the repeater could be overcome, by only allowing traffic to be transmitted on the network segments where it was necessary, keeping unnecessary traffic to a minimum.

To understand how a bridge works, consider what you would have to know to pass broadcasts. For that matter, what information do you already have at your disposal? When you first turn on a bridge, it is basically blind. The bridge has no idea which machines are on the network and has to figure out which ports it is listening to. But it has memory and a set of rules by which it lives. A bridge only passes data from one segment or port to another based on the following conditions:

If the destination physical address is on the same port as the source physical address, simply retransmit the request onto the same network segment.
If the destination physical address is on a different port than the source physical address, transmit that packet to the port on which the destination physical address resides.
If the destination physical address is unknown (not in the table) or it is a broadcast, pass the broadcast to the other ports, make a note of the source’s physical address, and indicate in memory the port on which it resides.

Every time a packet is sent and received on a port, the bridge is responsible for identifying the port on which the source address lives. After this has been determined, that physical address is mapped to the port in the bridge’s internal tables.

The advantage of being able to do this is that the number of machines on a network can essentially be doubled, tripled, and so on, depending on where the traffic patterns are and which machines communicate most with each other. Obviously, on some networks in which resources are centralized, this is not much help, but on networks in which resources are distributed in functional groups, it makes a great deal of difference.

Recall from the discussions on broadcasts that when a machine broadcasts a question, it also includes its source MAC address, or physical address. This is true regardless of the communications protocol being utilized. Why would NetBEUI or TCP/IP care about a source address if it is sending a broadcast? The initiator of the communication sends their physical address so that the receiver does not have to broadcast back. In terms of broadcast frames, bridges do not initially help much, because they simply retransmit the broadcast onto the segments and networks to which they are connected. They do come in handy after the source and destination addresses have been discovered. Bridges use the source addresses to build tables in memory of which addresses correspond to its ports. Look at the example shown in Figure 5.5 to see how this works.

As the example indicates, the bridge senses the voltage on the wire and decides whether it will rebroadcast the message based on its table. Bridges rebroadcast broadcasts by default; however, after they find source and destination addresses are on the same segment, they simply retransmit on that segment. If machine A tries to communicate with machine D, the router would mark machine D’s physical address in network 2 and would know to pass any frames destined for physical address 5 to network 2. In this way, you can isolate local traffic from other segments, but when machines need to communicate with machines on another segment, they can do that as well. The tables that the bridges keep make them smart enough to look at the frames and determine where they are supposed to go. Different types of bridges learn their tables in different ways, but they all perform essentially the same task. Bridges are typically associated with bus-based and ring-based networks, and can serve as primitive gateways between these networks, reformatting frames from one and placing them on the network of the other.

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