The function of the bridge in the computer world and the real world, in both, bridges connect two or more separate parts to bridge the gaps. If we compare local networks to small and independent islands, bridges connect these islands so that their residents can access each other.
Perhaps Figure 1-1 can convey the idea of a bridge in the network world. Many organizations have multiple LANs and tend to connect them together. Local area networks (LAN) can be connected through devices that operate in the data link layer and are called bridges.
First of all: many universities and different departments of companies have their own LAN so that they can connect their personal computers, workstations and servers. Since different departments of an institution pursue different goals, therefore, a different LAN is implemented in each department, regardless of what other departments do.
Sooner or later, these LANs need to interact and communicate with each other. In this example, the emergence of many LANs was due to the choice and freedom of their owners. Second, organizations may be geographically scattered in buildings with significant distances.
Having several separate LANs in each building and connecting them through “bridges” and high-speed links may be cheaper than running a single cable between all sites. The third is that: sometimes, to adjust the load and adjust the traffic, it is necessary to divide a logical and single LAN into several smaller LANs.
For example: in many universities, thousands of workstations are available to students and faculty. Generally, the files are stored in the machines of the file server and are transferred and loaded on their machines according to the user’s request. The very large scale of this system prevents all workstations from being placed in a single local network because the required bandwidth will be very high.
Instead, several LANs connected by “bridges” are used. Each LAN network includes a group of stations and its own file server, so that most of the traffic is limited in the area of a single LAN and a lot of load will not be added to the backbone of the network.
Fourth, in some situations, although a single local network is sufficient in terms of load volume, the physical distance between distant machines is very large. The only solution is to divide the LAN into several parts and install a bridge between them. By using the bridge, the physical distance of the entire network can be increased. The fifth is that: the issue of trustworthiness; On a single LAN, a failed node generating a continuous stream of busy output can cripple the entire network.
Bridges can be placed at critical points so that a damaged and confused node cannot disrupt the entire system. Unlike a repeater that reproduces its input unconditionally, a bridge can be programmed to make conscious decisions about what to conduct or not to conduct.
Sixth are: Bridges can help information security in an organization. Most LAN interface cards have a mode called promiscuous mode, in which all current frames on the network are received, not the frames that were sent to the address.
By placing bridges in different places and making sure that sensitive information is not directed to uncertain parts, the system manager can isolate parts of the network from other parts so that their traffic does not find its way out. And it should not be in the hands of uncertain people. Figure 2-1 shows the connection of several independent local area networks by the bridge along with the corresponding symbols. (Often, in many articles, the bridge is marked with a hexagon.) Note: A switch can act as a bridge.
Bridges check “data link layer” addresses for data routing. Since the content of the data field is not supposed to be processed from the frames that should be directed, therefore these frames can be IPv4 packets (which are currently used on the Internet), IPv6 (which will be used on the Internet in the future) will be used), carry Apple Talk, ATM, OSI, or any other type of packets.
Unlike the bridge, “routers” check the address inside the packets and based on that, they direct them (routing). If we want to compare the route finder with a bridge in the real world in a popular and inaccurate interpretation, the bridge directly connects separate islands, while the route finder, like a boat, transports its passengers from one side to the other after boarding them. do
Of course, the bridge will increase the speed of the passengers, but the boat is always available anywhere on the island and will increase the maneuverability of the passenger!! It is important. What are the reasons why a single organization has several scattered LANs?
After examining why bridges are needed, let us address the question of how they function? Armani’s goal is for the bridges to be completely transparent, meaning that a machine can be moved from one part of the network to another without any changes in hardware, software or configuration tables. It should also be possible for all the machines in one part of the network to exchange information with the machines in the other part, regardless of their LAN type. This goal is sometimes achieved, but not always!
Perhaps, in the simplest scenario, the operation of the bridge can be understood as follows: machine A in a local network prepares a packet to send to host machine B in another network that is connected through a bridge. This packet passes through the MAC layer and the MAC header is added to its beginning. This data unit is transmitted on the channel and received by the bridge; After the frame reaches the bridge, the work of receiving it starts from the physical layer and the data is directed to the lower layer above.
After the received frame is processed in layer 2 and the MAC addresses are checked, the bridge determines on which output it should transmit the frame. Finally, a new frame is created and transmitted on the local network of the destination.
Note that a bridge that connects K different networks has K MAC sub-layers and k number of physical layers and communication channels. So far, it seems that transferring frames from one LAN to another is simple, but this is not the case.
Perhaps by examining the above scenario, you came to the conclusion that what the bridge does is the same as what a switch does, and nothing has been added to your information. Only the new name of the bridge is placed on the same switch that you saw its operation! You are both right and wrong! If you assumed that the bridge is a device that is supposed to connect two Ethernet networks, you are right and the bridge has nothing to do with the switch.
But in a broader definition, you should consider the bridge as a tool that is supposed to connect two heterogeneous networks (such as Ethernet and wireless or Ethernet and ring) and the frame after entering the bridge and before being transferred to the destination network should Change the nature and change the entire MAC process. Such a thing does not come from a switch. Generally, all the ports of a layer 2 switch (such as an Ethernet switch) are of the same type, and the frames do not change during transmission from the switch.
The heterogeneity of the networks and the intrinsic conditions governing each network (such as the conditions governing the wireless or wireless environment) will create many problems that the bridge must handle. Therefore, from now on, consider Ethernet switches as bridges that only connect Ethernet-type networks. A bridge is a layer 2 switch, but it is superior and more complex than an Ethernet switch
