Virtual LANs and Network Topologies

When discussing local area networks, most network administrators think of the physical topology of the LANthat is, the switches, servers, and workstations, and how they connect to form the LAN. The physical topology, as you learned in Chapter 2, "Overview of Network Topologies," doesn't have to match the logical topology of the LAN. As an early example, the Token-Bus network topology (IEEE 802.4 standard) uses a single coaxial cable to connect computers into a LAN. However, the order in which individual computers gain access to this shared cable is not the order in which they exist on the cable. In Figure 9.1, you can see that six computers are connected to a single cable. In this example, you can assume that the computers are numbered in a manner that represents their actual network address (Token-Bus addresses actually can range in size from 2 to 5 bytes). For this figure, the numbers 16 are used instead of the network address. In a token-passing network, a token frame is passed from one computer to another, and it is this token frame that gives a computer the right to transmit data on the network.

Figure 9.1. An early Token-Bus network uses a token frame to determine which computer can transmit data on the cable.

Although in this figure it might seem logical that the token frame would be passed from Computer 1 to Computer 6, and then to Computer 3, that is not how Token-Bus networks function. The physical topology is a linear bus, in which a message broadcast on the cable by Computer 1 travels down the wire until it reaches the terminator that is placed after Computer 5 at the end of the segment.

The logical topology of a Token-Bus network, however, is that of a ring. Although all computers on the same cable segment can "hear" the broadcast that every other computer makes, communications take place in an orderly manner. The token frame is "passed" in numerical address order from Computer 1 to Computer 2, then to Computer 3, and so on. This example is intended to show you the difference between a logical and a physical topology. The physical layout of the network is a linear bus. The logical topology of this network is a ring.

So what does this have to do with virtual LANs? A lot. Early LAN technologies, such as Ethernet, were limited in their size and distance by the physical topology of the LAN. You can read about this in Chapter 13, "Ethernet: The Universal Standard." Even Token-Ring networks are limited in size based on the physical topology of the network. Switches, as you learned in the preceding chapter, enable you to greatly expand the number of computers you can place on a LAN, and you can use high-speed communication links between switches to greatly expand the distance of a LAN.

However, using switches to create a huge LAN solves only the problem of the broadcast domain and the number of devices that can be attached to the network. In a modern networking environment, there are other factors to consider, such as security, configuration, and management. Perhaps you don't want all your computers connected to the same LAN, though they may be in close proximity to the same LAN switch. The more computers you have on a single LAN, the greater the odds are that a security breach will occurgiving an intruder access to other computers on the LAN.

Virtual LANs, which can be created using switches, enable you to separate the physical topology from the logical topology. That is, although you might have all your computers interconnected using a switch or several switches, appropriately configured switches make it possible to configure individual virtual LANs that are independent of the physical topology. Legacy hubs allow all computers on the LAN to see every network frame that is transmitted by every other computer in the same broadcast domain. Switches make connections only between the transmitting computer and the switch port that will get the network frame to its eventual destination. By limiting the network frame to just the sending and receiving stations, and the switches that stand between them, you take a big step toward preventing eavesdropping on the LAN.

Note

If you have not yet read the preceding chapter, "Network Switches," you should do so before attempting to understand the concepts covered in this chapter. For those readers who have been working in the networking industry for a long time, be aware that switches have replaced hubs in every LAN except for the oldest because switches provide a dramatic increase in performance versus cost. If you don't understand how a switch works, you will probably not get a lot of useful information out of this chapter.