Switch Troubleshooting and Management
You troubleshoot a switch just like you troubleshoot a hub. If the switch has a link light (or LED), be sure it's on, indicating that the port is operating as it should and receiving a signal from the network adapter attached to the client computer (or another switch, as the case might be). Management software for the switch can be based on the SNMP or RMON specifications, or it might be proprietary in nature. In either case, all but the low-end home-office switches provide the capability to examine, test, and set parameters for each port on the switch.
For example, if you have a client computer connected to the switch, and the client's network adapter is autosensing, meaning that it can determine the network speed, it might not be compatible with the autosensing functionality of the switch. In that case, you might have to manually configure the switch port to match the higher speed that the network adapter can support or manually configure the network adapter to run at the switch's speed and duplex mode.
For more information about troubleshooting switches using SNMP and RMON, see Chapter 49, "Network Testing and Analysis Tools."
Chapter 9. Virtual LANs
SOME OF THE MAIN TOPICS IN THIS CHAPTER ARE
Chapter 8, "Network Switches," discussed switches that can be used to reduce the broadcast domain limit imposed by earlier networking technologies, such as hubs and bridges. LAN switches can be used to solve more problems than just reducing network traffic, however. This chapter discusses how switches can be used to create virtual LANs, or VLANs. Besides reducing the broadcast domain, switches configured for use in a VLAN can be used to solve many other problems:
Although a router can also be used to reduce a broadcast domain and create separate subnets in a network, switching technology works at a much faster pace. Thus, using VLANs in your network probably can enable you to get rid of a few slow routers that currently are being used to segment a LAN.
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
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
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
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
However, using switches to create a huge LAN
Virtual LANs, which can be created using switches, enable you to
. That is, although you might have all your computers
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