Previous Table of Contents Next Tree Trunks As I mentioned in Hour 9, "Ethernet Basics," full duplex communication between a switch and a node such as a server is possible. This is true of both Token-Ring and Ethernet switches. This makes the effective bandwidth double what it might ordinarily be between the server and the switch: 32Mbps for Token-Ring and 200Mbps for Fast Ethernet. Full duplex is really cool for connecting servers or high-utilization users to switches, but switch-to-switch communications might need even more bandwidth than this provides, because a switch is responsible for many more users than just one. Accordingly, some switches are starting to have the capability to use multiple lines to communicate in between two switches. This is known as trunking. This allows you to add as much bandwidth as might be necessary to connect two switches. You can think of this as adding lanes to a highway. Because the switches in Figure 14.6 have six 100Mbps segments apiece, the aggregate throughput (or combined amount of stuff going through these switches) could be 600Mbps. To make sure that there's not a choke point between the two switches, you might connect three full-duplex trunks between the switches, giving you a 600Mbps superhighway between the switches. [14-06t.jpg] Figure 14.6 Full duplex and trunking can be a great solution if you need to connect high-capacity switches. Because trunking is relatively new on the scene-and full duplex is not terribly long in the tooth either-you should definitely suspect compatibility issues if you implement either one or both and your network starts acting screwy. Summary You can think of routers and switches as being cousins. Routers switch packets at the network protocol (TCP/IP or IPX/SPX) level, whereas switches route packets at the data link (Ethernet or Token-Ring) level. Switches are usually wire-speed devices, whereas routers can handle delay (as is the case over a wide-area connection). Switches are typically used for one geographically separate area; routers are used to connect geograph-ically disperse areas. A switch can be used as a hub replacement, allowing users to enjoy an unshared line to communicate to other devices on the network. The documentation that comes with your switch or router should become bathroom reading material if you really want to be able to handle problems. Of course, the theory is really important, but router and switch theory by itself won't do you a lot of good; you'll also need to know specific informational and configuration commands for your devices. Routers are much more complex than switches, and they communicate back and forth quite a lot via routing protocols. Switches, on the other hand, are pretty dumb, and they only talk to each other to avoid a duplicate path to a workstation. Both switches and routers, however, are more often than not hybrid devices that can route packets at a protocol level or switch packets at a data link level, all depending on how you configure them. Workshop Q&A Q My boss has been saying that we should chuck all our routing and switching gear and get in some layer-three switches. What the heck is she talking about? A Let's first define what a layer-three switch does. To do so, we need to review that icky OSI seven-layer cake I talked about earlier. Here are the OSI layers in order: o Physical o Data link o Network o Transport o Session o Presentation o Application Without getting heavy into it, layer-three switching refers to the network layer, meaning the network protocol (TCP/IP or IPX/SPX, for example). And a device for taking in stuff and deciding what to do with it based on the network protocol is what? Yep, a router. Layer-three switch is just a new marketing term for a very fast router. So, the question for you is this: Do you need a new router? Q I tried to connect to my router/switch with a serial cable. I'm pretty sure that I'm using a null modem cable, but every time I use my communications program, all I see is a bunch of ç characters. Why is my device speaking French to me? Does it think I'm cute or something? A Don't worry, your device isn't making a pass at you; it's not even speaking French! Any time you see garbage characters like these on a serial connection, it probably means you're running with incorrect communication parameters. If you're using the correct default parameters, as specified by your documentation, somebody has probably improved the connection by ramping up the connection speed and hasn't told you about it. Try 19200bps, 38400bps, or 57600bps. If those don't work, you can always try changing the stop bits or parity-but realize that people usually don't monkey with these on the device end. This problem is typically the result of a speed issue. Previous Table of Contents Next