Previous Table of Contents Next Switch Theory When I was a boy, we didn't have any of this fancy switching nonsense. We walked uphill in the snow both directions to our networks and used bridges to connect them, and we liked it! Once upon a time, smaller switches were called bridges because they bridged two networks together. Today's switches can connect dozens of segments, but they still follow the rules that were invented for their smaller forebears. Many switches are now used as replacement hubs-shared media becomes very annoying if you have many shared media errors (such as collisions), and you don't get the full capacity of the pipe. As such, they don't speak network protocols the way that routers do. A switch operates by learning the MAC addresses of each station attached to it. If one MAC address on one segment wants to talk to another MAC address on another segment, the switch connects them together on a private circuit, much like a switchboard operator of old would connect various cables and plugs to appropriate outlets in order to connect various people's conversations together. However, unlike the switchboard operator, the switch could care less about the content of the conversation; it just blithely connects folks who want to talk to each other and separates out the other conversations. This separation of conversations is why folks use switching as a way to speed up their networks. ______________________________________________________________ Experiencing strange problems on your large switched network? Check out how many simultaneous MAC addresses your switches support-older switches didn't support very many and won't be able to keep large numbers of end stations, which can manifest itself in PC lockups and disconnects. ______________________________________________________________ Does each end station need to know that a switch is in the mix? Nope. This is because the switch will respond when one end station's calls for the other. In other words, Station A in Figure 14.4 wants to talk to Station B, which is on the other side of the switch. Station B never sees the switch; instead, it sees the unmodified frame from Station A, which the switch has lobbed from one segment to another. The frame has a source of A and a destination of B; the switch's MAC addresses never enter the picture. Scary, yet very neat. [14-04t.jpg] Figure 14.4 How two stations talk via a switch. ______________________________________________________________ Always remember that there's a segment on each switch port (whether it's shown or not). In other words, when you see a diagram showing a point-to-point link between a PC and a switch, or a server and a switch, it's unlikely that someone will show the segment in between the node and the switch; instead, just one line between the switch and the node will be shown. However, the segment is there! ______________________________________________________________ Wait and Switch There are two kinds of switching methods: o Cut through Like it sounds, cut through switching starts transmitting the packet as it comes through, and it's very fast. Of course, if the packet ends up being an error packet, the switch has just introduced an error on the other end. o Store and forward With store and forward switching, the switch receives the entire packet and then forwards it on to the destination segment. When errors occur here, the switch has the option of dropping the packet rather than propagating the error. Of course, because the switch waits for the entire packet to be received before forwarding it to the destination, there's delay involved. Store and forward versus cut through involves a tradeoff of reliability versus speed; if you're experiencing problems with your switched network, this is something to keep in mind. The price of speed can sometimes be more than you're willing to pay. Broadcast News If you have large numbers of programs, servers, or print servers that rely on data link-level broadcasts (that is, data link traffic destined to all stations on a network), remember that each broadcast no longer goes to one segment. When switching segments together, a broadcast on one propagates to all segments. This can result in large amounts of traffic and can cause unwanted problems. ______________________________________________________________ You'll want to think about when to switch and when to route. Two networks with different protocol addresses do not share data link-level broadcasts. ______________________________________________________________ Again, a switch is really dumb. It doesn't know anything about the conversation on the phone-it merely knows who's doing the talking. Can the switch be configured to divide two different segments based on protocols? Sure, but then the switch is basically acting as a router. Trees Save You One important aspect of switch operation is what happens when switches are connected together. Because there really isn't any routing protocol when a switch is acting as a switch, what happens if Switch 1 is connected to Switch 2, and Switch 3 is connected to both Switch 1 and 2? Sounds like something evil waiting to happen, doesn't it? Well, you're right, evil is indeed in the offing. As you can see from Figure 14.5, there are two paths from Station A to Station B. Because Switch 1 forwards the frame from Station A on to the next network, it seems as if Switch 2 and Switch 3 would pick up conversation from A and try to forward it to B. If these shenanigans are allowed to go on unchecked, all pandemonium will break loose: The packets will be forwarded forever and ever, your network utilization will go way up, and the bridges will become overworked and go on strike. [14-05t.jpg] Figure 14.5 Without the spanning tree protocol, the bridges would cause a loop that would cause the network to malfunction. This situation can be avoided by a bridge protocol called spanning tree. With this protocol, a special packet is sent out by each bridge or switch on the network; each switch learns about each other switch and stops this kind of nonsense before it even begins by blocking one of the interfaces, such as the one between Switch 1 and Switch 3. If the network changes by a switch being taken out of service, or by a switch losing its mind, the spanning tree will attempt to adjust automatically. All switches that you buy nowadays will give you the option to do this (or will do it automatically), but some (really old) switches won't, and they can cause problems. In this case, you have two options: Don't connect your switches in a loop or stop using the older stuff. Previous Table of Contents Next