Previous Table of Contents Next Hour 14 Router and Switch Basics Although a Beethoven symphony sounds much different than a Bach invention, both follow the same fundamental rules of theory and composition. Although there's an awful lot of room for creativity and inventiveness within the scope of theory and composition rules, if these rules aren't followed, composers may end up with something that sounds like trash cans in the alley. With the dozens of hardware-based routers and switches on the market, it would be difficult, if not impossible, to address the specific ways that each of them operates. Fortunately, they, too, follow a basic theory and composition germane to routers and switches; I'll discuss the theory behind routing and switching in this hour and throw in a couple of practical suggestions. Combine this with your router or switch documentation, and you'll have a pretty good one-two punch that should take out many of your router- or switch-based problems. Theory and Practice Why talk about routing and switching in the same breath? Well, fundamentally, a router and a switch operate in pretty much the same way: Each receives a packet on one of its interfaces and spits the packet out on another interface. It's just that the network layer for a router is different than the layer for a switch; routers work on the protocol layer (for example, TCP/IP), whereas switches work on the data link layer (that is, they talk directly to network cards). ______________________________________________________________ When network geeks refer to network conversations in layers, they're referring to the OSI (Open Standards Institute) model of networking, which consists of an imaginary cake with the following seven layers: o Physical o Data link o Network o Transport o Session o Presentation o Application Note that not all these layers have one-to-one representations in real-life networks. Some of this is really ivory-tower stuff, but some of it is useful when thinking about your real-life network. For example, physical layer refers to the electronic rules and signaling that go on in an Ethernet wire, whereas data link layer refers to how network cards have card-to-card (MAC) conversations. Network layer refers to your network protocol (TCP/IP or IPX/SPX). Application layer refers to things such as services (Web and FTP services, for example). All the other stuff is really geeky, and we won't go into it. Whew! ______________________________________________________________ ______________________________________________________________ By definition, both routers and switches are referred to as multihomed hosts-that is, they have more than one network interface connected to more than one network segment. You can think of this as routers and switches having more than one network they call home. Isn't that sweet? ______________________________________________________________ In reality, although routers and switches are spoken about in different breaths, most large switches have routing capabilities, and most advanced routers have switching (or what used to be called bridging) capabilities. Therefore, for practical considerations, it's worth realizing that your switch may be configured to perform routing functions, and that your router may be configured to perform switching functions. See Figure 14.1 for the practical difference between a router and a switch (note the different IP addresses of the networks connected to the router versus the networks connected to the switch). [14-01t.jpg] Figure 14.1 The basic difference between a router and a switch: All the networks connected to a switch have the same network numbers. ______________________________________________________________ A switch is usually a wire-speed device that's able to spit out packets as fast as it sucks them in. This means that you usually don't switch over a slow link (that is, wide-area link). The reason for this is that data link communication is usually very, very timing dependent-which makes sense, because data link stuff is usually on the same "party line" without a "monkey in the middle." Therefore, you don't want to potentially mess up communications by messing with the timing. This is one instance where a switch and a router don't typically interchange, and where you usually want to use a router somewhere in the mix (see Figure 14.2). ______________________________________________________________ [14-02t.jpg] Figure 14.2 Wide-area applications typically have a router in the middle. Previous Table of Contents Next