Internetworking
Internetworking Basics
Before exploring internetworking models and the OSI model’s specifications, you need to grasp the big picture and the answer to this burning question: Why is it so important to learn Cisco internetworking anyway? Networks and networking have grown exponentially over the past 20 years, and understandably so. They’ve had to evolve at light speed just to keep up with huge increases in basic, mission-critical user needs, (e.g. simple sharing data and printers), as well as greater burdens like multimedia remote presentations and conferencing. Unless everyone who needs
to share network resources is located in the same office space—an increasingly uncommon situation—the challenge is to connect relevant networks so all users can share the wealth of whatever services and resources are required. Figure 1.1 shows a basic local area network (LAN) that’s connected using a hub, which is basically just an antiquated device that connects wires together. Keep in mind that a simple network like this would be considered one collision domain and one broadcast domain. No worries if you have no idea what I mean by that because coming up soon, I’m going to talk about collision and broadcast domains enough to make you dream about them!
FIGuRE 1.1 A very basic network
SallyBob HEY SALLY?
Hub
Okay, things really can’t get much simpler than this. And yes, though you can still find this configuration in some home networks, even many of those as well as the smallest business networks are more complicated today. As we move through this book, I’ll just keep building upon this tiny network a bit at a time until we arrive at some really nice, robust and current network designs—the types that will help you get your certification and a job! But as I said, we’ll get there one step at a time, so let’s get back to the network shown in Figure 1.1 with this scenario: Bob wants to send Sally a file, and to complete that goal in this kind of network, he’ll simply broadcast that he’s looking for her, which is basically just shouting out over the network. Think of it like this, Bob walking out of his house and yelling down a street called Chaos Court in order to contact Sally. This might work if Bob and Sally were the only ones living there, but not so much if it’s crammed with homes and all the others living there are always hollering up and down the street to their neighbors just like Bob. Nope, Chaos Court would absolutely live up to its name, with all those residents going off whenever they felt like it—and believe it or not, our networks actually still work this way to a degree! So, given a choice, would you stay in Chaos, or would you pull up stakes and move on over to a nice new modern community called Broadway Lanes, which offers plenty of amenities and room for your home plus future additions all on nice, wide streets that can easily handle all present and future traffic? Good choice… so did Sally, who now lives a much quieter life, getting letters (packets) from Bob instead of a headache! The scenario I just described brings me to the basic point of what this book and the Cisco certification objectives are really all about. My goal of showing you how to create efficient networks and segment them correctly in order to minimize all the chaotic yelling and screaming going on in them is a universal theme throughout my CCENT and CCNA series books. It’s just inevitable that you’ll have to break up a large network into a bunch
of smaller ones at some point to match a network’s equally inevitable growth, and as that expansion occurs, user response time simultaneously dwindles to a frustrating crawl. But if you master the vital technology and skills I have in store for you in this series, you’ll be well equipped to rescue your network and its users by creating an efficient new network neighborhood to give them key amenities like the bandwidth they need to meet their evolving demands. And this is no joke; most of us think of growth as good—and it can be—but as many of us experience daily when commuting to work, school, etc., it can also mean your LAN’s traffic congestion can reach critical mass and grind to a complete halt! Again, the solution to this problem begins with breaking up a massive network into a number of smaller ones—something called network segmentation. This concept is a lot like planning a new community or modernizing an existing one. More streets are added, complete with new intersections and traffic signals, plus post offices with official maps documenting all those street names and directions on how to get to each are built. You’ll need to effect new laws to keep order to it all and provide a police station to protect this nice new neighborhood as well. In a networking neighborhood environment, all of this is carried out using devices like routers, switches, and bridges. So let’s take a look at our new neighborhood now, because the word has gotten out; many more hosts have moved into it, so it’s time to upgrade that new high-capacity infrastructure that we promised to handle the increase in population. Figure 1.2 shows a network that’s been segmented with a switch, making each network segment that connects to the switch its own separate collision domain. Doing this results in a lot less yelling!
FIGuRE 1.2 A switch can break up collision domains.
SallyBob JohnHEY JOHN!
Hub Switch
S1
This is a great start, but I really want you to make note of the fact that this network is still one, single broadcast domain, meaning that we’ve really only decreased our screaming and yelling, not eliminated it. For example, if there’s some sort of vital announcement that everyone in our neighborhood needs to hear about, it will definitely still get loud! You can see that the hub used in Figure 1.2 just extended the one collision domain from the switch port. The result is that John received the data from Bob but, happily, Sally did not. This is good because Bob intended to talk with John directly, and if he had needed to send a broadcast instead, everyone, including Sally, would have received it, possibly causing unnecessary congestion.
Here’s a list of some of the things that commonly cause LAN traffic congestion: uu Too many hosts in a collision or broadcast domain uu Broadcast storms uu Too much multicast traffic uu Low bandwidth uu Adding hubs for connectivity to the network uu A bunch of ARP broadcasts Take another look at Figure 1.2 and make sure you see that I extended the main hub from Figure 1.1 to a switch in Figure 1.2. I did that because hubs don’t segment a network; they just connect network segments. Basically, it’s an inexpensive way to connect a couple of PCs, and again, that’s great for home use and troubleshooting, but that’s about it! As our planned community starts to grow, we’ll need to add more streets with traffic control, and even some basic security. We’ll achieve this by adding routers because these convenient devices are used to connect networks and route packets of data from one network to another. Cisco became the de facto standard for routers because of its unparalleled selection of high-quality router products and fantastic service. So never forget that by default, routers are basically employed to efficiently break up a broadcast domain—the set of all devices on a network segment, which are allowed to “hear” all broadcasts sent out on that specific segment. Figure 1.3 depicts a router in our growing network, creating an internetwork and breaking up broadcast domains.
FIGuRE 1.3 Routers create an internetwork.
I LOVE SHOUTING! . . . HEY EVERYONE!
Sure is nice and quiet here.
S2 S2 R1
The network in Figure 1.3 is actually a pretty cool little network. Each host is connected to its own collision domain because of the switch, and the router has created two broadcast domains. So now our Sally is happily living in peace in a completely different neighborhood, no longer subjected to Bob’s incessant shouting! If Bob wants to talk with Sally, he has to send a packet with a destination address using her IP address—he cannot broadcast for her! But there’s more… routers provide connections to wide area network (WAN) services as well via a serial interface for WAN connections—specifically, a V.35 physical interface on a
Let me make sure you understand why breaking up a broadcast domain is so important. When a host or server sends a network broadcast, every device on the network must read and process that broadcast—unless you have a router. When the router’s interface receives this broadcast, it can respond by basically saying, “Thanks, but no thanks,” and discard the broadcast without forwarding it on to other networks. Even though routers are known for breaking up broadcast domains by default, it’s important to remember that they break up collision domains as well. There are two advantages to using routers in your network: uu They don’t forward broadcasts by default. uu They can filter the network based on layer 3, Network layer, information such as an IP address. Here are four ways a router functions in your network: uu Packet switching uu Packet filtering uu Internetwork communication uu Path selection I’ll tell you all about the various layers later in this chapter, but for now, it’s helpful to think of routers as layer 3 switches. Unlike plain-vanilla layer 2 switches, which forward or filter frames, routers (layer 3 switches) use logical addressing and provide an important capacity called packet switching. Routers can also provide packet filtering via access lists, and when routers connect two or more networks together and use logical addressing (IP or IPv6), you then have an internetwork. Finally, routers use a routing table, which is essentially a map of the internetwork, to make best path selections for getting data to its proper destination and properly forward packets to remote networks. Conversely, we don’t use layer 2 switches to create internetworks because they don’t break up broadcast domains by default. Instead, they’re employed to add functionality to a network LAN. The main purpose of these switches is to make a LAN work better—to optimize its performance—providing more bandwidth for the LAN’s users. Also, these switches don’t forward packets to other networks like routers do. Instead, they only “switch” frames from one port to another within the switched network. And don’t worry, even though you’re probably thinking, “Wait—what are frames and packets?” I promise to completely fill you in later in this chapter. For now, think of a packet as a package containing data. Okay, so by default, switches break up collision domains, but what are these things? Collision domain is an Ethernet term used to describe a network scenario in which one device sends a packet out on a network segment and every other device on that same segment is forced to pay attention no matter what. This isn’t very efficient because if a different device tries to transmit at the same time, a collision will occur, requiring both devices to retransmit, one at a time—not good! This happens a lot in a hub environment, where each host segment connects to a hub that represents only one collision domain and a single broadcast domain. By contrast, each and every port on a switch represents its own collision domain, allowing network traffic to flow much more smoothly.
The term bridging was introduced before routers and switches were implemented, so it’s pretty common to hear people referring to switches as bridges. That’s because bridges and switches basically do the same thing—break up collision domains on a LAN. Note to self that you cannot buy a physical bridge these days, only LAN switches, which use bridging technologies. This does mean that you’ll still hear Cisco and others refer to LAN switches as multiport bridges now and then. But does it mean that a switch is just a multiple-port bridge with more brainpower? Well, pretty much, only there are still some key differences. Switches do provide a bridging function, but they do that with greatly enhanced management ability and features. Plus, most bridges had only 2 or 4 ports, which is severely limiting. Of course, it was possible to get your hands on a bridge with up to 16 ports, but that’s nothing compared to the hundreds of ports available on some switches!
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