Before we explore internetworking models and the specifications of the OSI reference model, you’ve got to understand the big picture and learn the answer to the key question, Why is it so important to learn Cisco internet working?
Networks and networking have grown exponentially over the last 15 years—understandably so. They’ve had to evolve at light speed just to keep up with huge increases in basic mission critical user needs such as sharing data and printers as well as more advanced demands such as videoconferencing. Unless everyone who needs to share network resources is located in the same office area (an increasingly uncommon situation), the challenge is to connect the sometimes many relevant networks together so all users can share the networks’ wealth.
Starting with a look at Figure 1.1, you get a picture of a basic LAN network that’s connected together using a hub. This network is actually one collision domain and one broadcast domain—but no worries if you have no idea what this means because I’m going to talk about both collision and broadcast domains so much throughout this whole chapter, you’ll probably even dream about them!
Okay, about Figure 1.1… How would you say the PC named Bob communicates with the PC named Sally? Well, they’re both on the same LAN connected with a multiport repeater (a hub). So does Bob just send out a data message, “Hey Sally, you there?” or does Bob use Sally’s IP address and put things more like, “Hey 192.168.0.3, are you there?” Hopefully, you picked the IP address option, but even if you did, the news is still bad—both answers are wrong!
Why? Because Bob is actually going to use Sally’s MAC address (known as a hardware address), which is burned right into the network card of Sally’s PC, to get ahold of her.
Great, but how does Bob get Sally’s MAC address since Bob knows only Sally’s name and doesn’t even have her IP address yet? Bob is going to start with name resolution (hostname to IP address resolution), something that’s usually accomplished using Domain Name Service (DNS). And of note, if these two are on the same LAN, Bob can just broadcast to Sally asking her for the information (no DNS needed)—welcome to Microsoft Windows (Vista included)!
Here’s an output from a network analyzer depicting a simple name resolution process from Bob to Sally:
As I already mentioned, since the two hosts are on a local LAN, Windows (Bob) will just
broadcast to resolve the name Sally (the destination 192.168.0.255 is a broadcast address).
Let’s take a look at the rest of the information:
EthernetII,Src:192.168.0.2(00:14:22:be:18:3b),Dst:Broadcast (ff:ff:ff:ff:ff:ff)
What this output shows is that Bob knows his own MAC address and source IP address but
not Sally’s IP address or MAC address, so Bob sends a broadcast address of all fs for the MAC
address (a Data Link layer broadcast) and an IP LAN broadcast of 192.168.0.255. Again,
don’t freak—you’re going to learn all about broadcasts in Chapter 3, “Subnetting, Variable
Length Subnet Masks (VLSMs), and Troubleshooting TCP/IP.”
Before the name is resolved, the first thing Bob has to do is broadcast on the LAN to get
Sally’s MAC address so he can communicate to her PC and resolve her name to an IP address:
Next, check out Sally’s response:
Okay sweet— Bob now has both Sally’s IP address and her MAC address! These are both listed as the source address at this point because this information was sent from Sally back to Bob. So,finally, Bob has all the goods he needs to communicate with Sally. And just so you know, I’m going to tell you all about ARP and show you exactly how Sally’s IP address was resolved to a MAC address a little later in Chapter 6, “IP Routing.”
By the way, I want you to understand that Sally still had to go through the same resolution processes to communicate back to Bob—sounds crazy, huh? Consider this a welcome to IPv4 and basic networking with Windows (and we haven’t even added a router yet!).
To complicate things further, it’s also likely that at some point you’ll have to break up one large network into a bunch of smaller ones because user response will have dwindled to a slow crawl as the network grew and grew. And with all that growth, your LAN’s traffic congestion has reached epic proportions. The answer to this is breaking up a really big network into a number of smaller ones-something called network segmentation. You do this by using devices like routers,switches, and bridges. Figure 1.2 displays a network that’s been segmented with a switch so each network segment connected to the switch is now a separate collision domain. But make note of the fact that this network is still one broadcast domain.
Keep in mind that the hub used in Figure 1.2 just extended the one collision domain from the
switch port. Here’s a list of some of the things that commonly cause LAN traffic congestion:
Now routers are used to connect networks together and route packets of data from one network to another. Cisco became the de facto standard of routers because of its high-quality router products, great selection, and fantastic service. Routers, by default, break up a broadcast domain —the set of all devices on a network segment that hear all the broadcasts sent on that segment. Figure 1.3 shows a router in our little network that creates an internetwork and breaks up broadcast domains.
frames, routers (layer 3 switches) use logical addressing and provide what is called packet switching. Routers can also provide packet filtering by using access lists, and when routers connect two or more networks together and use logical addressing (IP or IPv6), this is called an internetwork. Last, routers use a routing table (map of the internetwork) to make path selections and to forward packets to remote networks.
Conversely, switches aren’t used to create internetworks (they do not break up broadcast domains by default); they’re employed to add functionality to a network LAN. The main purpose of a switch is to make a LAN work better—to optimize its performance—providing more bandwidth for the LAN’s users. And switches don’t forward packets to other networks as routers do. Instead, they only “switch” frames from one port to another within the switched network. Okay, you may be thinking, “Wait a minute, what are frames and packets?” I’ll tell you all about them later in this chapter, I promise!
By default, switches break up collision domains. This is an Ethernet term used to describe a network scenario wherein one particular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it. At the same time, a different device tries to transmit, leading to a collision, after which both devices must retransmit, one at a time. Not very efficient! This situation is typically found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. By contrast, each and every port on a switch represents its own collision domain.
The term bridging was introduced before routers and hubs were implemented, so it’s pretty common to hear people referring to bridges as switches. That’s because bridges and switches basically do the same thing—break up collision domains on a LAN (in reality, you cannot buy a physical bridge these days, only LAN switches, but they use bridging technologies, so Cisco still calls them multiport bridges).
So what this means is that a switch is basically just a multiple-port bridge with more brainpower, right? Well, pretty much, but there are differences. Switches do provide this function, but they do so with greatly enhanced management ability and features. Plus, most of the time, bridges only had 2 or 4 ports. Yes, you could get your hands on a bridge with up to 16 ports, but that’s nothing compared to the hundreds available on some switches!
Figure 1.4 shows how a network would look with all these internetwork devices in place. Remember that the router will not only break up broadcast domains for every LAN interface, it will break up collision domains as well.
When you looked at Figure 1.4, did you notice that the router is found at center stage and that it connects each physical network together? We have to use this layout because of the older technologies involved–—bridges and hubs.
On the top internetwork in Figure 1.4, you’ll notice that a bridge was used to connect the hubs to a router. The bridge breaks up collision domains, but all the hosts connected to both hubs are still crammed into the same broadcast domain. Also, the bridge only created two collision domains, so each device connected to a hub is in the same collision domain as every other device connected to that same hub. This is actually pretty lame, but it’s still better than having one collision domain for all hosts.
Notice something else: The three hubs at the bottom that are connected also connect to the router, creating one collision domain and one broadcast domain. This makes the bridged network look much better indeed!
The network in Figure 1.3 is a pretty cool network. Each host is connected to its own collision domain, and the router has created two broadcast domains. And don’t forget that the router provides connections to WAN services as well! The router uses something called a serial interface for WAN connections, specifically, a V.35 physical interface on a Cisco router.
Breaking up a broadcast domain is important because when a host or server sends a network broadcast, every device on the network must read and process that broadcast—unless you’ve got 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 of using routers in your network:
Okay, here I’ve placed the LAN switches at the center of the network world so the routers are connecting only logical networks together. If I implemented this kind of setup, I’ve created virtual LANs (VLANs), something I’m going to tell you about in Chapter 9, “Virtual LANs (VLANs).” So don’t stress. But it is really important to understand that even though you have a switched network, you still need a router to provide your inter-VLAN communication, or internetworking. Don’t forget that!
Obviously, the best network is one that’s correctly configured to meet the business requirements of the company it serves. LAN switches with routers, correctly placed in the network, are the best network design. This book will help you understand the basics of routers and switches so you can make tight, informed decisions on a case-by-case basis.
Let’s go back to Figure 1.4 again. Looking at the figure, how many collision domains and broadcast domains are in this internetwork? Hopefully, you answered nine collision domains and three broadcast domains! The broadcast domains are definitely the easiest to see because only routers break up broadcast domains by default. And since there are three connections, that gives you three broadcast domains. But do you see the nine collision domains? Just in case that’s a no, I’ll explain. The all-hub network is one collision domain; the bridge network equals three collision domains. Add in the switch network of five collision domains—one for each switch port—and you’ve got a total of nine.
Now, in Figure 1.5, each port on the switch is a separate collision domain and each VLAN is a separate broadcast domain. But you still need a router for routing between VLANs. How many collision domains do you see here? I’m counting 10—remember that connections between the switches are considered a collision domain!
Networks and networking have grown exponentially over the last 15 years—understandably so. They’ve had to evolve at light speed just to keep up with huge increases in basic mission critical user needs such as sharing data and printers as well as more advanced demands such as videoconferencing. Unless everyone who needs to share network resources is located in the same office area (an increasingly uncommon situation), the challenge is to connect the sometimes many relevant networks together so all users can share the networks’ wealth.
Starting with a look at Figure 1.1, you get a picture of a basic LAN network that’s connected together using a hub. This network is actually one collision domain and one broadcast domain—but no worries if you have no idea what this means because I’m going to talk about both collision and broadcast domains so much throughout this whole chapter, you’ll probably even dream about them!
Why? Because Bob is actually going to use Sally’s MAC address (known as a hardware address), which is burned right into the network card of Sally’s PC, to get ahold of her.
Great, but how does Bob get Sally’s MAC address since Bob knows only Sally’s name and doesn’t even have her IP address yet? Bob is going to start with name resolution (hostname to IP address resolution), something that’s usually accomplished using Domain Name Service (DNS). And of note, if these two are on the same LAN, Bob can just broadcast to Sally asking her for the information (no DNS needed)—welcome to Microsoft Windows (Vista included)!
Here’s an output from a network analyzer depicting a simple name resolution process from Bob to Sally:
broadcast to resolve the name Sally (the destination 192.168.0.255 is a broadcast address).
Let’s take a look at the rest of the information:
EthernetII,Src:192.168.0.2(00:14:22:be:18:3b),Dst:Broadcast (ff:ff:ff:ff:ff:ff)
What this output shows is that Bob knows his own MAC address and source IP address but
not Sally’s IP address or MAC address, so Bob sends a broadcast address of all fs for the MAC
address (a Data Link layer broadcast) and an IP LAN broadcast of 192.168.0.255. Again,
don’t freak—you’re going to learn all about broadcasts in Chapter 3, “Subnetting, Variable
Length Subnet Masks (VLSMs), and Troubleshooting TCP/IP.”
Before the name is resolved, the first thing Bob has to do is broadcast on the LAN to get
Sally’s MAC address so he can communicate to her PC and resolve her name to an IP address:
By the way, I want you to understand that Sally still had to go through the same resolution processes to communicate back to Bob—sounds crazy, huh? Consider this a welcome to IPv4 and basic networking with Windows (and we haven’t even added a router yet!).
To complicate things further, it’s also likely that at some point you’ll have to break up one large network into a bunch of smaller ones because user response will have dwindled to a slow crawl as the network grew and grew. And with all that growth, your LAN’s traffic congestion has reached epic proportions. The answer to this is breaking up a really big network into a number of smaller ones-something called network segmentation. You do this by using devices like routers,switches, and bridges. Figure 1.2 displays a network that’s been segmented with a switch so each network segment connected to the switch is now a separate collision domain. But make note of the fact that this network is still one broadcast domain.
switch port. Here’s a list of some of the things that commonly cause LAN traffic congestion:
- Too many hosts in a broadcast domain
- Broadcast storms
- Multicasting
- Low bandwidth
- Adding hubs for connectivity to the network
- A bunch of ARP or IPX traffic (IPX is a Novell protocol that is like IP, but really, really
- chatty. Typically not used in today’s networks.)
Now routers are used to connect networks together and route packets of data from one network to another. Cisco became the de facto standard of routers because of its high-quality router products, great selection, and fantastic service. Routers, by default, break up a broadcast domain —the set of all devices on a network segment that hear all the broadcasts sent on that segment. Figure 1.3 shows a router in our little network that creates an internetwork and breaks up broadcast domains.
frames, routers (layer 3 switches) use logical addressing and provide what is called packet switching. Routers can also provide packet filtering by using access lists, and when routers connect two or more networks together and use logical addressing (IP or IPv6), this is called an internetwork. Last, routers use a routing table (map of the internetwork) to make path selections and to forward packets to remote networks.
Conversely, switches aren’t used to create internetworks (they do not break up broadcast domains by default); they’re employed to add functionality to a network LAN. The main purpose of a switch is to make a LAN work better—to optimize its performance—providing more bandwidth for the LAN’s users. And switches don’t forward packets to other networks as routers do. Instead, they only “switch” frames from one port to another within the switched network. Okay, you may be thinking, “Wait a minute, what are frames and packets?” I’ll tell you all about them later in this chapter, I promise!
By default, switches break up collision domains. This is an Ethernet term used to describe a network scenario wherein one particular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it. At the same time, a different device tries to transmit, leading to a collision, after which both devices must retransmit, one at a time. Not very efficient! This situation is typically found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. By contrast, each and every port on a switch represents its own collision domain.
The term bridging was introduced before routers and hubs were implemented, so it’s pretty common to hear people referring to bridges as switches. That’s because bridges and switches basically do the same thing—break up collision domains on a LAN (in reality, you cannot buy a physical bridge these days, only LAN switches, but they use bridging technologies, so Cisco still calls them multiport bridges).
So what this means is that a switch is basically just a multiple-port bridge with more brainpower, right? Well, pretty much, but there are differences. Switches do provide this function, but they do so with greatly enhanced management ability and features. Plus, most of the time, bridges only had 2 or 4 ports. Yes, you could get your hands on a bridge with up to 16 ports, but that’s nothing compared to the hundreds available on some switches!
Figure 1.4 shows how a network would look with all these internetwork devices in place. Remember that the router will not only break up broadcast domains for every LAN interface, it will break up collision domains as well.
On the top internetwork in Figure 1.4, you’ll notice that a bridge was used to connect the hubs to a router. The bridge breaks up collision domains, but all the hosts connected to both hubs are still crammed into the same broadcast domain. Also, the bridge only created two collision domains, so each device connected to a hub is in the same collision domain as every other device connected to that same hub. This is actually pretty lame, but it’s still better than having one collision domain for all hosts.
Notice something else: The three hubs at the bottom that are connected also connect to the router, creating one collision domain and one broadcast domain. This makes the bridged network look much better indeed!
Breaking up a broadcast domain is important because when a host or server sends a network broadcast, every device on the network must read and process that broadcast—unless you’ve got 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 of using routers in your network:
- They don’t forward broadcasts by default.
- They can filter the network based on layer 3 (Network layer) information (e.g., IP address). Four router functions in your network can be listed as follows:
- Packet switching
- Packet filtering
- Internetwork communication
- Path selection
Remember that routers are really switches; they’re actually what we call layer 3 switches (we’ll talk about layers later in this chapter). Unlike layer 2 switches, which forward or filter
The best network connected to the router is the LAN switch network on the left. Why? Because each port on that switch breaks up collision domains. But it’s not all good—all devices are still in the same broadcast domain. Do you remember why this can be a really bad thing? Because all devices must listen to all broadcasts transmitted, that’s why. And if your broadcast domains are too large, the users have less bandwidth and are required to process more broadcasts, and network response time will slow to a level that could cause office riots.
Once we have only switches in our network, things change a lot! Figure 1.5 shows the network that is typically found today.
Obviously, the best network is one that’s correctly configured to meet the business requirements of the company it serves. LAN switches with routers, correctly placed in the network, are the best network design. This book will help you understand the basics of routers and switches so you can make tight, informed decisions on a case-by-case basis.
Let’s go back to Figure 1.4 again. Looking at the figure, how many collision domains and broadcast domains are in this internetwork? Hopefully, you answered nine collision domains and three broadcast domains! The broadcast domains are definitely the easiest to see because only routers break up broadcast domains by default. And since there are three connections, that gives you three broadcast domains. But do you see the nine collision domains? Just in case that’s a no, I’ll explain. The all-hub network is one collision domain; the bridge network equals three collision domains. Add in the switch network of five collision domains—one for each switch port—and you’ve got a total of nine.
Now, in Figure 1.5, each port on the switch is a separate collision domain and each VLAN is a separate broadcast domain. But you still need a router for routing between VLANs. How many collision domains do you see here? I’m counting 10—remember that connections between the switches are considered a collision domain!
No comments:
Post a Comment