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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 internetworking?
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 relevant networks
together so all users can share the networks’ wealth.
Addressing
Starting with a look at Figure 1.1, you get a picture of a basic local area network (LAN)
that’s connected 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 that
you’ll probably even dream about them!
FIGURE 1 . 1
The basic network
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 Internet Protocol (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
Media Access
The basic network allows devices to share information.
The term computer language refers to binary code (0s or 1s).
The two hosts above communicate using hardware or MAC addresses.
(Hub)
Bob Sally
4
Chapter 1
Internetworking
Control (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:
Time Source Destination Protocol Info
53.892794 192.168.0.2 192.168.0.255 NBNS Name query NB
SALLY
<00>
Time Source Destination Protocol Info
53.892794 192.168.0.2 192.168.0.255 NBNS Name query NB
SALLY
<00>
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
f
s 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, “IP Subnetting, Variable
Length Subnet Masks (VLSMs), Troubleshooting IP, and Introduction to NAT.”
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:
Time Source Destination Protocol Info
5.153054 192.168.0.2 Broadcast ARP Who has 192.168.0.3? Tell 192.168.0.2
Next, check out Sally’s response:
Time Source Destination Protocol Info
5.153403 192.168.0.3 192.168.0.2 ARP 192.168.0.3 is at 00:0b:db:99:d3:5e
5.53.89317 192.168.0.3 192.168.0.2 NBNS Name query response NB
192.168.0.3
Okay, sweet—Bob now has both Sally’s IP address and her MAC address! Both are 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!).
Internetworking Basics
5
Hubs, Bridges, Routers, and Switches
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 times will have dwindled to a slow
crawl as the network grew and grew. And with all that growth, your LAN’s traffic congestion
will have reached epic proportions. The answer to this problem is breaking up that 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 shows 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.
FIGURE 1 . 2
A switch can replace the hub, breaking up collision domains.
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:
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 routed protocol that is like IP, but really,
really chatty. Typically, IPX is not used in today’s networks.)
Take another look at Figure 1.2. Did you notice that I replaced the main hub from Figure 1.1
with a switch? Whether you did or didn’t, I did that because hubs don’t segment a network; they
just connect network segments together. So basically, it’s an inexpensive way to connect a couple
of PCs together, which is great for home use and troubleshooting, but that’s about it!
(Hub)
Switch
6
Chapter 1
Internetworking
Now, routers are
used to connect networks 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.
FIGURE 1 . 3
Routers create an internetwork.
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 wide area network (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:
They don’t forward broadcasts by default.
They can filter the network based on layer 3 (Network layer) information (like the
IP address).
A router creates an internetwork and
provides connections to WAN services.
Switch
Switch
Serial 0
Internetworking Basics
7
Routers can be used for the following four functions in your network:
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
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), you have what
is called an internetwork. Finally, routers use a routing table (map of the internetwork) to
make path selections and to forward packets to remote networks.
In this book, I’ll just talk about IP addressing. If you’d like to know more about
IPv6, pick up a copy of Sybex’s
CCNA: Cisco Certified Network Associate
Study Guide
. There’s a whole chapter on IPv6.
Conversely, layer 2 switches, the ones we usually call just plain switches, aren’t used to
create internetworks because they do not break up broadcast domains by default; 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. And these 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.
Collision domain
is an Ethernet term used
to describe a network scenario in which one device sends a packet on a network segment and
every other device on the same segment is forced to pay attention to it. If, at the same time, a
different device tries to transmit, a collision occurs and 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.
Switches create separate collision domains within a single broadcast domain.
Routers provide a separate broadcast domain for each interface.
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).
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Chapter 1
Internetworking
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 a bridging
function, but they do so with greatly enhanced management ability and features. Plus, most of
the time, bridges had only 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 of ports available on some switches!
You would use a bridge in a network to reduce collisions within broadcast
domains and to increase the number of collision domains in your network.
Doing this provides more bandwidth for users. And keep in mind that using
hubs in your Ethernet network can contribute to congestion. As always, plan
your network design carefully!
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.
FIGURE 1 . 4
Internetworking devices
Router
Bridge
Switch
Internetworking Basics 9
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 created only 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 interconnected hubs at the bottom of the figure also connect
to the router. This setup creates one collision domain and one broadcast domain and
makes the bridged network, with its two collision domains, look much better indeed!
Although bridges/switches are used to segment networks, they will not
isolate broadcast or multicast packets.
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.
Network response time eventually 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.
FIGURE 1 . 5 Switched networks creating an internetwork
Router
10 Chapter 1 Internetworking
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 that you don’t have to worry about in the ICND1 objectives.
VLANs are covered in depth in the Sybex CCNA Study Guide. But it is really important
to understand that even in 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. The best network design is one in which LAN switches with
routers are used, and correctly placed in the network. 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. Look 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!
So now that you’ve gotten an introduction to internetworking and the various devices that
live in an internetwork, it’s time to head into internetworking models.
Should I Just Replace All My Hubs with Switches?
You’re a network administrator at a large company in San Jose. The boss comes to you and
says that he got your requisition to buy a switch and he’s not sure about approving the
expense; do you really need it?
Well, if you can replace all the hubs with switches, sure—why not? Switches really add a lot
of functionality to a network that hubs just don’t have. But most of us don’t have an unlimited
budget. Hubs still can create a nice network—that is, of course, if you design and implement
the network correctly.
Let’s say that you have 40 users plugged into four hubs, 10 users each. At this point, the hubs
are all connected together so that you have one large collision domain and one large broadcast
domain. If you can afford to buy just one switch and plug each hub into a switch port,
and plug the servers into the switch, you’ll have four collision domains and one broadcast
domain. Not great, but for the price of one switch, your network is a much better thing. So,
go ahead! Put that requisition in to buy all new switches. What do you have to lose?
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