copyrighted material

copyrighted material
Internetworking
Chapter
1
The CCNA exam topics covered in this
chapter include the following:
ÛÛ
Describe how a network works
Describe the purpose and functions of various network devices
NN
Select the components required to meet a network
AL
NN
Use the OSI and TCP/IP models and their associated
TE
NN
RI
specification
protocols to explain how data flows in a network
Describe common networked applications including web
MA
NN
applications
NN
Describe the purpose and basic operation of the protocols in
NN
D
the OSI and TCP models
Describe the impact of applications (Voice over IP and Video
TE
over IP) on a network
Interpret network diagrams
GH
NN
NN
Describe the components required for network and Internet
communications
CO
PY
RI
NN
NN
Identify and correct common network problems at layers 1, 2,
3, and 7 using a layered model approach
Differentiate between LAN/WAN operation and features
ÛÛ
Configure, verify, and troubleshoot a switch with VLANs
and interswitch communications
NN
Explain network segmentation and basic traffic
management concepts
ÛÛ
Implement an IP addressing scheme and IP Services to
meet network requirements in a medium-size Enterprise
branch office network
NN
Explain the operation and benefits of using DHCP and DNS
ÛÛ
Configure, verify, and troubleshoot basic router operation
and routing on Cisco devices
Welcome to the exciting world of internetworking. This
first chapter will really help you review your understanding of basic internetworking by focusing on how to connect
networks together using Cisco routers and switches. This chapter was written with an
assumption that you have already achieved your CompTIA Network+ certification or
have the equivalent knowledge, and based on this, I will review internetworking only for
the purpose of fully grasping the Cisco CCENT and/or CCNA objectives needed to help
you achieve your certifications.
First, you need to know exactly what an internetwork is, right? You create an internetwork when you connect two or more networks via a router and configure a logical network
addressing scheme with a protocol such as IP or IPv6.
I’ll be reviewing the following in this chapter:
■■
Internetworking basics
■■
Network segmentation
■■
■■
How bridges, switches, and routers are used to physically and logically
segment a network
How routers are employed to create an internetwork
I’m also going to dissect the Open Systems Interconnection (OSI) model and describe
each part to you in detail because you really need a good grasp of it for the solid foundation
upon which you’ll build your Cisco networking knowledge. The OSI model has seven hierarchical layers that were developed to enable different networks to communicate reliably
between disparate systems. Since this book is centering upon all things CCNA, it’s crucial
for you to understand the OSI model as Cisco sees it, so that’s how I’ll be presenting the
seven layers to you.
After you finish reading this chapter, you’ll encounter 20 review questions and three
written labs. These are given to you to really lock the information from this chapter into
your memory. So don’t skip them!
To find up-to-the-minute updates for this chapter, please see www.lammle.com
or www.sybex.com/go/ccna7e.
Internetworking Basics 3
Internetworking Basics
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 20 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 can’t remember what this means right now, because I’m going
to talk so much about both collision and broadcast domains throughout this chapter and in
Chapter 2 that 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 send a data message like this: “Hey 192.168.0.3,
are you there?” Possibly 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.
Fi g u r e 1 .1 ​ ​The basic network
Bob
Sally
(Hub)
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.
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Great, but how does Bob get Sally’s MAC address if he 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!
Here’s an output from a network analyzer depicting a simple initiation process from
Bob to Sally:
Source
Destination
Protocol
Info
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)
and Sally will let Bob know her address is 192.168.0.3 (analyzer output not shown). 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, “Introduction to TCP/IP.”
Now Bob has to broadcast on the LAN to get Sally’s MAC address so he can finally
communicate to her PC and send data:
Source
Destination Protocol Info
192.168.0.2 Broadcast ARP Who has 192.168.0.3? Tell 192.168.0.2
Next, check out Sally’s response:
Source
Destination
Protocol Info
192.168.0.3 192.168.0.2 ARP 192.168.0.3 is at 00:0b:db:99:d3:5e
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! 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 Address Resolution Protocol (ARP) and show you
exactly how Sally’s IP address was resolved to a MAC address in Chapter 8, “IP Routing.”
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 that 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.
Internetworking Basics 5
Fi g u r e 1 . 2 ​ ​A switch can replace the hub, breaking up collision domains.
Switch
(Hub)
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:
NN
Too many hosts in a broadcast or collision domain
NN
Broadcast storms
NN
Too much multicast traffic
NN
Low bandwidth
NN
Adding hubs for connectivity to the network
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, the reason I did that is 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!
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.
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.
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Fi g u r e 1 . 3 ​ ​Routers create an internetwork.
Serial 0
Switch
Router
A router creates an internetwork and
provides connections to WAN services.
Switch
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:
NN
NN
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:
NN
Packet switching
NN
Packet filtering
NN
Internetwork communication
NN
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 (or 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
Internetworking Basics 7
an internetwork. Lastly, 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. If at the
same time a different device tries to transmit, leading to a collision, 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 but 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 and vice versa. 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 refers to them as 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!
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 network can contribute to congestion on your Ethernet network.
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.
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Fi g u r e 1 . 4 ​ ​Internetworking devices
Bridge
Switch
Router
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!
Although bridges/switches are used to segment networks, they will not
isolate broadcast or multicast packets.
Internetworking Basics 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.
Fi g u r e 1 . 5 ​ ​Switched networks creating an internetwork
Router
Okay, here I’ve placed the LAN switches at the center of the network world so the router
is 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 11. So don’t stress.
But it is really important to understand that even though you have a switched network, you
still need a router (or layer 3 switch) 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 good, 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.
9
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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!
Should I Replace My Existing 10/100Mbps 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 all new switches and is not sure about approving
the expense; do you really need it?
Well, if you can, absolutely! The newest switches really add a lot of functionality to a network that older 10/100Mbps switches just don’t have (yes, five-year-old switches are considered just plain old today). But most of us don’t have an unlimited budget to buy all new
gigabit switches. 10/100Mbps switches can still create a nice network—that is, of course,
if you design and implement the network correctly—but you’ll still have to replace these
switches eventually.
So do you need 1Gbps or better switch ports for all your users, servers, and other devices?
Yes, you absolutely need new higher-end switches! With the new Windows networking
stack and the IPv6 revolution shortly ahead of us, the server and hosts are no longer the
bottlenecks of our internetworks. Our routers and switches are! We need at a minimum
gigabit to the desktop and on every router interface—10Gbps would be better, or even
higher if you can afford it.
So, go ahead! Put that requisition in to buy all new switches.
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.
Internetworking Models
When networks first came into being, computers could typically communicate only
with computers from the same manufacturer. For example, companies ran either a complete DECnet solution or an IBM solution—not both together. In the late 1970s, the
Open Systems Interconnection (OSI) reference model was created by the International
Organization for Standardization (ISO) to break this barrier.
The OSI model was meant to help vendors create interoperable network devices
and software in the form of protocols so that different vendor networks could work
Internetworking Models 11
with each other. Like world peace, it’ll probably never happen completely, but it’s still a
great goal.
The OSI model is the primary architectural model for networks. It describes how data
and network information are communicated from an application on one computer through
the network media to an application on another computer. The OSI reference model breaks
this approach into layers.
In the following section, I am going to explain the layered approach and how we can use
this approach to help us troubleshoot our internetworks.
The Layered Approach
A reference model is a conceptual blueprint of how communications should take place. It
addresses all the processes required for effective communication and divides these processes
into logical groupings called layers. When a communication system is designed in this manner,
it’s known as layered architecture.
Think of it like this: You and some friends want to start a company. One of the
first things you’ll do is sit down and think through what tasks must be done, who
will do them, the order in which they will be done, and how they relate to each other.
Ultimately, you might group these tasks into departments. Let’s say you decide to have
an order-taking department, an inventory department, and a shipping department.
Each of your departments has its own unique tasks, keeping its staff members busy and
requiring them to focus on only their own duties.
In this scenario, I’m using departments as a metaphor for the layers in a communication
system. For things to run smoothly, the staff of each department will have to trust and rely
heavily upon the others to do their jobs and competently handle their unique responsibilities.
In your planning sessions, you would probably take notes, recording the entire process to
facilitate later discussions about standards of operation that will serve as your business blueprint, or reference model.
Once your business is launched, your department heads, each armed with the part
of the blueprint relating to their own department, will need to develop practical methods
to implement their assigned tasks. These practical methods, or protocols, will need to be
compiled into a standard operating procedures manual and followed closely. Each of the
various procedures in your manual will have been included for different reasons and have
varying degrees of importance and implementation. If you form a partnership or acquire
another company, it will be imperative that its business protocols—its business blueprint—
match yours (or at least be compatible with it).
Similarly, software developers can use a reference model to understand computer communication processes and see what types of functions need to be accomplished on any one
layer. If they are developing a protocol for a certain layer, all they need to concern themselves with is that specific layer’s functions, not those of any other layer. Another layer and
protocol will handle the other functions. The technical term for this idea is binding. The
communication processes that are related to each other are bound, or grouped together, at
a particular layer.
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Advantages of Reference Models
The OSI model is hierarchical, and the same benefits and advantages can apply to any
layered model. The primary purpose of all such models, especially the OSI model, is to
allow different vendors’ networks to interoperate.
Advantages of using the OSI layered model include, but are not limited to, the following:
NN
NN
NN
NN
NN
It divides the network communication process into smaller and simpler components,
thus aiding component development, design, and troubleshooting.
It allows multiple-vendor development through standardization of network components.
It encourages industry standardization by defining what functions occur at each layer
of the model.
It allows various types of network hardware and software to communicate.
It prevents changes in one layer from affecting other layers, so it does not hamper
development.
The OSI Reference Model
One of the greatest functions of the OSI specifications is to assist in data transfer between
disparate hosts—meaning, for example, that they enable us to transfer data between a Unix
host and a PC or a Mac.
The OSI isn’t a physical model, though. Rather, it’s a set of guidelines that application
developers can use to create and implement applications that run on a network. It also
provides a framework for creating and implementing networking standards, devices, and
internetworking schemes.
The OSI has seven different layers, divided into two groups. The top three layers define
how the applications within the end stations will communicate with each other and with
users. The bottom four layers define how data is transmitted end to end. Figure 1.6 shows
the three upper layers and their functions, and Figure 1.7 shows the four lower layers and
their functions.
When you study Figure 1.6, understand that the user interfaces with the computer at
the Application layer and also that the upper layers are responsible for applications communicating between hosts. Remember that none of the upper layers knows anything about
networking or network addresses. That’s the responsibility of the four bottom layers.
In Figure 1.7, you can see that it’s the four bottom layers that define how data is transferred through a physical wire or through switches and routers. These bottom layers also
determine how to rebuild a data stream from a transmitting host to a destination host’s
application.
The following network devices operate at all seven layers of the OSI model:
NN
Network Management Stations (NMSs)
NN
Web and application servers
The OSI Reference Model NN
Gateways (not default gateways)
NN
Network hosts
13
Fi g u r e 1 . 6 ​ ​The upper layers
Application
• Provides a user interface
Presentation
• Presents data
• Handles processing such as encryption
• Keeps different applications’
• data separate
Session
Transport
Network
Data Link
Physical
Fi g u r e 1 . 7 ​ ​The lower layers
Transport
• Provides reliable or unreliable delivery
• Performs error correction before retransmit
Network
• Provides logical addressing,
• which routers use for path determination
Data Link
• Combines packets into bytes and bytes into frames
• Provides access to media using MAC address
• Performs error detection not correction
Physical
• Moves bits between devices
• Specifies voltage, wire speed,
• and pin-out of cables
Basically, the ISO is pretty much the Emily Post of the network protocol world. Just as
Ms. Post wrote the book setting the standards—or protocols—for human social interaction,
the ISO developed the OSI reference model as the precedent and guide for an open network
protocol set. Defining the etiquette of communication models, it remains today the most
popular means of comparison for protocol suites.
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The OSI reference model has the following seven layers:
NN
Application layer (layer 7)
NN
Presentation layer (layer 6)
NN
Session layer (layer 5)
NN
Transport layer (layer 4)
NN
Network layer (layer 3)
NN
Data Link layer (layer 2)
NN
Physical layer (layer 1)
Figure 1.8 shows a summary of the functions defined at each layer of the OSI model.
Fi g u r e 1 . 8 ​ ​Layer functions
With this in hand, you’re now ready to explore each layer’s function in detail.
The Application Layer
The Application layer of the OSI model marks the spot where users actually communicate to the computer. This layer comes into play only when it’s apparent that access to the
network is going to be needed soon. Take the case of Internet Explorer (IE). You could
uninstall every trace of networking components from a system, such as TCP/IP, NIC card,
and so on, and you could still use IE to view a local HTML document—no problem. But
things would definitely get messy if you tried to do something like view an HTML document that must be retrieved using HTTP or nab a file with FTP or TFTP. That’s because IE
will respond to requests such as those by attempting to access the Application layer. And
what’s happening is that the Application layer is acting as an interface between the actual
application program—which isn’t at all a part of the layered structure—and the next layer
down by providing ways for the application to send information down through the protocol stack. In other words, IE doesn’t truly reside within the Application layer—it interfaces
with Application layer protocols when it needs to deal with remote resources.
The OSI Reference Model 15
The Application layer is also responsible for identifying and establishing the availability
of the intended communication partner and determining whether sufficient resources for
the intended communication exist.
These tasks are important because computer applications sometimes require more than
only desktop resources. Often, they’ll unite communicating components from more than
one network application. Prime examples are file transfers and email as well as enabling
remote access, network management activities, client/server processes, and information
location. Many network applications provide services for communication over enterprise
networks, but for present and future internetworking, the need is fast developing to reach
beyond the limits of current physical networking.
It’s important to remember that the Application layer is acting as an
interface between the actual application programs. This means that
Microsoft Word, for example, does not reside at the Application layer
but instead interfaces with the Application layer protocols. Chapter 3 will
present some programs that actually reside at the Application layer—for
example, FTP and TFTP.
The Presentation Layer
The Presentation layer gets its name from its purpose: It presents data to the Application
layer and is responsible for data translation and code formatting.
This layer is essentially a translator and provides coding and conversion functions.
A successful data-transfer technique is to adapt the data into a standard format before
transmission. Computers are configured to receive this generically formatted data and
then convert the data back into its native format for actual reading (for example, EBCDIC
to ASCII). By providing translation services, the Presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of
another one.
The OSI has protocol standards that define how standard data should be formatted. Tasks
like data compression, decompression, encryption, and decryption are associated with this
layer. Some Presentation layer standards are involved in multimedia operations too.
The Session Layer
The Session layer is responsible for setting up, managing, and then tearing down sessions
between Presentation layer entities. This layer also provides dialog control between devices,
or nodes. It coordinates communication between systems and serves to organize their communication by offering three different modes: simplex, half duplex, and full duplex. To sum
up, the Session layer basically keeps different applications’ data separate from other applications’ data.
16 Chapter 1 Internetworking
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The Transport Layer
The Transport layer segments and reassembles data into a data stream. Services located in
the Transport layer segment and reassemble data from upper-layer applications and unite it
into the same data stream. They provide end-to-end data transport services and can establish
a logical connection between the sending host and destination host on an internetwork.
Some of you are probably familiar with TCP and UDP already. (But if you’re not, no
worries—I’ll tell you all about them in Chapter 3.) If so, you know that both work at the
Transport layer and that TCP is a reliable service and UDP is not. This means that application developers have more options because they have a choice between the two protocols
when working with TCP/IP protocols.
The Transport layer is responsible for providing mechanisms for multiplexing upperlayer applications, establishing sessions, and tearing down virtual circuits. It also hides
details of any network-dependent information from the higher layers by providing transparent data transfer.
The term reliable networking can be used at the Transport layer. It means
that acknowledgments, sequencing, and flow control will be used.
The Transport layer can be connectionless or connection oriented. However, Cisco is
mostly concerned with you understanding the connection-oriented portion of the Transport
layer. The following sections will provide the skinny on the connection-oriented (reliable)
protocol of the Transport layer.
Flow Control
Data integrity is ensured at the Transport layer by maintaining flow control and by allowing applications to request reliable data transport between systems. Flow control prevents
a sending host on one side of the connection from overflowing the buffers in the receiving
host—an event that can result in lost data. Reliable data transport employs a connectionoriented communications session between systems, and the protocols involved ensure that
the following will be achieved:
NN
The segments delivered are acknowledged back to the sender upon their reception.
NN
Any segments not acknowledged are retransmitted.
NN
Segments are sequenced back into their proper order upon arrival at their destination.
NN
A manageable data flow is maintained in order to avoid congestion, overloading, and
data loss.
The purpose of flow control is to provide a means for the receiver to
govern the amount of data sent by the sender.
The OSI Reference Model 17
Connection-Oriented Communication
In reliable transport operation, a device that wants to transmit sets up a connection-oriented
communication session with a remote device by creating a session. The transmitting device
first establishes a connection-oriented session with its peer system, which is called a call
setup or a three-way handshake. Data is then transferred; when the transfer is finished, a
call termination takes place to tear down the virtual circuit.
Figure 1.9 depicts a typical reliable session taking place between sending and receiving
systems. Looking at it, you can see that both hosts’ application programs begin by notifying
their individual operating systems that a connection is about to be initiated. The two operating
systems communicate by sending messages over the network confirming that the transfer is
approved and that both sides are ready for it to take place. After all of this required synchronization takes place, a connection is fully established and the data transfer begins (this virtual
circuit setup is called overhead!).
Fi g u r e 1 . 9 ​ ​Establishing a connection-oriented session
The Three-Way Handshake
Sender
Receiver
SYN
SYN/ACK
ACK
Connection Established
Data Transfer
(Send Bytes of Segments)
While the information is being transferred between hosts, the two machines periodically
check in with each other, communicating through their protocol software to ensure that all
is going well and that the data is being received properly.
Here’s a summary of the steps in the connection-oriented session—the three-way
handshake—pictured in Figure 1.9:
NN
NN
NN
The first “connection agreement” segment is a request for synchronization.
The next segments acknowledge the request and establish connection parameters—
the rules—between hosts. These segments request that the receiver’s sequencing is
synchronized here as well so that a bidirectional connection is formed.
The final segment is also an acknowledgment. It notifies the destination host that
the connection agreement has been accepted and that the actual connection has been
established. Data transfer can now begin.
18 Chapter 1 Internetworking
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Sounds pretty simple, but things don’t always flow so smoothly. Sometimes during a
transfer, congestion can occur because a high-speed computer is generating data traffic a
lot faster than the network can handle transferring. A bunch of computers simultaneously
sending datagrams through a single gateway or destination can also botch things up nicely.
In the latter case, a gateway or destination can become congested even though no single
source caused the problem. In either case, the problem is basically akin to a freeway bottleneck—too much traffic for too small a capacity. It’s not usually one car that’s the problem;
there are simply too many cars on that freeway.
Okay, so what happens when a machine receives a flood of datagrams too quickly
for it to process? It stores them in a memory section called a buffer. But this buffering
action can solve the problem only if the datagrams are part of a small burst. If not,
and the datagram deluge continues, a device’s memory will eventually be exhausted, its
flood capacity will be exceeded, and it will react by discarding any additional datagrams
that arrive.
No huge worries here, though. Because of the transport function, network flood control
systems really work quite well. Instead of dumping data and allowing data to be lost, the
transport can issue a “not ready” indicator to the sender, or source, of the flood (as shown in
Figure 1.10). This mechanism works kind of like a stoplight, signaling the sending device to
stop transmitting segment traffic to its overwhelmed peer. After the peer receiver processes
the segments already in its memory reservoir—its buffer—it sends out a “ready” transport
indicator. When the machine waiting to transmit the rest of its datagrams receives this “go”
indicator, it resumes its transmission.
Fi g u r e 1 .1 0 ​ ​Transmitting segments with flow control
Sender
Receiver
Transmit
Buffer full
Not ready—
STOP!
Segments
processed
GO!
Transmit
The OSI Reference Model 19
In fundamental, reliable, connection-oriented data transfer, datagrams are delivered to
the receiving host in exactly the same sequence they’re transmitted—and the transmission
fails if this order is breached! If any data segments are lost, duplicated, or damaged along
the way, a failure will occur. This problem is solved by having the receiving host acknowledge that it has received each and every data segment.
A service is considered connection oriented if it has the following characteristics:
NN
A virtual circuit is set up (e.g., a three-way handshake).
NN
It uses sequencing.
NN
It uses acknowledgments.
NN
It uses flow control.
The types of flow control are buffering, windowing, and congestion
avoidance.
Windowing
Ideally, data throughput happens quickly and efficiently. And as you can imagine, it would
be slow if the transmitting machine had to wait for an acknowledgment after sending each
segment. But because there’s time available after the sender transmits the data segment
and before it finishes processing acknowledgments from the receiving machine, the sender
uses the break as an opportunity to transmit more data. The quantity of data segments
(measured in bytes) that the transmitting machine is allowed to send without receiving an
acknowledgment for them is called a window.
Windows are used to control the amount of outstanding, unacknowledged
data segments.
So the size of the window controls how much information is transferred from one end to
the other. While some protocols quantify information by observing the number of packets,
TCP/IP measures it by counting the number of bytes.
As you can see in Figure 1.11, there are two window sizes—one set to 1 and one set to 3.
When you’ve configured a window size of 1, the sending machine waits for an
acknowledgment for each data segment it transmits before transmitting another. If you’ve
configured a window size of 3, it’s allowed to transmit three data segments before an
acknowledgment is received.
In this simplified example, both the sending and receiving machines are workstations. In
reality, this is not done in simple numbers but in the amount of bytes that can be sent.
If a receiving host fails to receive all the bytes that it should acknowledge, the
host can improve the communication session by decreasing the window size.
20 Chapter 1 Internetworking
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Fi g u r e 1 .11 ​ ​Windowing
Acknowledgments
Reliable data delivery ensures the integrity of a stream of data sent from one machine to
the other through a fully functional data link. It guarantees that the data won’t be duplicated or lost. This is achieved through something called positive acknowledgment with
retransmission—a technique that requires a receiving machine to communicate with the
transmitting source by sending an acknowledgment message back to the sender when it
receives data. The sender documents each segment measured in bytes; it then sends and
waits for this acknowledgment before sending the next segment round of bytes. When
it sends a segment, the transmitting machine starts a timer and retransmits if it expires
before an acknowledgment is returned from the receiving end.
In Figure 1.12, the sending machine transmits segments 1, 2, and 3. The receiving node
acknowledges it has received them by requesting segment 4. When it receives the acknowledgment, the sender then transmits segments 4, 5, and 6. If segment 5 doesn’t make it to the
destination, the receiving node acknowledges that event with a request for the segment to be
resent. The sending machine will then resend the lost segment and wait for an acknowledgment, which it must receive in order to move on to the transmission of segment 7.
The Network Layer
The Network layer (also called layer 3) manages device addressing, tracks the location of
devices on the network, and determines the best way to move data, which means that the
The OSI Reference Model 21
Network layer must transport traffic between devices that aren’t locally attached. Routers
(layer 3 devices) are specified at the Network layer and provide the routing services within
an internetwork.
Fi g u r e 1 .1 2 ​ ​Transport layer reliable delivery
Sender
1
2
3
4
Receiver
5
6
1
2
3
4
5
6
Send 1
Send 2
Send 3
Ack 4
Send 4
Send 5
Connection lost!
Send 6
Ack 5
Send 5
Ack 7
It happens like this: First, when a packet is received on a router interface, the destination
IP address is checked. If the packet isn’t destined for that particular router, it will look up the
destination network address in the routing table. Once the router chooses an exit interface,
the packet will be sent to that interface to be framed and sent out on the local network. If the
router can’t find an entry for the packet’s destination network in the routing table, the router
drops the packet.
Two types of packets are used at the Network layer: data and route updates.
Data packets Used to transport user data through the internetwork. Protocols used to
support data traffic are called routed protocols; examples of routed protocols are IP and
IPv6. You’ll learn about IP addressing in Chapters 3 and 4 and IPv6 in Chapter 15.
Route update packets Used to update neighboring routers about the networks connected
to all routers within the internetwork. Protocols that send route update packets are called
routing protocols; examples of some common ones are RIP, RIPv2, EIGRP, and OSPF.
Route update packets are used to help build and maintain routing tables on each router.
Figure 1.13 shows an example of a routing table.
The routing table used in a router includes the following information:
Network addresses Protocol-specific network addresses. A router must maintain a routing
table for individual routed protocol because each routed protocol keeps track of a network
with a different addressing scheme (IP, IPv6, and IPX, for example). Think of it as a street
22 Chapter 1 Internetworking
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sign in each of the different languages spoken by the residents that live on a particular street.
So, if there were American, Spanish, and French folks on a street named Cat, the sign would
read Cat/Gato/Chat.
Interface The exit interface a packet will take when destined for a specific network.
Metric The distance to the remote network. Different routing protocols use different
ways of computing this distance. I’m going to cover routing protocols in Chapters 8 and 9,
but for now, know that some routing protocols (namely RIP) use something called a hop
count (the number of routers a packet passes through en route to a remote network), while
others use bandwidth, delay of the line, or even tick count (1⁄18 of a second).
Fi g u r e 1 .1 3 ​ ​Routing table used in a router
1.0
3.0
1.1
3.1
1.3
2.1
2.2
3.3
E0
S0
S0
E0
1.2
3.2
Routing Table
NET INT Metric
1
E0
0
2
S0
0
3
S0
1
Routing Table
NET INT Metric
1
S0
1
2
S0
0
3
E0
0
And as I mentioned earlier, routers break up broadcast domains, which means that by
default, broadcasts aren’t forwarded through a router. Do you remember why this is a good
thing? Routers also break up collision domains, but you can also do that using layer 2 (Data
Link layer) switches. Because each interface in a router represents a separate network, it must
be assigned unique network identification numbers, and each host on the network connected
to that router must use the same network number. Figure 1.14 shows how a router works in
an internetwork.
Here are some points about routers that you should really commit to memory:
NN
NN
NN
Routers, by default, will not forward any broadcast or multicast packets.
Routers use the logical address in a Network layer header to determine the next hop
router to forward the packet to.
Routers can use access lists, created by an administrator, to control security on the
types of packets that are allowed to enter or exit an interface.
The OSI Reference Model NN
23
Routers can provide layer 2 bridging functions if needed and can simultaneously route
through the same interface.
NN
Layer 3 devices (routers in this case) provide connections between virtual LANs (VLANs).
NN
Routers can provide quality of service (QoS) for specific types of network traffic.
Switching and VLANs and are covered in Chapter 10, “Layer 2 Switching and
Spanning Tree Protocol (STP),” and Chapter 11, “Virtual LANs (VLANs).”
Fi g u r e 1 .1 4 ​ ​A router in an internetwork
FastEthernet0/0
Serial0
FastEthernet0/1
WAN Services
Internet
Each router interface is a broadcast domain.
Routers break up broadcast domains by
default and provide WAN services.
The Data Link Layer
The Data Link layer provides the physical transmission of the data and handles error notification, network topology, and flow control. This means that the Data Link layer will ensure
that messages are delivered to the proper device on a LAN using hardware addresses and will
translate messages from the Network layer into bits for the Physical layer to transmit.
The Data Link layer formats the message into pieces, each called a data frame, and adds
a customized header containing the hardware destination and source address. This added
information forms a sort of capsule that surrounds the original message in much the same way
that engines, navigational devices, and other tools were attached to the lunar modules of the
Apollo project. These various pieces of equipment were useful only during certain stages of
space flight and were stripped off the module and discarded when their designated stage was
complete. Data traveling through networks is similar.
Figure 1.15 shows the Data Link layer with the Ethernet and IEEE specifications. When
you check it out, notice that the IEEE 802.2 standard is used in conjunction with and adds
functionality to the other IEEE standards.
It’s important for you to understand that routers, which work at the Network layer, don’t
care at all about where a particular host is located. They’re only concerned about where networks are located and the best way to reach them—including remote ones. Routers are totally
obsessive when it comes to networks. And for once, this is a good thing! It’s the Data Link
layer that’s responsible for the actual unique identification of each device that resides on a
local network.
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Fi g u r e 1 .1 5 ​ ​Data Link layer
For a host to send packets to individual hosts on a local network as well as transmit packets between routers, the Data Link layer uses hardware addressing. Each time a packet is sent
between routers, it’s framed with control information at the Data Link layer, but that information is stripped off at the receiving router and only the original packet is left completely
intact. This framing of the packet continues for each hop until the packet is finally delivered
to the correct receiving host. It’s really important to understand that the packet itself is never
altered along the route; it’s only encapsulated with the type of control information required
for it to be properly passed on to the different media types.
The IEEE Ethernet Data Link layer has two sublayers:
Media Access Control (MAC) 802.3 Defines how packets are placed on the media.
Contention media access is “first come/first served” access where everyone shares the
same bandwidth—hence the name. Physical addressing is defined here as well as logical topologies. What’s a logical topology? It’s the signal path through a physical topology. Line discipline, error notification (not correction), ordered delivery of frames, and
optional flow control can also be used at this sublayer.
Logical Link Control (LLC) 802.2 Responsible for identifying Network layer protocols
and then encapsulating them. An LLC header tells the Data Link layer what to do with a
packet once a frame is received. It works like this: A host will receive a frame and look in the
LLC header to find out where the packet is destined—say, the IP protocol at the Network
layer. The LLC can also provide flow control and sequencing of control bits.
The switches and bridges I talked about near the beginning of the chapter both work at
the Data Link layer and filter the network using hardware (MAC) addresses. We will look
at these in the following section.
Switches and Bridges at the Data Link Layer
Layer 2 switching is considered hardware-based bridging because it uses specialized hardware
called an application-specific integrated circuit (ASIC). ASICs can run up to gigabit speeds
with very low latency rates.
Latency is the time measured from when a frame enters a port to when it
exits a port.
The OSI Reference Model 25
Bridges and switches read each frame as it passes through the network. The layer 2
device then puts the source hardware address in a filter table and keeps track of which
port the frame was received on. This information (logged in the bridge’s or switch’s filter
table) is what helps the machine determine the location of the specific sending device.
Figure 1.16 shows a switch in an internetwork.
Fi g u r e 1 .1 6 ​ ​A switch in an internetwork
1 2 3 4
Each segment has its own collision domain.
All segments are in the same broadcast domain.
The real estate business is all about location, location, location, and it’s the same way for
both layer 2 and layer 3 devices. Though both need to be able to negotiate the network, it’s
crucial to remember that they’re concerned with very different parts of it. Primarily, layer 3
machines (such as routers) need to locate specific networks, whereas layer 2 machines (switches
and bridges) need to eventually locate specific devices. So, networks are to routers as individual
devices are to switches and bridges. And routing tables that “map” the internetwork are for
routers as filter tables that “map” individual devices are for switches and bridges.
After a filter table is built on the layer 2 device, it will forward frames only to the segment
where the destination hardware address is located. If the destination device is on the same
segment as the frame, the layer 2 device will block the frame from going to any other segments. If the destination is on a different segment, the frame can be transmitted only to that
segment. This is called transparent bridging.
When a switch interface receives a frame with a destination hardware address that
isn’t found in the device’s filter table, it will forward the frame to all connected segments.
If the unknown device that was sent the “mystery frame” replies to this forwarding
action, the switch updates its filter table regarding that device’s location. But in the event
the destination address of the transmitting frame is a broadcast address, the switch will
forward all broadcasts to every connected segment by default.
All devices that the broadcast is forwarded to are considered to be in the same broadcast
domain. This can be a problem; layer 2 devices propagate layer 2 broadcast storms that choke
performance, and the only way to stop a broadcast storm from propagating through an internetwork is with a layer 3 device—a router.
26 Chapter 1 Internetworking
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The biggest benefit of using switches instead of hubs in your internetwork is that each
switch port is actually its own collision domain. (Conversely, a hub creates one large collision
domain.) But even armed with a switch, you still don’t break up broadcast domains by default.
Neither switches nor bridges will do that. They’ll simply forward all broadcasts instead.
Another benefit of LAN switching over hub-centered implementations is that each device
on every segment plugged into a switch can transmit simultaneously—at least, they can as
long as there is only one host on each port and a hub isn’t plugged into a switch port. As
you might have guessed, hubs allow only one device per network segment to communicate
at a time.
The Physical Layer
Finally arriving at the bottom, we find that the Physical layer does two things: It sends bits
and receives bits. Bits come only in values of 1 or 0—a Morse code with numerical values.
The Physical layer communicates directly with the various types of actual communication
media. Different kinds of media represent these bit values in different ways. Some use audio
tones, while others employ state transitions—changes in voltage from high to low and low
to high. Specific protocols are needed for each type of media to describe the proper bit patterns to be used, how data is encoded into media signals, and the various qualities of the
physical media’s attachment interface.
The Physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems. This
layer is also where you identify the interface between the data terminal equipment (DTE)
and the data communication equipment (DCE). (Some old phone-company employees still
call DCE data circuit-terminating equipment.) The DCE is usually located at the service provider, while the DTE is the attached device. The services available to the DTE are most often
accessed via a modem or channel service unit/data service unit (CSU/DSU).
The Physical layer’s connectors and different physical topologies are defined by the OSI
as standards, allowing disparate systems to communicate. The CCNA objectives are only
interested in the IEEE Ethernet standards.
Hubs at the Physical Layer
A hub is really a multiple-port repeater. A repeater receives a digital signal and reamplifies
or regenerates that signal and then forwards the digital signal out all active ports without
looking at any data. An active hub does the same thing. Any digital signal received from a
segment on a hub port is regenerated or reamplified and transmitted out all other ports on
the hub. This means all devices plugged into a hub are in the same collision domain as well
as in the same broadcast domain. Figure 1.17 shows a hub in a network.
Hubs, like repeaters, don’t examine any of the traffic as it enters and is then transmitted
out to the other parts of the physical media. Every device connected to the hub, or hubs, must
listen if a device transmits. A physical star network—where the hub is a central device and
cables extend in all directions out from it—is the type of topology a hub creates. Visually,
the design really does resemble a star, whereas Ethernet networks run a logical bus topology,
meaning that the signal has to run through the network from end to end.
Summary 27
Hubs and repeaters can be used to enlarge the area covered by a single LAN
segment, although I do not recommend this. LAN switches are affordable for
almost every situation.
Fi g u r e 1 .17 ​ ​A hub in a network
A
B
C
D
All devices in the same collision domain.
All devices in the same broadcast domain.
Devices share the same bandwidth.
Summary
Whew! I know this seemed like the chapter that wouldn’t end, but it did—and you made it
through! You’re now armed with a ton of fundamental information; you’re ready to build
upon it and are well on your way to certification.
I started by discussing simple, basic networking and the differences between collision
and broadcast domains.
I then discussed the OSI model—the seven-layer model used to help application developers
design applications that can run on any type of system or network. Each layer has its special
jobs and select responsibilities within the model to ensure that solid, effective communications
do, in fact, occur. I provided you with complete details of each layer and discussed how Cisco
views the specifications of the OSI model.
In addition, each layer in the OSI model specifies different types of devices, and I
described these different devices used at each layer.
Remember that hubs are Physical layer devices and repeat the digital signal to all segments
except the one from which it was received. Switches segment the network using hardware
addresses and break up collision domains. Routers break up broadcast domains (and collision
domains) and use logical addressing to send packets through an internetwork.
28 Chapter 1 Internetworking
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Exam Essentials
Identify the possible causes of LAN traffic congestion. Too many hosts in a broadcast
domain, broadcast storms, multicasting, and low bandwidth are all possible causes of LAN
traffic congestion.
Describe the difference between a collision domain and a broadcast domain. Collision
domain is an Ethernet term used to describe a network collection of devices in which one
particular device sends a packet on a network segment, forcing every other device on that
same segment to pay attention to it. On a broadcast domain, a set of all devices on a network segment hear all broadcasts sent on that segment.
Differentiate a MAC address and an IP address and describe how and when each address
type is used in a network. A MAC address is a hexadecimal number identifying the physical connection of a host. MAC addresses are said to operate on layer 2 of the OSI model. IP
addresses, which can be expressed in binary or decimal format, are logical identifiers that are
said to be on layer 3 of the OSI model. Hosts on the same physical segment locate one another
with MAC addresses, while IP addresses are used when they reside on different LAN segments
or subnets. Even when the hosts are in different subnets, a destination IP address will be converted to a MAC address when the packet reaches the destination network via routing.
Understand the difference between a hub, a bridge, a switch, and a router. Hubs create one
collision domain and one broadcast domain. Bridges break up collision domains but create
one large broadcast domain. They use hardware addresses to filter the network. Switches
are really just multiple-port bridges with more intelligence. They break up collision domains
but create one large broadcast domain by default. Switches use hardware addresses to filter
the network. Routers break up broadcast domains (and collision domains) and use logical
addressing to filter the network.
Identify the functions and advantages of routers. Routers perform packet switching, filtering,
and path selection, and they facilitate internetwork communication. One advantage of routers
is that they reduce broadcast traffic.
Differentiate connection-oriented and connectionless network services and describe
how each is handled during network communications Connection-oriented services
use acknowledgments and flow control to create a reliable session. More overhead is
used than in a connectionless network service. Connectionless services are used to send
data with no acknowledgments or flow control. This is considered unreliable.
Define the OSI layers, understand the function of each, and describe how devices and networking protocols can be mapped to each layer. You must remember the seven layers of
the OSI model and what function each layer provides. The Application, Presentation, and
Session layers are upper layers and are responsible for communicating from a user interface to an application. The Transport layer provides segmentation, sequencing, and virtual
circuits. The Network layer provides logical network addressing and routing through an
internetwork. The Data Link layer provides framing and placing of data on the network
medium. The Physical layer is responsible for taking 1s and 0s and encoding them into a
digital signal for transmission on the network segment.
Written Labs 29
Written Labs
In this section, you’ll complete the following labs to make sure you’ve got the information
and concepts contained within them fully dialed in:
Lab 1.1: OSI Questions
Lab 1.2: Defining the OSI Layers and Devices
Lab 1.3: Identifying Collision and Broadcast Domains
(The answers to the written labs can be found following the answers to the review
questions for this chapter.)
Written Lab 1.1: OSI Questions
Answer the following questions about the OSI model:
1. Which layer chooses and determines the availability of communicating partners along with
the resources necessary to make the connection, coordinates partnering applications, and
forms a consensus on procedures for controlling data integrity and error recovery?
2. Which layer is responsible for converting data packets from the Data Link layer into
electrical signals?
3. At which layer is routing implemented, enabling connections and path selection between
two end systems?
4. Which layer defines how data is formatted, presented, encoded, and converted for use on
the network?
5. Which layer is responsible for creating, managing, and terminating sessions between
applications?
6. Which layer ensures the trustworthy transmission of data across a physical link and is
primarily concerned with physical addressing, line discipline, network topology, error
notification, ordered delivery of frames, and flow control?
7. Which layer is used for reliable communication between end nodes over the network
and provides mechanisms for establishing, maintaining, and terminating virtual circuits;
transport-fault detection and recovery; and controlling the flow of information?
8. Which layer provides logical addressing that routers will use for path determination?
9. Which layer specifies voltage, wire speed, and pinout cables and moves bits between devices?
10. Which layer combines bits into bytes and bytes into frames, uses MAC addressing, and
provides error detection?
11. Which layer is responsible for keeping the data from different applications separate on the
network?
30 Chapter 1 Internetworking
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12. Which layer is represented by frames?
13. Which layer is represented by segments?
14. Which layer is represented by packets?
15. Which layer is represented by bits?
16. Put the following in order of encapsulation:
Packets
Frames
Bits
Segments
17. Which layer segments and reassembles data into a data stream?
18. Which layer provides the physical transmission of the data and handles error notification,
network topology, and flow control?
19. Which layer manages device addressing, tracks the location of devices on the network, and
determines the best way to move data?
20. What is the bit length and expression form of a MAC address?
Written Lab 1.2: Defining the OSI Layers and Devices
Fill in the blanks with the appropriate layer of the OSI or hub, switch, or router device.
Description
This device sends and receives information about the
Network layer.
This layer creates a virtual circuit before transmitting between
two end stations.
This device uses hardware addresses to filter a network.
Ethernet is defined at these layers.
This layer supports flow control, sequencing, and
acknowledgments.
This device can measure the distance to a remote network.
Logical addressing is used at this layer.
Hardware addresses are defined at this layer.
Device or OSI Layer
Written Labs Description
Device or OSI Layer
This device creates one big collision domain and one large
broadcast domain.
This device creates many smaller collision domains, but the
network is still one large broadcast domain.
This device can never run full duplex.
This device breaks up collision domains and broadcast domains.
Written Lab 1.3: Identifying Collision
and Broadcast Domains
1. In the following exhibit, identify the number of collision domains and broadcast domains
in each specified device. Each device is represented by a letter:
A. Hub
B. Bridge
C. Switch
D. Router
A
C
Hub
Switch
Router
B
Bridge
D
31
32 Chapter 1 Internetworking
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Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional questions,
please see this book’s introduction.
1. A receiving host has failed to receive all of the segments that it should acknowledge. What
can the host do to improve the reliability of this communication session?
A. Send a different source port number.
B. Restart the virtual circuit.
C. Decrease the sequence number.
D. Decrease the window size.
2. When a station sends a transmission to the MAC address ff:ff:ff:ff:ff:ff, what type
of transmission is it?
A. Unicast
B. Multicast
C. Anycast
D. Broadcast
3. Which layer 1 devices can be used to enlarge the area covered by a single LAN segment?
(Choose two.)
A. Switch
B. NIC
C. Hub
D. Repeater
E. RJ45 transceiver
4. Segmentation of a data stream happens at which layer of the OSI model?
A. Physical
B. Data Link
C. Network
D. Transport
Review Questions 33
5. Which of the following describe the main router functions? (Choose four.)
A. Packet switching
B. Collision prevention
C. Packet filtering
D. Broadcast domain enlargement
E. Internetwork communication
F. Broadcast forwarding
G. Path selection
6. Routers operate at layer ___. LAN switches operate at layer ___. Ethernet hubs operate at
layer ___. Word processing operates at layer ___.
A. 3, 3, 1, 7
B. 3, 2, 1, none
C. 3, 2, 1, 7
D. 2, 3, 1, 7
E. 3, 3, 2, none
7. When data is encapsulated, which is the correct order?
A. Data, frame, packet, segment, bit
B. Segment, data, packet, frame, bit
C. Data, segment, packet, frame, bit
D. Data, segment, frame, packet, bit
8. Why does the data communication industry use the layered OSI reference model?
(Choose two.)
A. It divides the network communication process into smaller and simpler components,
thus aiding component development, design, and troubleshooting.
B. It enables equipment from different vendors to use the same electronic components,
thus saving research and development funds.
C. It supports the evolution of multiple competing standards and thus provides business
opportunities for equipment manufacturers.
D. It encourages industry standardization by defining what functions occur at each layer
of the model.
E. It provides a framework by which changes in functionality in one layer require changes
in other layers.
9. What are two purposes for segmentation with a bridge?
A. To add more broadcast domains
B. To create more collision domains
C. To add more bandwidth for users
D. To allow more broadcasts for users
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10. Which of the following is not a cause of LAN congestion?
A. Too many hosts in a broadcast domain
B. Adding switches for connectivity to the network
C. Broadcast storms
D. Low bandwidth
11. If a switch has three computers connected to it, with no VLANs present, how many broadcast and collision domains is the switch creating?
A. Three broadcast and one collision
B. Three broadcast and three collision
C. One broadcast and three collision
D. One broadcast and one collision
12. Acknowledgments, sequencing, and flow control are characteristics of which OSI layer?
A. Layer 2
B. Layer 3
C. Layer 4
D. Layer 7
13. Which of the following are types of flow control? (Choose all that apply.)
A. Buffering
B. Cut-through
C. Windowing
D. Congestion avoidance
E. VLANs
14. If a hub has three computers connected to it, how many broadcast and collision domains is
the hub creating?
A. Three broadcast and one collision
B. Three broadcast and three collision
C. One broadcast and three collision
D. One broadcast and one collision
15. What is the purpose of flow control?
A. To ensure that data is retransmitted if an acknowledgment is not received
B. To reassemble segments in the correct order at the destination device
C. To provide a means for the receiver to govern the amount of data sent by the sender
D. To regulate the size of each segment
Review Questions 35
16. Which three statements are true about the operation of a full-duplex Ethernet network?
A. There are no collisions in full-duplex mode.
B. A dedicated switch port is required for each full-duplex node.
C. Ethernet hub ports are preconfigured for full-duplex mode.
D. In a full-duplex environment, the host network card must check for the availability of
the network media before transmitting.
E. The host network card and the switch port must be capable of operating in fullduplex mode.
17. Which of the following is not a benefit of reference models such as the OSI model?
A. It allows changes on one layer to affect operations on all other layers as well.
B. It divides the network communication process into smaller and simpler components,
thus aiding component development, design, and troubleshooting.
C. It allows multiple-vendor development through standardization of network components.
D. It allows various types of network hardware and software to communicate.
18. Which of the following devices do not operate at all levels of the OSI model?
A. Network management stations (NMSs)
B. Routers
C. Web and application servers
D. Network hosts
19. When an HTTP document must be retrieved from a location other than the local machine,
what layer of the OSI model must be accessed first?
A. Presentations
B. Transport
C. Application
D. Network
20. Which layer of the OSI model offers three different modes of communication: simplex,
half duplex, and full duplex?
A. Presentation
B. Transport
C. Application
D. Session
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Answers to Review Questions
1. D. A receiving host can control the transmitter by using flow control (TCP uses windowing
by default). By decreasing the window size, the receiving host can slow down the transmitting host so the receiving host does not overflow its buffers.
2. D. A transmission to the MAC address ff:ff:ff:ff:ff:ff is a broadcast transmission
to all stations.
3. C, D. Not that you really want to enlarge a single collision domain, but a hub (multiport
repeater) will provide this for you.
4. D. The Transport layer receives large data streams from the upper layers and breaks these
up into smaller pieces called segments.
5. A, C, E, G. Routers provide packet switching, packet filtering, internetwork communication, and path selection. Although routers do create or terminate collision domains, this is
not the main purpose of a router, so option B is not a correct answer to this question.
6. B. Routers operate at layer 3. LAN switches operate at layer 2. Ethernet hubs operate at
layer 1. Word processing applications communicate to the Application layer interface, but
do not operate at layer 7, so the answer would be none.
7. C. The encapsulation method is data, segment, packet, frame, bit.
8. A, D. The main advantage of a layered model is that it can allow application developers to
change aspects of a program in just one layer of the layer model’s specifications. Advantages
of using the OSI layered model include, but are not limited to, the following: It divides the
network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting; it allows multiple-vendor development
through standardization of network components; it encourages industry standardization by
defining what functions occur at each layer of the model; it allows various types of network
hardware and software to communicate; and it prevents changes in one layer from affecting
other layers, so it does not hamper development.
9. B, C. Every port on a switch creates a collision domain, which provides more bandwidth
for users.
10. B. Adding switches for connectivity to the network would reduce LAN congestion rather
than cause LAN congestion.
11. C. If a switch has three computers connected to it, with no VLANs present, one broadcast
and three collision domains are created.
12. C. A reliable Transport layer connection uses acknowledgments to make sure all data is
transmitted and received reliably. A reliable connection is defined by a virtual circuit that
uses acknowledgments, sequencing, and flow control, which are characteristics of the
Transport layer (layer 4).
Answers to Review Questions 37
13. A, C, D. The common types of flow control are buffering, windowing, and congestion
avoidance.
14. D. If a hub has three computers connected to it, one broadcast and one collision domain
is created.
15. C. Flow control allows the receiving device to control the transmitter so the receiving
device’s buffer does not overflow.
16. A, B, E. Full duplex means you are using both wire pairs simultaneously to send and receive
data. You must have a dedicated switch port for each node, which means you will not have
collisions. Both the host network card and the switch port must be capable and set to work
in full-duplex mode.
17. A. Reference models prevent, rather than allow, changes on one layer to affect operations
on other layers as well, so the model doesn’t hamper development.
18. B. Routers operate no higher than layer 3 of the OSI model.
19. C. When an HTTP document must be retrieved from a location other than the local
machine, the Application layer must be accessed first.
20. D. The Session layer of the OSI model offers three different modes of communication:
simplex, half duplex, and full duplex.
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Answers to Written Lab 1.1
1. The Application layer is responsible for finding the network resources broadcast from a
server and adding flow control and error control (if the application developer chooses).
2. The Physical layer takes frames from the Data Link layer and encodes the 1s and 0s
into a digital signal for transmission on the network medium.
3. The Network layer provides routing through an internetwork and logical addressing.
4. The Presentation layer makes sure that data is in a readable format for the Applica-
tion layer.
5. The Session layer sets up, maintains, and terminates sessions between applications.
6. PDUs at the Data Link layer are called frames and provide physical addressing, plus
other options to place packets on the network medium.
7. The Transport layer uses virtual circuits to create a reliable connection between two hosts.
8. The Network layer provides logical addressing, typically IP addressing and routing.
9. The Physical layer is responsible for the electrical and mechanical connections
between devices.
10. The Data Link layer is responsible for the framing of data packets.
11. The Session layer creates sessions between different hosts’ applications.
12. The Data Link layer frames packets received from the Network layer.
13. The Transport layer segments user data.
14. The Network layer creates packets out of segments handed down from the Trans-
port layer.
15. The Physical layer is responsible for transporting 1s and 0s (bits) in a digital signal.
16. Segments, packets, frames, bits
17. Transport
18. Data Link
19. Network
20. 48 bits (6 bytes) expressed as a hexadecimal number
Answers to Written Lab 1.3 39
Answers to Written Lab 1.2
Description
Device or OSI Layer
This device sends and receives information about the
Network layer.
Router
This layer creates a virtual circuit before transmitting between
two end stations.
Transport
This device uses hardware addresses to filter a network.
Bridge or switch
Ethernet is defined at these layers.
Data Link and Physical
This layer supports flow control, sequencing, and
acknowledgments.
Transport
This device can measure the distance to a remote network.
Router
Logical addressing is used at this layer.
Network
Hardware addresses are defined at this layer.
Data Link (MAC sublayer)
This device creates one big collision domain and one large
broadcast domain.
Hub
This device creates many smaller collision domains, but the
network is still one large broadcast domain.
Switch or bridge
This device can never run full duplex.
Hub
This device breaks up collision domains and broadcast domains. Router
Answers to Written Lab 1.3
1. Hub: One collision domain, one broadcast domain
2. Bridge: Two collision domains, one broadcast domain
3. Switch: Four collision domains, one broadcast domain
4. Router: Three collision domains, three broadcast domains
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