Senior Acquisitions Editor: Kenyon Brown Development Editor: Kim Wimpsett

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 the 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 as well as collision

domains and use logical addressing to send packets through an

internetwork.

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. With a broadcast domain, a set of all

devices on a network hears all broadcasts sent on all segments.

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.

Understand the difference between a hub, a bridge, a switch,

and a router. A hub creates one collision domain and one broadcast

domain. A bridge breaks up collision domains but creates 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. Bridges and 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

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

You can find the answers to these labs in Appendix A, “Answers to

Written Labs.”

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 cable pinouts 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?

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.  Rearrange 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 logical device addressing, tracks the location of

devices on the internetwork, 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

Device or

OSI Layer

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.

This device creates one collision domain and one

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

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

www.lammle.com/ccna

.

You can find the answers to these questions in Appendix B, “Answers to

Review Questions.”

1.  Which of the following statements is/are true with regard to the

device shown here? (Choose all that apply.)

A.  It includes one collision domain and one broadcast domain.

B.  It includes 10 collision domains and 10 broadcast domains.

C.  It includes 10 collision domains and one broadcast domain.

D.  It includes one collision domain and 10 broadcast domains.

2.  With respect to the OSI model, which one of the following is the

correct statement about PDUs?

A.  A segment contains IP addresses.

B.  A packet contains IP addresses.

C.  A segment contains MAC addresses.

D.  A packet contains MAC addresses.

3.  You are the Cisco administrator for your company. A new branch

office is opening and you are selecting the necessary hardware to

support the network. There will be two groups of computers, each

organized by department. The Sales group computers will be assigned

IP addresses ranging from 192.168.1.2 to 192.168.1.50. The

Accounting group will be assigned IP addresses ranging from 10.0.0.2

to 10.0.0.50. What type of device should you select to connect the two

groups of computers so that data communication can occur?

A.  Hub

B.  Switch

C.  Router

D.  Bridge

4.  The most effective way to mitigate congestion on a LAN would be to

________.

A.  Upgrade the network cards

B.  Change the cabling to CAT 6

C.  Replace the hubs with switches

D.  Upgrade the CPUs in the routers

5.  In the following work area, draw a line from the OSI model layer to its

PDU.

6.  What is a function of the WLAN Controller?

A.  To monitor and control the incoming and outgoing network traffic

B.  To automatically handle the configuration of wireless access points

C.  To allow wireless devices to connect to a wired network

D.  To connect networks and intelligently choose the best paths

between networks

7.  You need to provide network connectivity to 150 client computers that

will reside in the same subnetwork, and each client computer must be

allocated dedicated bandwidth. Which device should you use to

accomplish the task?

A.  Hub

B.  Switch

C.  Router

D.  Bridge

8.  In the following work area, draw a line from the OSI model layer

definition on the left to its description on the right.

Layer

Description

Transport Framing

Physical

End-to-end connection

Data Link Routing

Network

Conversion to bits

9.  What is the function of a firewall?

A.  To automatically handle the configuration of wireless access points

B.  To allow wireless devices to connect to a wired network

C.  To monitor and control the incoming and outgoing network traffic

D.  To connect networks and intelligently choose the best paths

between networks

10.  Which layer in the OSI reference model is responsible for determining

the availability of the receiving program and checking to see whether

enough resources exist for that communication?

A.  Transport

B.  Network

C.  Presentation

D.  Application

11.  Which of the following correctly describe steps in the OSI data

encapsulation process? (Choose two.)

A.  The Transport layer divides a data stream into segments and may

add reliability and flow control information.

B.  The Data Link layer adds physical source and destination

addresses and an FCS to the segment.

C.  Packets are created when the Network layer encapsulates a frame

with source and destination host addresses and protocol-related

control information.

D.  Packets are created when the Network layer adds layer 3 addresses

and control information to a segment.

E.  The Presentation layer translates bits into voltages for

transmission across the physical link.

12.  Which of the following layers of the OSI model was later subdivided

into two layers?

A.  Presentation

B.  Transport

C.  Data Link

D.  Physical

13.  What is a function of an access point (AP)?

A.  To monitor and control the incoming and outgoing network traffic

B.  To automatically handle the configuration of wireless access point

C.  To allow wireless devices to connect to a wired network

D.  To connect networks and intelligently choose the best paths

between networks

14.  A_________is an example of a device that operates only at the

physical layer.

A.  Hub

B.  Switch

C.  Router

D.  Bridge

15.  Which of the following is not a benefit of using a reference model?

A.  It divides the network communication process into smaller and

simpler components.

B.  It encourages industry standardization.

C.  It enforces consistency across vendors.

D.  It allows various types of network hardware and software to

communicate.

16.  Which of the following statements is not true with regard to routers?

A.  They forward broadcasts by default.

B.  They can filter the network based on Network layer information.

C.  They perform path selection.

D.  They perform packet switching.

17.  Switches break up_______domains, and routers break

up_______domains.

A.  broadcast, broadcast

B.  collision, collision

C.  collision, broadcast

D.  broadcast, collision

18.  How many collision domains are present in the following diagram?

A.  8

B.  9

C.  10

D.  11

19.  Which of the following layers of the OSI model is not involved in

defining how the applications within the end stations will

communicate with each other as well as with users?

A.  Transport

B.  Application

C.  Presentation

D.  Session

20.  Which of the following is the only device that operates at all layers of

the OSI model?

A.  Network host

B.  Switch

C.  Router

D.  Bridge

Chapter 2

Ethernet Networking and Data Encapsulation

THE FOLLOWING ICND1 EXAM TOPICS ARE

COVERED IN THIS CHAPTER:

Network Fundamentals

1.6 Select the appropriate cabling type based on implementation

requirements

1.4 Compare and contrast collapsed core and three-tier

architectures

LAN Switching Technologies

2.2 Interpret Ethernet frame format

Before we begin exploring a set of key

foundational topics like the TCP/IP DoD model, IP addressing,

subnetting, and routing in the upcoming chapters, I really want you to

grasp the big picture of LANs conceptually. The role Ethernet plays in

today’s networks as well as what Media Access Control (MAC) addresses

are and how they are used are two more critical networking basics you’ll

want a solid understanding of as well.

We’ll cover these important subjects and more in this chapter, beginning

with Ethernet basics and the way MAC addresses are used on an Ethernet

LAN, and then we’ll focus in on the actual protocols used with Ethernet at

the Data Link layer. To round out this discussion, you’ll also learn about

some very important Ethernet specifications.

You know by now that there are a whole bunch of different devices

specified at the various layers of the OSI model and that it’s essential to

be really familiar with the many types of cables and connectors employed

to hook them up to the network correctly. I’ll review the types of cabling

used with Cisco devices in this chapter, demonstrate how to connect to a

router or switch, plus show you how to connect a router or switch via a

console connection.

I’ll also introduce you to a vital process of encoding data as it makes its

way down the OSI stack, known as encapsulation.

I’m not nagging at all here—okay, maybe just a little, but promise that

you’ll actually work through the four written labs and 20 review questions

I added to the end of this chapter just for you. You’ll be so happy you did

because they’re written strategically to make sure all the important

material covered in this chapter gets locked in, vault-tight into your

memory. So don’t skip them!

To find up-to-the-minute updates for this chapter, please see

www.lammle.com/ccna

or the book’s web page via

www.sybex.com/go/ccna

.

Ethernet Networks in Review

Ethernet is a contention-based media access method that allows all hosts

on a network to share the same link’s bandwidth. Some reasons it’s so

popular are that Ethernet is really pretty simple to implement and it

makes troubleshooting fairly straightforward as well. Ethernet is also

readily scalable, meaning that it eases the process of integrating new

technologies into an existing network infrastructure, like upgrading from

Fast Ethernet to Gigabit Ethernet.

Ethernet uses both Data Link and Physical layer specifications, so you’ll

be presented with information relative to both layers, which you’ll need to

effectively implement, troubleshoot, and maintain an Ethernet network.

Collision Domain

In Chapter 1, “Internetworking,” you learned that the Ethernet term

collision domain refers to a network scenario wherein one device sends a

frame out on a physical network segment forcing every other device on

the same segment to pay attention to it. This is bad because if two devices

on a single physical segment just happen to transmit simultaneously, it

will cause a collision and require these devices to retransmit. Think of a

collision event as a situation where each device’s digital signals totally

interfere with one another on the wire.

Figure 2.1

shows an old, legacy

network that’s a single collision domain where only one host can transmit

at a time.

FIGURE 2.1

Legacy collision domain design

The hosts connected to each hub are in the same collision domain, so if

one of them transmits, all the others must take the time to listen for and

read the digital signal. It is easy to see how collisions can be a serious

drag on network performance, so I’ll show you how to strategically avoid

them soon!

Okay—take another look at the network pictured in

Figure 2.1

. True, it

has only one collision domain, but worse, it’s also a single broadcast

domain—what a mess! Let’s check out an example, in

Figure 2.2

, of a

typical network design still used today and see if it’s any better.

FIGURE 2.2

A typical network you’d see today

Because each port off a switch is a single collision domain, we gain more

bandwidth for users, which is a great start. But switches don’t break up

broadcast domains by default, so this is still only one broadcast domain,

which is not so good. This can work in a really small network, but to

expand it at all, we would need to break up the network into smaller

broadcast domains or our users won’t get enough bandwidth! And you’re

probably wondering about that device in the lower-right corner, right?

Well, that’s a wireless access point, which is sometimes referred as an AP

(which stands for access point). It’s a wireless device that allows hosts to

connect wirelessly using the IEEE 802.11 specification and I added it to

the figure to demonstrate how these devices can be used to extend a

collision domain. But still, understand that APs don’t actually segment

the network, they only extend them, meaning our LAN just got a lot

bigger, with an unknown amount of hosts that are all still part of one

measly broadcast domain! This clearly demonstrates why it’s so

important to understand exactly what a broadcast domain is, and now is

a great time to talk about them in detail.

Broadcast Domain

Let me start by giving you the formal definition: broadcast domain refers

to a group of devices on a specific network segment that hear all the

broadcasts sent out on that specific network segment.

But even though a broadcast domain is usually a boundary delimited by

physical media like switches and routers, the term can also refer to a

logical division of a network segment, where all hosts can communicate

via a Data Link layer, hardware address broadcast.

Figure 2.3

shows how a router would create a broadcast domain

boundary.

Here you can see there are two router interfaces giving us two broadcast

domains, and I count 10 switch segments, meaning we’ve got 10 collision

domains.

The design depicted in

Figure 2.3

is still in use today, and routers will be

around for a long time, but in the latest, modern switched networks, it’s

important to create small broadcast domains. We achieve this by building

virtual LANs (VLANs) within our switched networks, which I’ll

demonstrate shortly. Without employing VLANs in today’s switched

environments, there wouldn’t be much bandwidth available to individual

users. Switches break up collision domains with each port, which is

awesome, but they’re still only one broadcast domain by default! It’s also

one more reason why it’s extremely important to design our networks

very carefully.