Senior Acquisitions Editor: Kenyon Brown Development Editor: Kim Wimpsett

Part 1

ICND1

Chapter 1

Internetworking

THE FOLLOWING ICND1 EXAM TOPICS ARE

COVERED IN THIS CHAPTER:

Network Fundamentals

1.3 Describe the impact of infrastructure components in an

enterprise network

1.3.a Firewalls

1.3.b Access points

1.3.c Wireless controllers

1.5 Compare and contrast network topologies

1.5.a Star

1.5.b Mesh

1.5.c Hybrid

Welcome to the exciting world of

internetworking. This first chapter will serve as an internetworking

review by focusing on how to connect networks together using Cisco

routers and switches, and I’ve written it with the assumption that you

have some simple basic networking knowledge. The emphasis of this

review will be on the Cisco CCENT and/or CCNA Routing and Switching

(CCNA R/S) objectives, on which you’ll need a solid grasp in order to

succeed in getting your certifications.

Let’s start by defining exactly what an internetwork is: 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.

We’ll also dissect the Open Systems Interconnection (OSI) model, and I’ll

describe each part of it to you in detail because you really need complete,

reliable knowledge of it. Understanding the OSI model is key for the solid

foundation you’ll need to build upon with the more advanced Cisco

networking knowledge gained as you become increasingly more skilled.

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 review questions

and 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/ccna

or the book’s web page via

www.sybex.com/go/ccna

.

Internetworking Basics

Before exploring internetworking models and the OSI model’s

specifications, you need to grasp the big picture and the answer to this

burning question: Why is it so important to learn Cisco internetworking

anyway?

Networks and networking have grown exponentially over the past 20

years, and understandably so. They’ve had to evolve at light speed just to

keep up with huge increases in basic, mission-critical user needs (e.g., the

simple sharing of data and printers) as well as greater burdens like

multimedia remote presentations and conferencing. Unless everyone who

needs to share network resources is located in the same office space—an

increasingly uncommon situation—the challenge is to connect relevant

networks so all users can share the wealth of whatever services and

resources are required.

Figure 1.1

shows a basic local area network (LAN) that’s connected using

a hub, which is basically just an antiquated device that connects wires

together. Keep in mind that a simple network like this would be

considered one collision domain and one broadcast domain. No worries if

you have no idea what I mean by that because coming up soon, I’m going

to talk about collision and broadcast domains enough to make you dream

about them!

FIGURE 1.1

A very basic network

Things really can’t get much simpler than this. And yes, though you can

still find this configuration in some home networks, even many of those

as well as the smallest business networks are more complicated today. As

we move through this book, I’ll just keep building upon this tiny network

a bit at a time until we arrive at some really nice, robust, and current

network designs—the types that will help you get your certification and a

job!

But as I said, we’ll get there one step at a time, so let’s get back to the

network shown in

Figure 1.1

with this scenario: Bob wants to send Sally a

file, and to complete that goal in this kind of network, he’ll simply

broadcast that he’s looking for her, which is basically just shouting out

over the network. Think of it like this: Bob walks out of his house and

yells down a street called Chaos Court in order to contact Sally. This

might work if Bob and Sally were the only ones living there, but not so

much if it’s crammed with homes and all the others living there are

always hollering up and down the street to their neighbors just like Bob.

Nope, Chaos Court would absolutely live up to its name, with all those

residents going off whenever they felt like it—and believe it or not, our

networks actually still work this way to a degree! So, given a choice,

would you stay in Chaos Court, or would you pull up stakes and move on

over to a nice new modern community called Broadway Lanes, which

offers plenty of amenities and room for your home plus future additions

all on nice, wide streets that can easily handle all present and future

traffic? If you chose the latter, good choice… so did Sally, and she now

lives a much quieter life, getting letters (packets) from Bob instead of a

headache!

The scenario I just described brings me to the basic point of what this

book and the Cisco certification objectives are really all about. My goal of

showing you how to create efficient networks and segment them correctly

in order to minimize all the chaotic yelling and screaming going on in

them is a universal theme throughout my CCENT and CCNA series books.

It’s just inevitable that you’ll have to break up a large network into a

bunch of smaller ones at some point to match a network’s equally

inevitable growth, and as that expansion occurs, user response time

simultaneously dwindles to a frustrating crawl. But if you master the vital

technology and skills I have in store for you in this series, you’ll be well

equipped to rescue your network and its users by creating an efficient

new network neighborhood to give them key amenities like the

bandwidth they need to meet their evolving demands.

And this is no joke; most of us think of growth as good—and it can be—

but as many of us experience daily when commuting to work, school, etc.,

it can also mean your LAN’s traffic congestion can reach critical mass and

grind to a complete halt! Again, the solution to this problem begins with

breaking up a massive network into a number of smaller ones—

something called network segmentation. This concept is a lot like

planning a new community or modernizing an existing one. More streets

are added, complete with new intersections and traffic signals, plus post

offices are built with official maps documenting all those street names

and directions on how to get to each. You’ll need to effect new laws to

keep order to it all and provide a police station to protect this nice new

neighborhood as well. In a networking neighborhood environment, all of

this is carried out using devices like routers, switches, and bridges.

So let’s take a look at our new neighborhood now, because the word has

gotten out; many more hosts have moved into it, so it’s time to upgrade

that new high-capacity infrastructure that we promised to handle the

increase in population.

Figure 1.2

shows a network that’s been segmented

with a switch, making each network segment that connects to the switch

its own separate collision domain. Doing this results in a lot less yelling!

FIGURE 1.2

A switch can break up collision domains.

This is a great start, but I really want you to make note of the fact that this

network is still one, single broadcast domain, meaning that we’ve really

only decreased our screaming and yelling, not eliminated it. For example,

if there’s some sort of vital announcement that everyone in our

neighborhood needs to hear about, it will definitely still get loud! You can

see that the hub used in

Figure 1.2

just extended the one collision domain

from the switch port. The result is that John received the data from Bob

but, happily, Sally did not. This is good because Bob intended to talk with

John directly, and if he had needed to send a broadcast instead, everyone,

including Sally, would have received it, possibly causing unnecessary

congestion.

Here’s a list of some of the things that commonly cause LAN traffic

congestion:

Too many hosts in a collision or broadcast domain

Broadcast storms

Too much multicast traffic

Low bandwidth

Adding hubs for connectivity to the network

A bunch of ARP broadcasts

Take another look at

Figure 1.2

and make sure you see that I extended the

main hub from

Figure 1.1

to a switch in

Figure 1.2

. I did that because hubs

don’t segment a network; they just connect network segments. Basically,

it’s an inexpensive way to connect a couple of PCs, and again, that’s great

for home use and troubleshooting, but that’s about it!

As our planned community starts to grow, we’ll need to add more streets

with traffic control, and even some basic security. We’ll achieve this by

adding routers because these convenient devices are used to connect

networks and route packets of data from one network to another. Cisco

became the de facto standard for routers because of its unparalleled

selection of high-quality router products and fantastic service. So never

forget that by default, routers are basically employed to efficiently break

up a broadcast domain—the set of all devices on a network segment,

which are allowed to “hear” all broadcasts sent out on that specific

segment.

Figure 1.3

depicts a router in our growing network, creating an

internetwork and breaking up broadcast domains.

FIGURE 1.3

Routers create an internetwork.

The network in

Figure 1.3

is actually a pretty cool little network. Each

host is connected to its own collision domain because of the switch, and

the router has created two broadcast domains. So now our Sally is happily

living in peace in a completely different neighborhood, no longer

subjected to Bob’s incessant shouting! If Bob wants to talk with Sally, he

has to send a packet with a destination address using her IP address—he

cannot broadcast for her!

But there’s more… routers provide connections to wide area network

(WAN) services as well via a serial interface for WAN connections—

specifically, a V.35 physical interface on a Cisco router.

Let me make sure you understand why breaking up a broadcast domain is

so important. When a host or server sends a network broadcast, every

device on the network must read and process that broadcast—unless you

have a router. When the router’s interface receives this broadcast, it can

respond by basically saying, “Thanks, but no thanks,” and discard the

broadcast without forwarding it on to other networks. Even though

routers are known for breaking up broadcast domains by default, it’s

important to remember that they break up collision domains as well.

There are two advantages to using routers in your network:

They don’t forward broadcasts by default.

They can filter the network based on layer 3 (Network layer)

information such as an IP address.

Here are four ways a router functions in your network:

Packet switching

Packet filtering

Internetwork communication

Path selection

I’ll tell you all about the various layers later in this chapter, but for now,

it’s helpful to think of routers as layer 3 switches. Unlike plain-vanilla

layer 2 switches, which forward or filter frames, routers (layer 3 switches)

use logical addressing and provide an important capacity called packet

switching. Routers can also provide packet filtering via access lists, and

when routers connect two or more networks together and use logical

addressing (IP or IPv6), you then have an internetwork. Finally, routers

use a routing table, which is essentially a map of the internetwork, to

make best path selections for getting data to its proper destination and

properly forward packets to remote networks.

Conversely, we don’t use layer 2 switches to create internetworks because

they don’t break up broadcast domains by default. Instead, they’re

employed to add functionality to a network LAN. The main purpose of

these switches is to make a LAN work better—to optimize its performance

—providing more bandwidth for the LAN’s users. Also, these switches

don’t forward packets to other networks like routers do. Instead, they

only “switch” frames from one port to another within the switched

network. And don’t worry, even though you’re probably thinking, “Wait—

what are frames and packets?” I promise to completely fill you in later in

this chapter. For now, think of a packet as a package containing data.

Okay, so by default, switches break up collision domains, but what are

these things? Collision domain is an Ethernet term used to describe a

network scenario in which one device sends a packet out on a network

segment and every other device on that same segment is forced to pay

attention no matter what. This isn’t very efficient because if a different

device tries to transmit at the same time, a collision will occur, requiring

both devices to retransmit, one at a time—not good! This happens a lot in

a hub environment, where each host segment connects to a hub that

represents only one collision domain and a single broadcast domain. By

contrast, each and every port on a switch represents its own collision

domain, allowing network traffic to flow much more smoothly.

Switches create separate collision domains within a single

broadcast domain. Routers provide a separate broadcast domain for

each interface. Don’t let this ever confuse you!

The term bridging was introduced before routers and switches were

implemented, so it’s pretty common to hear people referring to switches

as bridges. That’s because bridges and switches basically do the same

thing—break up collision domains on a LAN. Note to self that you cannot

buy a physical bridge these days, only LAN switches, which use bridging

technologies. This does not mean that you won’t still hear Cisco and

others refer to LAN switches as multiport bridges now and then.

But does it mean that a switch is just a multiple-port bridge with more

brainpower? Well, pretty much, only there are still some key differences.

Switches do provide a bridging function, but they do that with greatly

enhanced management ability and features. Plus, most bridges had only 2

or 4 ports, which is severely limiting. Of course, it was possible to get

your hands on a bridge with up to 16 ports, but that’s nothing compared

to the hundreds of ports available on some switches!

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 never forget that using hubs in your Ethernet network can

contribute to congestion. As always, plan your network design

carefully!

Figure 1.4

shows how a network would look with all these internetwork

devices in place. Remember, a router doesn’t just break up broadcast

domains for every LAN interface, it breaks up collision domains too.

FIGURE 1.4

Internetworking devices

Looking at

Figure 1.4

, did you notice that the router has the center stage

position and connects each physical network together? I’m stuck with

using this layout because of the ancient bridges and hubs involved. I

really hope you don’t run across a network like this, but it’s still really

important to understand the strategic ideas that this figure represents!

See that bridge up at the top of our internetwork shown in

Figure 1.4

? It’s

there 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. That bridge also created only three collision

domains, one for each port, which means that each device connected to a

hub is in the same collision domain as every other device connected to

that same hub. This is really lame and to be avoided if possible, but it’s

still better than having one collision domain for all hosts! So don’t do this

at home; it’s a great museum piece and a wonderful example of what not

to do, but this inefficient design would be terrible for use in today’s

networks! It does show us how far we’ve come though, and again, the

foundational concepts it illustrates are really important for you to get.

And I want you to notice something else: The three interconnected hubs

at the bottom of the figure also connect to the router. This setup creates

one collision domain and one broadcast domain and makes that bridged

network, with its two collision domains, look majorly better by contrast!

Don’t misunderstand… bridges/switches are used to segment

networks, but they will not isolate broadcast or multicast packets.

The best network connected to the router is the LAN switched 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 really bad? Because all

devices must listen to all broadcasts transmitted, that’s why! And if your

broadcast domains are too large, the users have less bandwidth and are

required to process more broadcasts. Network response time eventually

will slow to a level that could cause riots and strikes, so it’s important to

keep your broadcast domains small in the vast majority of networks

today.

Once there are only switches in our example network, things really

change a lot!

Figure 1.5

demonstrates a network you’ll typically stumble

upon today.

FIGURE 1.5

Switched networks creating an internetwork

Here I’ve placed the LAN switches at the center of this network world,

with the router connecting the logical networks. If I went ahead and

implemented this design, I’ll have created something called virtual LANs,

or VLANs, which are used when you logically break up broadcast

domains in a layer 2, switched network. It’s really important to

understand that even in a switched network environment, you still need a

router to provide communication between VLANs. Don’t forget that!

Still, clearly the best network design is the one that’s perfectly configured

to meet the business requirements of the specific company or client it

serves, and it’s usually one in which LAN switches exist in harmony with

routers strategically placed in the network. It’s my hope that this book

will help you understand the basics of routers and switches so you can

make solid, informed decisions on a case-by-case basis and be able to

achieve that goal! But I digress…

So let’s go back to

Figure 1.4

now for a minute and really scrutinize it

because I want to ask you this question: How many collision domains and

broadcast domains are really there in this internetwork? I hope you

answered nine collision domains and three broadcast domains! The

broadcast domains are definitely the easiest to spot because only routers

break up broadcast domains by default, and since there are three

interface 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 at the bottom is one collision domain; the bridge

network on top equals three collision domains. Add in the switch network

of five collision domains—one for each switch port—and you get a total of

nine!

While we’re at this, in

Figure 1.5

, each port on the switch is a separate

collision domain, and each VLAN would be a separate broadcast domain.

So how many collision domains do you see here? I’m counting 12—

remember that connections between the switches are considered a

collision domain! Since the figure doesn’t show any VLAN information,

we can assume the default of one broadcast domain is in place.

Before we move on to Internetworking Models, let’s take a look at a few

more network devices that we’ll find in pretty much every network today

as shown in

Figure 1.6

.

FIGURE 1.6

Other devices typically found in our internetworks today.

Taking off from the switched network in

Figure 1.5

, you’ll find WLAN

devices, including AP’s and wireless controllers, and firewalls. You’d be

hard pressed not to find these devices in your networks today.

Let’s look closer at these devices:

WLAN devices: These devices connect wireless devices such as

computers, printers, and tablets to the network. Since pretty much

every device manufactured today has a wireless NIC, you just need to

configure a basic access point (AP) to connect to a traditional wired

network.

Access Points or APs: These devices allow wireless devices to connect

to a wired network and extend a collision domain from a switch, and

are typically in their own broadcast domain or what we’ll refer to as a

Virtual LAN (VLAN). An AP can be a simple standalone device, but

today they are usually managed by wireless controllers either in house

or through the internet.

WLAN Controllers: These are the devices that network administrators

or network operations centers use to manage access points in medium

to large to extremely large quantities. The WLAN controller

automatically handles the configuration of wireless access points and

was typically used only in larger enterprise systems. However, with

Cisco’s acquisition of Meraki systems, you can easily manage a small

to medium sized wireless network via the cloud using their simple to

configure web controller system.

Firewalls: These devices are network security systems that monitor

and control the incoming and outgoing network traffic based on

predetermined security rules, and is usually an Intrusion Protection

System (IPS). Cisco Adaptive Security Appliance (ASA) firewall

typically establishes a barrier between a trusted, secure internal

network and the Internet, which is not secure or trusted. Cisco’s new

acquisition of Sourcefire put them in the top of the market with Next

Generation Firewalls (NGFW) and Next Generation IPS (NGIPS),

which Cisco now just calls Firepower. Cisco new Firepower runs on

dedicated appliances, Cisco’s ASA’s, ISR routers and even on Meraki

products.