CCNA Routing and Switching 200-125 Certification Guide

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By Lazaro (Laz) Diaz
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  1. Internetworking Models

About this book

Cisco Certified Network Associate (CCNA) Routing and Switching is one of the most important qualifications for keeping your networking skills up to date. CCNA Routing and Switching 200-125 Certification Guide covers topics included in the latest CCNA exam, along with review and practice questions. This guide introduces you to the structure of IPv4 and IPv6 addresses and examines in detail the creation of IP networks and sub-networks and how to assign addresses in the network.

You will then move on to understanding how to configure, verify, and troubleshoot layer 2 and layer 3 protocols.

In addition to this, you will discover the functionality, configuration, and troubleshooting of DHCPv4. Combined with router and router simulation practice, this certification guide will help you cover everything you need to know in order to pass the CCNA Routing and Switching 200-125 exam.

By the end of this book, you will explore security best practices, as well as get familiar with the protocols that a network administrator can use to monitor the network.

Publication date:
October 2018


Chapter 1. Internetworking Models

Before we begin our trek into the world of networking, let's take a quick look back on how it began. In 1962, J.C.R. Licklider and W. Clark coined the Galactic Network concept, which encompasses social interaction.

It wasn't until 1969 that the first version of ARPANET (internet) went online. It connected four devices from four different universities: the University of Utah, Stanford Research Institute, UCLA, and the UCSB.

So, in our world of information technology, we have been trying since the 60s to communicate with each other using different types of networks.

The Defense Department was one of the first institutions to develop a system by which we could communicate across the world in case a major catastrophe occurred. Yes, we had the phone system, post office, even through air and sea we could send information across the globe, but we were not satisfied with the scalability, interoperability, and efficiency of how these particular networks operated, and you basically had to have a PhD to even get close to one those monstrosities called computers.

Not until the mid-1980s did computer networks start to appear more in small to large businesses. This was due to the powers that be; they created the TCP/IP network suite and allowed the rest of world to transmit information throughout their business, making them more efficient at getting information into the hands of the people that need it, in a way that was quick and required little effort.

Let me explain that last line – required little effort: you could simply share a file, folder, or drive on your network and people could navigate to it and access that information without leaving the comfort of their desk.

So, networks came to the rescue; using a cabling system, internetworking devices, and protocols, we could do the same work at lightning speed.


The following topics will be covered in the following chapter:

  • Internetworking devices 
  • Network topologies
  • The OSI model

Internetworking devices

With all that said, let's begin by talking about internetworking devices and the role they play on the network.


This is the most intelligent device that exists on the network. It handles all the traffic in your network and sends it to the proper destination. Routers have an Internetworking Operating System (IOS) that allows the router to have a set of features that will allow you to configure it for the specifications needed on your network to get that data across:

Routers have the following components you need to be aware of, not only for your certification, but for real-world applications: ROM, RAM, NVRAM, and Flash—each of these components serves a unique purpose.

For now, you need to know that routers create multiple collision domains and multiple broadcast domains, and they work on layer three, or the network layer, of the OSI model. Don't fret; we will be getting to that shortly.



Switches come in different flavors, meaning they could have different functionalities depending on the IOS that they had and the needs of your network. For certification purposes, layer-two switches will be the focus of our studies, but we will briefly cover some layer-three switching features:

The main purpose of a switch on a network is functionality. The switch is where all your devices will be connected for them to communicate with each other, but the switch offers a lot of features we can use to our advantage, in making our network more efficient. The following bullet points concern some of those features:

  • VLANs
  • Switchport security
  • Spanning Tree Protocol
  • EtherChannel

And there is much more, depending on the IOS you have. The switch also has the same components as the router, but it maintains a VLAN database file that you need to be aware of. Once again, all of these features and their details will be revealed later in the book.


Bridges are like switches, but they are much more limited, with fewer ports, are software-based instead of hardware-based, and offer fewer features:

Bridges operates at layer two and their main function on the network is to segment the network. They also create multiple collision domains and broadcast domains.


Hubs are not used on a network in today's IT world. Hubs are unintelligent devices. They are a layer one device; their main function is to act like a multiport repeater. It will create one collision domain and one broadcast domain, which is a very bad thing, especially in an Ethernet network. But this will be explained in detail later.

Just remember not to use hubs in your network, because they will slow it down.

Network cabling

I know what you are saying, Cabling is not an internetworking device, but know that when building, repairing, or enhancing a network, the type of network cabling used is very important. The following diagram shows the typical CAT5e cabling used to connect end devices to internetworking devices to allow them to communicate. We will discuss cabling more in depth later, but for now just keep it in the back of your mind:


Network topologies

Alright, now that you have been introduced to the internetworking devices, let's talk about topologies. First, let's define what a topology is. There are two types of topologies: you have the physical topology, which is how the network is physically connected. The other is the logical topology, which is how the path of the data flows. It depends on several factors, such as routing protocols, internetworking devices used, and the bandwidth configured on the interfaces of those internetworking devices.

But let's begin with the basics.

The Bus topology

Bus topologies use a primary cable, to which all end devices are connected. The data travels along this cable, hence the name Bus. The problem is that, at the time this type of topology existed, we were using coaxial cabling that at speeds of 10 Mbps, which is considered slow using today's standards. It was considered a shared medium, because the bandwidth was divided up based on how many computers you had connected. The following diagram shows the basic structure of Bus topology:

In this topology, Ethernet technology was used, which uses an access method called Carrier Sense Multiple Access Collision Detection (CSMA/CD). CSMA/CD is the method in Ethernet that end devices use, to be able to transmit their data. As I explained previously, if a device hears any noise on the wire, it will not transmit, it will wait until all noise is gone and then it will send its data. It could be that one node or device does not hear the other device, and both end devices are attempting to send at the same time. That will cause a collision; at that point, a jamming signal is sent, packets are dropped, and a countdown begins to see who transmits; the one whose countdown ends gets to send first.

So, imagine not the five nodes that you see in the figure, but hundreds of nodes trying to communicate. It's insane, since this type of topology creates only one collision domain and one broadcast domain that is running on half-duplex. It was not scalable and hard to troubleshoot, hence, not feasible at all.

Besides all that, if you do not terminate both ends of the cable, you will create something called reflection, which the signal that is on the wire reflect onto the cable continuously, creates noise so no one can transmit. The same thing would happen if your cable were cut somewhere in between; that is why troubleshooting this network was a nightmare. But, let's put the icing on the cake: if you don't ground one side of the cable, if a power surge hits your cable, it could fry all your nodes attached to the cable.

The Bus topology was not going to become the wave of the future.

The Star topology

In this topology, all devices are connected to a central device, in this case a layer-two switch. This is still using the Ethernet access method of CSMA/CD. But, since the media that is transferring the data is a switch, each port on a switch is a private collision domain, so you can have full-duplex, which will allow you to send and receive data. If one of the cables from an end device breaks, only that device will not be able to communicate on the network:

Even though you have increased the number of collision domains and they are private collision domains, which allows for greater bandwidth, one problem still exists: you have, by default, one broadcast domain. This means that when someone transmits on the network, everyone connected to that device, or to be more specific, VLAN 1, which is the native VLAN that all end devices connect to, will also hear that noise and still slow down your network.


The good news is that with a layer two or layer three switch, you can create multiple VLANs. You can logically segment your network so that when someone transmits within their own VLAN, no one else hears that noise.

To explain the obvious about this Star topology, you might be thinking, Hey, that doesn't look like a star, and you would be right. Just because they called it a Star, does not mean you are going to design your physical network in such a manner. It simply means you are connecting your devices to a central point where all devices can communicate:

The preceding illustration shows the reality of a common network design. You will run your cable from the office, cubicle, or classroom to the communications closet and terminate your cable at the patch panel. This in turn gets connected to the switch using patch cables, which then gets connected to the router.

With all that said and illustrated, I hope that clears up the Star topology definition.


The Ring topology

As illustrated in the following diagram, a token ring network is represented as a circle or ring, but there is more to token ring networks. A token ring network uses a central device called a Multi-Station Access Unit or Media Access Unit (MAU) and its purpose is to connect all end devices to it:

The MAU is not circular; it is rectangular and one could say it looks like a switch. There is a huge difference between them; an MAU has two ports called Ring in and Ring out to connect to other MAUs and concentrator ports for the end devices.

This MAU connects all these devices in a logical circular pattern, but the physical topology is that of a star.

The type of access method is called token passing and is deterministic in nature, unlike Ethernet which is contention-based. By this, I mean a token ring has an empty, free-flowing token that goes around the network waiting for someone to seize the token and send data. Only the person with that token can transmit, and once the token is seized, no other token is generated. Therefore, no one else can transmit until that token has been released by the destination end device back into the network.



With the token ring, there are no collisions and it was reliable, but the speed of it was just too slow. Again, the popularity for designing, implementing, and using a token ring network simply did not catch on for use on LANs.

On WANs, we did have the Fiber Distributed Data Interface (FDDI) which used token ring technology and ran it up to gigabit speed. But, as you go through this book, the token ring will not be mentioned at all; it is considered an older technology and, for LANs, it is not used. Also, for your certification you will not need to know this information. Just think of it as information to have in your back pocket for interviews and dinner parties.


The OSI model

For anything to work properly and for us to understand how things work, we need to have some sort of standards or blueprints that will allow us to clear the concepts of how particular objects interoperate with each other.

So, for us to be able to network with different types of devices and understand what it takes to get information from a source to a destination, the International Standards Organization (ISO) came up with a conceptual blueprint called the OSI model. This model is in a seven-layer approach that helps us understand this concept and allows vendors to create devices that can interoperate with each other.

This conceptual layered blueprint gives each layer a responsibility; each layer has a job to do, specific to that layer. You can think of it as a company; every business has departments and each department is responsible for a specific role that the company requires to operate smoothly. If any department within the company fails to do their job, the company will fail to carry out its primary objective.

The cool thing is you can change employees within the department and, if they are trained or at least knowledgeable in their respective field, it will not affect the outcome of what that company is trying to do.

The same goes for networks, each layer of the OSI model has a job to do and if vendors make changes to one layer, it won't affect the other layer from doing its job.


Let's go ahead and look at this seven-layer OSI model:

Layer number

Layer name

Brief description



Works closest to the user, data



Deals with the format of the data



Keeps different applications' data separate



Provides reliable or unreliable delivery of information, segmentation



Provides logical addressing, which routers use to route traffic, packets


Data Link

Deals with frames, error correction, and uses the MAC address to access media



Deals with bits, voltage, cabling

Now that we have seen the OSI model, for certification purposes, you must know each layer number and name, not to mention be able to recognize or define what job that layer is responsible for.

So, let's break down the OSI model into two parts: the upper layers and the lower layers.

The upper layers

Looking at the following three upper layers, we can understand that these layers work closest with user interaction, and how it will communicate with other end devices.

So, let's start defining each layer, starting from the top and working our way down:


Layer number

Layer name

Brief description



Works closest to the user, data



Deals with the format of the data



Keeps different applications' data separate

The Application layer

This layer is the closest to the user, because it is the interface between an actual application and the next layer down.

People get confused with this layer because of its name. It does not mean that an application lives at that layer, such as IE or MS Word; it is the interface that allows the user to interact with it.

Any time we use any browser or Office application, the Application layer is involved, but that is not the only thing the Application layer does, it makes sure that the receiving end is ready to communicate and accept your incoming data.

So, for certification purposes, we need to remember the protocols that work on this layer: HTTP, HTTPS, FTP, TFTP, SNMP, DNS, POP, IMAP, TELNET, and any network service looking for communication across a large network.

The Presentation layer

This layer's function is very simple to remember: it is responsible for data translation and code formatting. When devices transmit information, it is coded in a certain format; an example used everywhere is ASCII, so when the data gets to its destination, it needs to understand this format, it should be able decode the ASCII, and present it to the Application layer so the user will be able to read it. A simpler example would be an Excel spreadsheet, or a picture taken with a proprietary software that you don't have. If you do not have the software installed on your computer and someone sends you a file with an extension of .xls, .doc, .ppt, and so on, your operating system will not understand it and simply place a generic icon wherever you save it, and if you try to open it you will get a dialogue box asking which program you would like to use to open the file with.

The Presentation layer is also responsible for key functions, such as data compression, decompression, encryption, and decryption.

The Session layer

The common definition for this layer is setting up, managing, and breaking down sessions between Presentation layer objects, and keeps user data separate. So, basically, it is like having a dialogue control while monitoring the type of mode the client/server communication has, such as full-duplex or half-duplex communication.

Full-duplex communication is pretty much like a conversation you would have with a person, or over the phone – it is two-way communication. Whereas, half-duplex is like a walkie-talkie; you talk, then you listen. So, you can either send or receive at any given time.

The lower layers

Simply stated, the following layers define how information will be transmitted from the source to the destination:



Provides reliable or unreliable delivery of information, segmentation



Provides logical addressing, which routers use to route traffic, packets


Data Link

Deals with frames, error correction, and uses a MAC address to access media



Deals with bits, voltage, cabling


We now have a better understanding of the OSI model. By breaking them up into two parts, we can see the overall picture of what they are trying to achieve. But we must go in deeper and break down the OSI into its individual layers.

The Transport layer

This is the layer that segments and reassembles data. Services that live on this layer take all data coming from the Application layer and combine it into a succinct data stream.

This layer holds two very important protocols: the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).

The TCP is known as the connection-oriented protocol, which means it will provide reliable transmission, compared to UDP, which does not.

Let's define what exactly Connection-Oriented Communication is. In reliable transmission, we have something called a three-way handshake. The process consists of the source sending a SYN packet to the receiver. If the receiver is ready, it will reply with a SYN/ACK, and then the sender replies with an ACK, then communication can occur and transfer data.

Let's see a visual:

Remember that your topologies and your internetworking devices have a lot do with it as well. In a Star topology, everyone is connected to a central device. If you use a hub, you are in a shared collision domain, running at half-duplex and it's Ethernet, which uses the CSMA/CD access method.

Your network will burn to the ground in no time. That is why the internetworking device you use, the cables you run, and the protocols you use play a very important role in your network.

Luckily for us, we have a fail-safe solution called flow control and windowing.

Flow control prevents the sending device from overflowing the buffers on the receiving side.

The protocols that are involved with reliable communications make sure the following happens:

  • Segments are acknowledged back to the sender
  • Segments that do not get acknowledged are retransmitted

Services that are considered to be connection oriented have the following characteristics:

  • Three-way handshake
  • Uses sequencing
  • Uses ACKs
  • Uses flow control



Windowing is the process to check how much information the receiver can handle in one segment.

This window is adjustable, based on how much information is coming in.

Imagine two people are unloading a truck full of boxes. You've got the sender, the guy on the truck you have the receiver, the guy on the warehouse floor. So, the unloading begins, one box at a time. After a while, the receiver says, Hey! Send two boxes at a time. The window got bigger so the receiver sends two boxes at a time and, if something happens along the way, the receiver will let the sender know, Hey I did not get that box, send it again.

The same principle is applied when using reliable networking, ACKs, NACKs, sequencing, and windowing.

The Network layer

The Network layer is my favorite layer. This layer is where all the routing of the packets that take place in your segment or remote segments. The Network layer works with routed protocols, such as IPv4 or IPv6, routing protocols, such as RIPv2, RIPng, EIGRP, EIGRP for IPv6 and OSPF, and OSPFv3. I will be explaining these protocols in more depth later in the book.

The Network layer creates a routing table that stores all the routes it learns from the routing protocols or static routes that one enters manually. By default, all routers know who they are connected to. So, when a source decides to send a packet to a destination not within its own segment, it will need a layer three device, such as a router, to send the information to the proper destination.

If a router receives a packet with a network destination that is not in the routing table, the router will simply drop the packet and send you an error statement: Destination host unreachable or you could get Request Timeout. These two errors have different meanings, the first is that an entry for that network was never found, and the second is that the destination router has no entry or path to get back.

So, clearly, when we configure routers or any layer-three device, we must be very careful when inputting the IP addresses and subnet mask on their interface. When you configure any routing protocol, make sure you input the network addresses you are directly connected to. Routers will always choose the shortest path to a destination based on its metric; this will determine the path the packet will take to the destination.


Let's define some of the terms used:

  • Routed protocols: These are the protocols that sit on an interface, such as IPv4 and IPv6. These protocols will have a subnetted scheme, so data can be routed by a routing protocol that chooses the appropriate network.
  • Routing protocols: These are the components that create the routing table based on their algorithm, which will use the routed protocol's IP information to obtain the network address, and then route protocols to the correct destination.
  • Metric: This is a measurement of how far the destination is from the source; depending on the routing protocol in use, it will use the shortest metric to get to the destination.

Let's continue to the next layer.

The Data Link layer

This layer provides the physical transmission of information, and handles flow control, physical network topology, and error notification. At the Data Link layer, each message is translated into a data frame and this frame will have customized information in it, such as the source and destination hardware address.

The Data Link layer does not perform any routing at layer two. It simply uses these physical addresses of the end devices to get from source to destination within the same segment.

Routers do not care about layer-two addressing, they are more concerned with layer-three addressing.

Be careful with that statement, because if you're using Ethernet technology, at this point layer-two addressing becomes very important to the router.

The Data Link layer is divided into two sublayers.

Media Access Control layer

In this sub layer, packets are placed on the media, depending on the technology used, such as Contention-Based or Token-Passing. As you know, physical addressing, that is, the MAC address or burned-in address of an NIC card, is used through the physical topology as well as the logical topology.



Logical Link Control (LLC)

Here, the responsibilities change to identifying network protocols and then passing them on to encapsulate them. The LLC header will always tell the Data-Link layer what to do with a packet once the frame is received.

The Physical layer

This layer is responsible for sending bits from the source to the destination on whatever media it is using.

Remember, even though in theory we say 0's and 1's, it is really electrical impulses that are generated and sent through the air as a Carrier Wave; or through cabling, that might need specific encoding and decoding, such as serial cables. In this layer, you'll find devices such as hubs, repeaters, amplifiers, cabling, even a modem at the client side, known as a channel service unit/data service unit.

As far as your certification is concerned, you only need to know IEEE basic information where the OSI is concerned.

One last thing I would like to leave you with before we move on to more exciting and adventurous topics is encapsulation and Protocol Data Units (PDUs). But a visual will be much better:

As packets flow down the OSI model, they will get encapsulated with the proper protocols, error corrections, and any other information they need to get to reach destination. Once they reach the destination, they will be de-encapsulated back into the original data format.

The process of encapsulation is a called data, segments, packets, frames, and bits or you could think of it as Don't, Stop, Pouring, Free, Beer.

These are called the PDUs, which communicates with their peer layer to make sure everything is in order.



We have learned quite a bit. We looked at basic topology types and devices on a network. Then we covered the OSI model. Finally, we looked at the different layers of the OSI model in detail.

In the next chapter, we will learn about fundamentals of Ethernet networking and data encapsulation.

About the Author

  • Lazaro (Laz) Diaz

    Lazaro (Laz) Diaz, as he prefers to be called, is a Cisco Instructor who has been in both the field and the teaching industry for almost two decades. He currently lives in the State of Florida, USA and migrated from Cuba at the age of six (6) years old. He is a Network Engineer and holds several certifications in CCNA R/S, CCNA Security, CCNA Voice, CCNP, Security+, Network+, A+, MCP, MCTS and the MOS.

    However, he is best known for his self-paced courses, teaching over 100K students around the globe. His first book is called The Only IP Book You Will Ever Need! However, he knows that this CCNA study guide will be a testament to his commitment in giving students an opportunity to achieve the CCNA R/S certification.

    Browse publications by this author

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