Senior Acquisitions Editor: Kenyon Brown Development Editor: Kim Wimpsett

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Todd Lammle CCNA Routing and Switching

FIGURE 2.22

PDU and layer addressing

Before we go further in our discussion of

Figure 2.22

, let’s discuss port

numbers and make sure you understand them. The Transport layer uses

port numbers to define both the virtual circuit and the upper-layer

processes, as you can see from

Figure 2.23

FIGURE 2.23

Port numbers at the Transport layer

When using a connection-oriented protocol like TCP, the Transport layer

takes the data stream, makes segments out of it, and establishes a reliable

session by creating a virtual circuit. It then sequences (numbers) each

segment and uses acknowledgments and flow control. If you’re using

TCP, the virtual circuit is defined by the source and destination port

number plus the source and destination IP address and called a socket.

Understand that the host just makes this up, starting at port number

1024 because 0 through 1023 are reserved for well-known port numbers.

The destination port number defines the upper-layer process or

application that the data stream is handed to when the data stream is

reliably rebuilt on the receiving host.

Now that you understand port numbers and how they are used at the

Transport layer, let’s go back to

Figure 2.22

. Once the Transport layer

header information is added to the piece of data, it becomes a segment

that’s handed down to the Network layer along with the destination IP

address. As you know, the destination IP address was handed down from

the upper layers to the Transport layer with the data stream and was

identified via name resolution at the upper layers—probably with DNS.

The Network layer adds a header and adds the logical addressing such as

IP addresses to the front of each segment. Once the header is added to

the segment, the PDU is called a packet. The packet has a protocol field

that describes where the segment came from (either UDP or TCP) so it

can hand the segment to the correct protocol at the Transport layer when

it reaches the receiving host.

The Network layer is responsible for finding the destination hardware

address that dictates where the packet should be sent on the local

network. It does this by using the Address Resolution Protocol (ARP)—

something I’ll talk about more in Chapter 3. IP at the Network layer looks

at the destination IP address and compares that address to its own source

IP address and subnet mask. If it turns out to be a local network request,

the hardware address of the local host is requested via an ARP request. If

the packet is destined for a host on a remote network, IP will look for the

IP address of the default gateway (router) instead.

The packet, along with the destination hardware address of either the

local host or default gateway, is then handed down to the Data Link layer.

The Data Link layer will add a header to the front of the packet and the

piece of data then becomes a frame. It’s called a frame because both a

header and a trailer are added to the packet, which makes it look like it’s

within bookends—a frame—as shown in

Figure 2.22

. The frame uses an

Ether-Type field to describe which protocol the packet came from at the

Network layer. Now a cyclic redundancy check is run on the frame, and

the answer to the CRC is placed in the Frame Check Sequence field found

in the trailer of the frame.

The frame is now ready to be handed down, one bit at a time, to the

Physical layer, which will use bit-timing rules to encode the data in a

digital signal. Every device on the network segment will receive the digital

signal and synchronize with the clock and extract the 1s and 0s from the

digital signal to build a frame. After the frame is rebuilt, a CRC is run to

make sure the frame is in proper order. If everything turns out to be all

good, the hosts will check the destination MAC and IP addresses to see if

the frame is for them.

If all this is making your eyes cross and your brain freeze, don’t freak. I’ll

be going over exactly how data is encapsulated and routed through an

internetwork later, in Chapter 9, “IP Routing.”

The Cisco Three-Layer Hierarchical Model

Most of us were exposed to hierarchy early in life. Anyone with older

siblings learned what it was like to be at the bottom of the hierarchy.

Regardless of where you first discovered the concept of hierarchy, most of

us experience it in many aspects of our lives. It’s hierarchy that helps us

understand where things belong, how things fit together, and what

functions go where. It brings order to otherwise complex models. If you

want a pay raise, for instance, hierarchy dictates that you ask your boss,

not your subordinate, because that’s the person whose role it is to grant

or deny your request. So basically, understanding hierarchy helps us

discern where we should go to get what we need.

Hierarchy has many of the same benefits in network design that it does in

other areas of life. When used properly, it makes networks more

predictable and helps us define which areas should perform certain

functions. Likewise, you can use tools such as access lists at certain levels

in hierarchical networks and avoid them at others.

Let’s face it: Large networks can be extremely complicated, with multiple

protocols, detailed configurations, and diverse technologies. Hierarchy

helps us summarize a complex collection of details into an

understandable model, bringing order from the chaos. Then, as specific

configurations are needed, the model dictates the appropriate manner in

which to apply them.

The Cisco hierarchical model can help you design, implement, and

maintain a scalable, reliable, cost-effective hierarchical internetwork.

Cisco defines three layers of hierarchy, as shown in

Figure 2.24

, each with

specific functions.

FIGURE 2.24

The Cisco hierarchical model

Each layer has specific responsibilities. Keep in mind that the three layers

are logical and are not necessarily physical devices. Consider the OSI

model, another logical hierarchy. Its seven layers describe functions but

not necessarily protocols, right? Sometimes a protocol maps to more than

one layer of the OSI model, and sometimes multiple protocols

communicate within a single layer. In the same way, when we build

physical implementations of hierarchical networks, we may have many

devices in a single layer, or there may be a single device performing

functions at two layers. Just remember that the definition of the layers is

logical, not physical!

So let’s take a closer look at each of the layers now.

The Core Layer

The core layer is literally the core of the network. At the top of the

hierarchy, the core layer is responsible for transporting large amounts of

traffic both reliably and quickly. The only purpose of the network’s core

layer is to switch traffic as fast as possible. The traffic transported across

the core is common to a majority of users. But remember that user data is

processed at the distribution layer, which forwards the requests to the

core if needed.

If there’s a failure in the core, every single user can be affected! This is

why fault tolerance at this layer is so important. The core is likely to see

large volumes of traffic, so speed and latency are driving concerns here.

Given the function of the core, we can now consider some design

specifics. Let’s start with some things we don’t want to do:

Never do anything to slow down traffic. This includes making sure you

don’t use access lists, perform routing between virtual local area

networks, or implement packet filtering.

Don’t support workgroup access here.

Avoid expanding the core (e.g., adding routers when the internetwork

grows). If performance becomes an issue in the core, give preference

to upgrades over expansion.

Here’s a list of things that we want to achieve as we design the core:

Design the core for high reliability. Consider data-link technologies

that facilitate both speed and redundancy, like Gigabit Ethernet with

redundant links or even 10 Gigabit Ethernet.

Design with speed in mind. The core should have very little latency.

Select routing protocols with lower convergence times. Fast and

redundant data-link connectivity is no help if your routing tables are

shot!

The Distribution Layer

The distribution layer is sometimes referred to as the workgroup layer

and is the communication point between the access layer and the core.

The primary functions of the distribution layer are to provide routing,

filtering, and WAN access and to determine how packets can access the

core, if needed. The distribution layer must determine the fastest way

that network service requests are handled—for example, how a file

request is forwarded to a server. After the distribution layer determines

the best path, it forwards the request to the core layer if necessary. The

core layer then quickly transports the request to the correct service.

The distribution layer is where we want to implement policies for the

network because we are allowed a lot of flexibility in defining network

operation here. There are several things that should generally be handled

at the distribution layer:

Routing

Implementing tools (such as access lists), packet filtering, and

queuing

Implementing security and network policies, including address

translation and firewalls

Redistributing between routing protocols, including static routing

Routing between VLANs and other workgroup support functions

Defining broadcast and multicast domains

Key things to avoid at the distribution layer are those that are limited to

functions that exclusively belong to one of the other layers!

The Access Layer

The access layer controls user and workgroup access to internetwork

resources. The access layer is sometimes referred to as the desktop layer.

The network resources most users need will be available locally because

the distribution layer handles any traffic for remote services.

The following are some of the functions to be included at the access layer:

Continued (from distribution layer) use of access control and policies

Creation of separate collision domains (microsegmentation/switches)

Workgroup connectivity into the distribution layer

Device connectivity

Resiliency and security services

Advanced technology capabilities (voice/video, etc.)

Technologies like Gigabit or Fast Ethernet switching are frequently seen

in the access layer.

I can’t stress this enough—just because there are three separate levels

does not imply three separate devices! There could be fewer or there

could be more. After all, this is a layered approach.

Summary

In this chapter, you learned the fundamentals of Ethernet networking,

how hosts communicate on a network. You discovered how CSMA/CD

works in an Ethernet half-duplex network.

I also talked about the differences between half- and full-duplex modes,

and we discussed the collision detection mechanism called CSMA/CD.

I described the common Ethernet cable types used in today’s networks in

this chapter as well, and by the way, you’d be wise to study that section

really well!

Important enough to not gloss over, this chapter provided an

introduction to encapsulation. Encapsulation is the process of encoding

data as it goes down the OSI stack.

Last, I covered the Cisco three-layer hierarchical model. I described in

detail the three layers and how each is used to help design and implement

a Cisco internetwork.

Exam Essentials

Describe the operation of Carrier Sense Multiple Access with

Collision Detection (CSMA/CD). CSMA/CD is a protocol that helps

devices share the bandwidth evenly without having two devices transmit

at the same time on the network medium. Although it does not eliminate

collisions, it helps to greatly reduce them, which reduces retransmissions,

resulting in a more efficient transmission of data for all devices.

Differentiate half-duplex and full-duplex communication and

define the requirements to utilize each method. Full-duplex

Ethernet uses two pairs of wires at the same time instead of one wire pair

like half-duplex. Full-duplex allows for sending and receiving at the same

time, using different wires to eliminate collisions, while half-duplex can

send or receive but not at the same time and still can suffer collisions. To

use full-duplex, the devices at both ends of the cable must be capable of

and configured to perform full-duplex.

Describe the sections of a MAC address and the information

contained in each section . The MAC, or hardware, address is a 48-bit

(6-byte) address written in a hexadecimal format. The first 24 bits, or 3

bytes, are called the organizationally unique identifier (OUI), which is

assigned by the IEEE to the manufacturer of the NIC. The balance of the

number uniquely identifies the NIC.

Identify the binary and hexadecimal equivalent of a decimal

number. Any number expressed in one format can also be expressed in

the other two. The ability to perform this conversion is critical to

understanding IP addressing and subnetting. Be sure to go through the

written labs covering binary to decimal to hexadecimal conversion.

Identify the fields in the Data Link portion of an Ethernet

frame. The fields in the Data Link portion of a frame include the

preamble, Start Frame Delimiter, destination MAC address, source MAC

address, Length or Type, Data, and Frame Check Sequence.

Identify the IEEE physical standards for Ethernet cabling. These

standards describe the capabilities and physical characteristics of various

cable types and include but are not limited to 10Base-2, 10Base-5, and

10Base-T.

Differentiate types of Ethernet cabling and identify their

proper application. The three types of cables that can be created from

an Ethernet cable are straight-through (to connect a PC’s or router’s

Ethernet interface to a hub or switch), crossover (to connect hub to hub,

hub to switch, switch to switch, or PC to PC), and rolled (for a console

connection from a PC to a router or switch).

Describe the data encapsulation process and the role it plays in

packet creation. Data encapsulation is a process whereby information

is added to the frame from each layer of the OSI model. This is also called

packet creation. Each layer communicates only with its peer layer on the

receiving device.

Understand how to connect a console cable from a PC to a

router and switch. Take a rolled cable and connect it from the COM

port of the host to the console port of a router. Start your emulations

program such as putty or SecureCRT and set the bits per second to 9600

and flow control to None.

Identify the layers in the Cisco three-layer model and describe

the ideal function of each layer. The three layers in the Cisco

hierarchical model are the core (responsible for transporting large

amounts of traffic both reliably and quickly), distribution (provides

routing, filtering, and WAN access), and access (workgroup connectivity

into the distribution layer).

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 2.1: Binary/Decimal/Hexadecimal Conversion

Lab 2.2: CSMA/CD Operations

Lab 2.3: Cabling

Lab 2.4: Encapsulation

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

Written Labs.”

Written Lab 2.1: Binary/Decimal/Hexadecimal Conversion

1. Convert from decimal IP address to binary format.

Complete the following table to express 192.168.10.15 in binary

format.

128 64 32 16 8 4 2 1 Binary

Complete the following table to express 172.16.20.55 in binary format.

128 64 32 16 8 4 2 1 Binary

Complete the following table to express 10.11.12.99 in binary format.

128 64 32 16 8 4 2 1 Binary

2. Convert the following from binary format to decimal IP address.