Senior Acquisitions Editor: Kenyon Brown Development Editor: Kim Wimpsett



Yüklə 22,5 Mb.
Pdf görüntüsü
səhifə13/69
tarix26.10.2019
ölçüsü22,5 Mb.
#29436
1   ...   9   10   11   12   13   14   15   16   ...   69
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.

Complete the following table to express

11001100.00110011.10101010.01010101 in decimal IP address format.



128 64 32 16 8 4 2 1 Decimal

Complete the following table to express

11000110.11010011.00111001.11010001 in decimal IP address format.



128 64 32 16 8 4 2 1 Decimal

Complete the following table to express

10000100.11010010.10111000.10100110 in decimal IP address

format.


128 64 32 16 8 4 2 1 Decimal

3.  Convert the following from binary format to hexadecimal.

Complete the following table to express

11011000.00011011.00111101.01110110 in hexadecimal.



128 64 32 16 8 4 2 1 Hexadecimal

Complete the following table to express

11001010.11110101.10000011.11101011 in hexadecimal.

128 64 32 16 8 4 2 1 Hexadecimal

Complete the following table to express

10000100.11010010.01000011.10110011 in hexadecimal.

128 64 32 16 8 4 2 1 Hexadecimal

Written Lab 2.2: CSMA/CD Operations

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) helps

to minimize collisions in the network, thereby increasing data

transmission efficiency. Place the following steps of its operation in the

order in which they occur after a collision.

All hosts have equal priority to transmit after the timers have expired.

Each device on the Ethernet segment stops transmitting for a short

time until the timers expire.

The collision invokes a random backoff algorithm.

A jam signal informs all devices that a collision occurred.



Written Lab 2.3: Cabling

For each of the following situations, determine whether a straight-

through, crossover, or rolled cable would be used.

1.  Host to host

2.  Host to switch or hub

3.  Router direct to host

4.  Switch to switch

5.  Router to switch or hub

6.  Hub to hub

7.  Hub to switch

8.  Host to a router console serial communication (COM) port

Written Lab 2.4: Encapsulation

Place the following steps of the encapsulation process in the proper order.

Packets or datagrams are converted to frames for transmission on the

local network. Hardware (Ethernet) addresses are used to uniquely

identify hosts on a local network segment.

Segments are converted to packets or datagrams, and a logical address

is placed in the header so each packet can be routed through an

internetwork.

User information is converted to data for transmission on the

network.


Frames are converted to bits, and a digital encoding and clocking

scheme is used.

Data is converted to segments, and a reliable connection is set up

between the transmitting and receiving hosts.



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.  In the accompanying graphic, what is the name for the section of the

MAC address marked as unknown?

A.  IOS


B.  OSI

C.  ISO


D.  OUI

2.  __________on an Ethernet network is the retransmission delay

that’s enforced when a collision occurs.

A.  Backoff

B.  Carrier sense

C.  Forward delay

D.  Jamming

3.  On which type of device could the situation shown in the diagram

occur?


A.  Hub

B.  Switch

C.  Router

D.  Bridge

4.  In the Ethernet II frame shown here, what is the function of the

section labeled “FCS”?

A.  Allows the receiving devices to lock the incoming bit stream.

B.  Error detection

C.  Identifies the upper-layer protocol

D.  Identifies the transmitting device

5.  A network interface port has collision detection and carrier sensing

enabled on a shared twisted-pair network. From this statement, what

is known about the network interface port?

A.  This is a 10 Mbps switch port.



B.  This is a 100 Mb/s switch port.

C.  This is an Ethernet port operating at half-duplex.

D.  This is an Ethernet port operating at full-duplex.

E.  This is a port on a network interface card in a PC.

6.  For what two purposes does the Ethernet protocol use physical

addresses? (Choose two.)

A.  To uniquely identify devices at layer 2

B.  To allow communication with devices on a different network

C.  To differentiate a layer 2 frame from a layer 3 packet

D.  To establish a priority system to determine which device gets to

transmit first

E.  To allow communication between different devices on the same

network

F.  To allow detection of a remote device when its physical address is



unknown

7.  Between which systems could you use a cable that uses the pinout

pattern shown here?

A.  With a connection from a switch to a switch

B.  With a connection from a router to a router

C.  With a connection from a host to a host

D.  With a connection from a host to a switch

8.  In an Ethernet network, under what two scenarios can devices



transmit? (Choose two.)

A.  When they receive a special token

B.  When there is a carrier

C.  When they detect that no other devices are sending

D.  When the medium is idle

E.  When the server grants access

9.  What type of cable uses the pinout shown here?

A.  Fiber optic

B.  Crossover Gigabit Ethernet cable

C.  Straight-through Fast Ethernet

D.  Coaxial

10.  When configuring a terminal emulation program, which of the

following is an incorrect setting?

A.  Bit rate: 9600

B.  Parity: None

C.  Flow control: None

D.  Data bits: 1

11.  Which part of a MAC address indicates whether the address is a

locally or globally administered address?

A.  FCS


B.  I/G bit

C.  OUI

D.  U/L bit

12.  What cable type uses the pinout arrangement shown below?

A.  Fiber optic

B.  Rolled

C.  Straight-through

D.  Crossover

13.  Which of the following is not one of the actions taken in the operation

of CSMA/CD when a collision occurs?

A.  A jam signal informs all devices that a collision occurred.

B.  The collision invokes a random backoff algorithm on the systems

involved in the collision.

C.  Each device on the Ethernet segment stops transmitting for a short

time until its backoff timer expires.

D.  All hosts have equal priority to transmit after the timers have

expired.


14.  Which of the following statements is false with regard to Ethernet?

A.  There are very few collisions in full-duplex mode.

B.  A dedicated switch port is required for each full-duplex node.

C.  The host network card and the switch port must be capable of

operating in full-duplex mode to use full-duplex.

D.  The default behavior of 10Base-T and 100Base-T hosts is 10 Mbps



half-duplex if the autodetect mechanism fails.

15.  In the following diagram, identify the cable types required for

connections A and B.

A.  A= crossover, B= crossover

B.  A= crossover, B= straight-through

C.  A= straight-through, B= straight-through

D.  A= straight-through, B= crossover

16.  In the following image, match the cable type to the standard with

which it goes.

1000Base-T

IEEE 802.3u

1000Base-SX IEEE 802.3

10Base-T

IEEE 802.3ab

100Base-TX IEEE 802.3z

17.  The cable used to connect to the console port on a router or switch is

called a _________cable.

A.  Crossover

B.  Rollover


C.  Straight-through

D.  Full-duplex

18.  Which of the following items does a socket comprise?

A.  IP address and MAC address

B.  IP address and port number

C.  Port number and MAC address

D.  MAC address and DLCI

19.  Which of the following hexadecimal numbers converts to 28 in

decimal?

A.  1c


B.  12

C.  15


D.  ab

20.  What cable type is shown in the following graphic?



A.  Fiber optic

B.  Rollover

C.  Coaxial

D.  Full-duplex



Yüklə 22,5 Mb.

Dostları ilə paylaş:
1   ...   9   10   11   12   13   14   15   16   ...   69




Verilənlər bazası müəlliflik hüququ ilə müdafiə olunur ©azkurs.org 2024
rəhbərliyinə müraciət

gir | qeydiyyatdan keç
    Ana səhifə


yükləyin