CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 26 potx

The devices that are used to connect network segments together include bridges, switches, routers, and gateways.. Figure 4-32 Bridges Segmenting a Network When a bridge receives a frame

the geographical area past what a single LAN can support, as shown in Figure 4-32

The devices that are used to connect network segments together include bridges, switches,

routers, and gateways Switches and bridges operate at the data link layer of the OSI

model The function of the bridge is to make intelligent decisions about whether or not

to pass signals on to the next segment of a network Bridges can also be used to connect

dissimilar protocols and media as with wireless bridges interconnecting Ethernet LANs

in a metropolitan area

Figure 4-32 Bridges Segmenting a Network

When a bridge receives a frame on the network, the destination MAC address is

looked up in the bridge table to determine whether to filter, flood, or copy the frame

onto another segment This decision process occurs as follows:

■ If the destination device is on the same segment as the frame, the bridge blocks

the frame from going on to other segments, as shown in Figure 4-33 This

pro-cess is known as filtering.

■ If the destination device is on a different segment, the bridge forwards the frame

to the appropriate segment, as shown in Figure 4-34

Bridge

Computer A 00-50-DA-0D-F5-2D 00-50-04-7C-2B-01Computer B 00-50-F1-12-8A-00Computer C

Segment 1

To Segment 1

To Segment 2

Computer D 00-50-C2-43-0F-1B 00-50-B5-00-92-8BComputer E 00-50-BA-41-44-3CComputer F

Segment 2

Figure 4-33 Bridges Segmenting a Network: Filtering

Figure 4-34 Bridges Segmenting a Network: Forwarding

Hh

Hh

Xc

In this example, a data packet originates from Computer V and its destination is Computer Xc The packet reaches its final destination and is not broadcast to other segments of the network.

Hh

Hh

Xc

In this example, a data packet originates from Computer V and its destination is Computer Hh The bridge checks its table to determine whether or not to allow the signal to continue to other segments of the network.

■ If the destination address is unknown to the bridge, the bridge forwards the

frame to all segments except the one on which it was received This process is

known as flooding.

If placed strategically, a bridge greatly improves network performance

Switches

Aswitch is sometimes described as a multiport bridge While a typical bridge might

have just two ports (linking two network segments), the switch can have multiple ports

depending on how many network segments are to be linked Like bridges, switches learn

certain information about the data packets that they receive from various computers

on the network They use this information to build forwarding tables to determine the

destination of data being sent by one computer to another computer on the network as

demonstrated in Figure 4-35

Figure 4-35 Switching Table

Although some similarities exist between the two, a switch is a more sophisticated

device than a bridge A bridge determines whether the frame is forwarded to the other

network segment based on the destination MAC address A switch has many ports

with many network segments connected to them A switch chooses the port to which

the destination device or workstation is connected Ethernet switches are becoming

popular connectivity solutions because, like bridges, they improve network performance

(speed and bandwidth)

Switching is a technology that alleviates congestion in Ethernet LANs by reducing

traf-fic and increasing bandwidth Switches often replace shared hubs because they work

with existing cable infrastructures, which improves performance with a minimum of

intrusion into an existing network

0260.8c01.1111

0260.8c01.2222

0260.8c01.1111

0260.8c01.2222

Interface E0 E0 E1 E1

MAC Address 0260.8c01.1111 0260.8c01.2222 0260.8c01.3333 0260.8c01.4444

Today, in data communications, all switching equipment performs two basic operations:

■ Switching data frames—The process by which a frame is received on an input

medium and then transmitted to an output medium

■ Maintenance of switching operations—Switches build and maintain switching

tables and search for loops

Switches operate at much higher speeds than bridges and can support other functional-ity, such as virtual LANs

An Ethernet switch has many benefits, such as allowing many users to communicate in parallel through the use of virtual circuits and dedicated network segments in a virtually collision-free environment, as shown in Figure 4-36 This arrangement maximizes the bandwidth available on the shared medium Another benefit is that moving to a switched LAN environment is very cost effective because existing hardware and cabling can be reused

Figure 4-36 Microsegmentation of the Network via Switches

Wireless Networking Media

A wireless network is an alternative method for connecting a LAN You don’t need

to run any cables, and you can easily move computers Wireless networks use radio frequency (RF), laser, infrared (IR), or satellite/microwaves to carry signals from one

Lab Activity LAN Switches Purchase

In this lab, you are introduced to the variety and prices of network compo-nents out in the market This lab looks specifically at Ethernet switches and NICs

All Traffic Visible on Network Segment

Shared SegmentBefore LAN Switch

After

Multiple Traffic Paths Within Switch

computer to another without a permanent cable connection Wireless signals are

elec-tromagnetic waves that travel through the air No physical medium is necessary for

wireless signals, making them a very versatile way to build a network

A common application of wireless data communication is for mobile use Some examples

of mobile use include commuters, airplanes, satellites, remote space probes, space

shuttles, and space stations

At the core of wireless communication are devices called transmitters and receivers

The source interacts with the transmitter that converts data to electromagnetic (EM)

waves that are then received by the receiver The receiver then converts these

electro-magnetic waves back into data for the destination For two-way communication, each

device requires a transmitter and a receiver Many networking device manufacturers

build the transmitter and receiver into a single unit called a transceiver or wireless

net-work card All devices in wireless LANs (WLANs) must have the appropriate wireless

network card installed

The two most common wireless technologies used for networking are infrared (IR)

and radio frequency (RF) IR technology has its weaknesses Workstations and digital

devices must be in the line of sight of the transmitter to operate An IR-based network

suits environments where all the digital devices that require network connectivity are

in one room IR networking technology can be installed quickly, but the data signals

can be weakened or obstructed by people walking across the room or by moisture in

the air However, new IR technologies that can work out of sight are being developed

RF technology allows devices to be in different rooms or even buildings The limited

range of the radio signals still restricts the use of this kind of network RF technology can

be on single or multiple frequencies A single radio frequency is subject to outside

inter-ference and geographic obstructions Furthermore, a single frequency is easily monitored

by others, which makes the transmissions of data insecure Spread spectrum avoids the

problem of insecure data transmission by using multiple frequencies to increase the

immunity to noise and to make it difficult for outsiders to intercept data transmissions

Security in the Wireless Environment

The exponential growth of networking, including wireless technologies, has led to

increased security risks Increasing the security means increasing the time spent

manag-ing the system

The first level of security in a wireless LAN consists of protecting the radio frequency

waveform itself Wireless access points radiate radio waves over a large area that

is not contained in a physical building, which makes the radio waves accessible to

eavesdroppers and thus increases vulnerability The radio waves of wireless bridges are

concentrated in a beam An eavesdropper must get into the beam path to intercept the communication Therefore, wireless access points usually require better security than wireless bridges

If you think someone might eavesdrop on your LAN radio links, encryption is the key The following sections discuss two wireless security approaches: wired equivalent privacy (WEP) and IEEE 802.1X or Extensible Authentication Protocol (EAP)

WEP WEP is the first step in addressing customer security concerns WEP is a security mech-anism, defined within the 802.11 standard, that is designed to protect the over-the-air transmission between wireless LAN access points and NICs The IEEE 802.11b requires 40-bit encryption keys However, many vendors, such as Cisco, support the optional 128-bit standard

The main goals of WEP are

■ Deny access to the network by unauthorized users who do not possess the appro-priate WEP key

■ Prevent the decoding of captured WLAN traffic that is WEP-encrypted without the possession of the WEP key

WEP uses the RC4 stream cipher that was invented by Ron Rivest of RSA Data Secu-rity, Inc., (RSADSI) for encryption The RC4 encryption algorithm is a symmetric-stream cipher that supports a variable-length key A symmetric cipher uses the same key for both encryption and decryption The key is the one piece of information that must be shared by both the encrypting and decrypting endpoints

Recently, encryption analysts have reported weaknesses in the authentication and WEP encryption schemes in the IEEE 802.11 WLAN standard Improvements on WEP have been developed to address the weaknesses found by encryption analysts However, it is not recommended to use WEP as a sole security mechanism for a WLAN WEP should

be supplemented with additional higher-level security mechanisms such as Virtual Private Networks (VPNs) or firewalls

802.1X/EAP

IEEE 802.1X/Extensible Authentication Protocol (EAP)is an alternative WLAN security approach to WEP, as specified by IEEE 802.11 IEEE 802.1X/EAP focuses on developing

a framework for providing centralized authentication and dynamic key distribution IEEE 802.1X is a standard for port-based network access control EAP allows wireless client adapters that can support different authentication types to communicate with dif-ferent back-end servers, such as Remote Authentication Dial-In User Service (RADIUS)

Cisco Systems has developed a derivation of EAP based on mutual authentication, called

Lightweight EAP (LEAP) Mutual authentication means that both the user and the access

point to which the user is attempting to connect must be authenticated before access

onto the corporate network is allowed Mutual authentication protects enterprises from

unauthorized access points serving as a potential entrance into the network

The Cisco LEAP authentication provides the following benefits:

■ Centralized authentication and key distribution

■ Large-scale enterprise WLAN deployment because of its broad operating system

support and dynamic key derivation

Host LAN Connectivity: NICs and Interfaces

In terms of appearance, a NIC, shown in Figure 4-37 and 4-38, is a printed circuit

board that fits into the expansion slot of a bus on a computer’s motherboard or

peripheral device It is also called a network adapter On laptop/notebook computers,

NICs are usually the size of a credit card Its function is to connect the host device to

the network medium

Figure 4-37 Network Interface Card (Circuit Board)

NICs operate at both Layer 1 and Layer 2 of the OSI model NICs are considered Layer 2

devices because each individual NIC throughout the world carries a unique code, called

a Media Access Control (MAC) address This address controls data communication for

the host on the network Layer 2 devices, such as a bridge or switch, use each individual

NIC’s MAC address This MAC address controls data communication for the host on

the network You learn more about the MAC address in later chapters As its name

implies, the NIC controls the host’s access to the medium For this reason, a NIC also

works at Layer 1 because it looks only at bits and not at any address information or

higher-level protocols NICs typically have the transceiver built-in

Figure 4-38 Network Interface Card (Media Connection)

In some cases, the type of connector on the NIC does not match the type of media that needs to be connected to it A good example is a Cisco 2500 router On the router, the Ethernet interface is an AUI connector and that connector needs to connect to a UTP CAT 5 Ethernet cable To do this, a transceiver (transmitter/receiver) is used The Ethernet transceiver provides the transmit/receive function (because none is built into the Ethernet interface) and at the same time converts one type of signal or connector to another (for example, to connect a 15-pin AUI interface to an RJ-45 jack)

In diagrams, NICs have no standardized symbol It is implied that, when networking devices are attached to network media, a NIC or NIC-like is device present Wherever

a dot is seen on a topology map, it represents either a NIC or an interface (port), which acts like a NIC

Workstation and Server Relationships

By using LAN and WAN technologies, many computers are interconnected to provide services to their users To accomplish this, networked computers take on different roles

or functions in relation to each other Some types of applications require computers

to function as equal partners Other types of applications distribute their work so that one computer functions to serve a number of others in an unequal relationship In either case, two computers typically communicate with each other by using request/ response protocols One computer issues a request for a service, and a second computer receives and responds to that request The requestor takes on the role of a client, and the responder takes on the role of a server

Peer-to-Peer Networks

In a peer-to-peer network, the networked computers act as equal partners, or peers, to

each other Peer-to-peer networks are also referred to as workgroups As peers, each

computer can take on the client function or the server function At one time, for example,

computer A might make a request for a file from computer B, which responds by

serv-ing the file to computer A Computer A functions as client, while B functions as the

server At a later time, computers A and B can reverse roles B, as client, makes a print

request of A, which has a shared printer attached, and A, as server, responds to the

request from B A and B stand in a reciprocal or peer relationship to each other

In a peer-to-peer network, individual users control their own resources They can

decide to share certain files with other users, as shown in Figure 4-39 and Figure 4-40

They might also require passwords before they allow others to access their resources

Because individual users make these decisions, no central point of control or

adminis-tration exists in the network In addition, individual users must back up their own

systems to be able to recover from data loss in case of failures When a computer acts

as a server, the user of that machine might experience reduced performance as the

machine serves the requests made by other systems

Figure 4-39 Sharing Files

Figure 4-40 Shared File

Peer-to-peer networks are relatively easy to install and operate No additional equip-ment is necessary beyond a suitable operating system installed on each computer Most modern desktop operating systems provide support for peer-to-peer networking Because users control their own resources, no dedicated administrators are needed

A peer-to-peer network works well with a small number of computers, perhaps ten

or fewer As networks grow, peer-to-peer relationships become increasingly difficult to coordinate and manage Because they do not scale well, their efficiency decreases rapidly

as the number of computers on the network increases Also, individual users control access to the resources on their computers, which means security might be difficult to maintain The client/server model of network can be used to overcome the limitations

of the peer-to-peer network

Client/Server Networks

In a client-server arrangement, network services are located on a dedicated computer called a server, which responds to the requests of clients, as shown in Figure 4-41 The server is a central computer that is continuously available to respond to a client’s requests for file, print, application, and other services Most network operating systems (NOSs) adopt the form of client-server relationships Typically, desktop computers function as clients and one or more computers with additional processing power, memory, and specialized software function as servers

Lab Activity Building a Peer-to-Peer Network

In this lab, you create a simple peer-to-peer network between two PCs You identify and locate the proper cable, configure workstation IP addresses and

test connectivity using the ping command You also share a folder on one PC

and access it with the other