Tài liệu CCNP Building Scalable Cisco Network ppt

• Given a network specification calling for either a single or a multi-homed interconnection into an ISP’s BGP network, configure the edge routers to properly interconnect into the BGP c

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Building Scalable Cisco Networks Introduction

Overview

Building Scalable Cisco Networks (BSCN) is an instructor-led course presented

by Cisco Systems, Inc training partners to their end-user customers This day course focuses on using Cisco routers connected in local area networks(LANs) and wide area networks (WANs) typically found at medium to largenetwork sites

five-Upon completion of this training course, you will be able to select andimplement the appropriate Cisco IOS services required to build a scalable routednetwork

This chapter highlights the course prerequisites and course highlights as well assome administrative issues It includes the following topics:

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1-2 Building Scalable Cisco Networks Copyright  1999, Cisco Systems, Inc.

Course Objectives

This section lists the course objectives

• Given a network specification calling for a scalable routed network that includes link state protocols and redistribution, implement the appropriate technologies

Upon completion of this course, you will be able to perform the following level tasks:

high-■ Select and configure a scalable IP address solution (including routesummarization) for a branch office environment, given a list ofspecifications

■ Select and implement the technologies necessary to redistribute between and

to support multiple, advanced, IP routing protocols, given a networkspecification

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Copyright  1999, Cisco Systems, Inc Introduction 1-3

Course Objectives (cont.)

• Given a network specification calling for either a single or a multi-homed

interconnection into an ISP’s BGP network, configure the edge routers to properly

interconnect into the BGP cloud

• Given a network specification calling for controlling access to networks or devices, or for minimizing overhead traffic, select and configure the appropriate access list features

■ Configure and test edge router connectivity (either single or multi-homedconnection) into a BGP network, given a network specification

■ Configure access lists, given a need to control access to devices and toselectively reduce overhead traffic in the network

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Course Objectives (cont.)

• Given various network specifications calling for multiple routed and routing protocols, implement case studies that reflect a scalable internetwork

■ Implement the results of case study discussions in a laboratory environment,given a specification containing multiple routed and routing protocols

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Course Topics

This section lists the topics that will be covered in this course

Legend

FastEthernet/

Ethernet Primary Secondary

Course Topics

BGP

AS #2 BGP

AS #1

The figure shows a high-level overview of a network that you should be able tobuild at the end of this class To accomplish this course goal, you will be taughthow to configure Cisco routers with Ethernet LAN and serial WAN interfaces.You will configure the following on a Cisco device:

■ Transmission Control Protocol/Internet Protocol (TCP/IP) and InternetProtocol (IP) addresses

■ Hierarchical addressing using variable length subnet masking (VLSM) androute summarization

■ Routing protocols: Enhanced Internet Gateway Routing Protocol (EIGRP),Open Shortest Path First (OSPF), and Border Gateway Protocol (BGP)

■ Redistribution between different routing protocols

■ Access lists to control IP traffic and routing updates

■ Serial WAN connections over interfaces that use High-Level Data LinkControl (HDLC) and Point-to-Point Protocol (PPP)

■ Serial WAN connections over subinterfaces that use Frame Relayencapsulation

Configuration, verification, and troubleshooting are done with Cisco IOSsoftware

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Prerequisites

This section lists the course’s prerequisites

• Understanding distance

vector routing protocol operation and configuring RIP and IGRP

• Determining when to use

static and default routes and enabling them on a Cisco router

• Working knowledge of the OSI

reference model and the hierarchical model

• Understanding of internetworking

fundamentals

Building Scalable Cisco Networks (BSCN)

• Operating and configuring

a Cisco IOS device

• Working knowledge of the

TCP/IP stack and how to configure a routed protocol, such as IP

Prerequisites

To fully benefit from BSCN, you should already possess certain prerequisiteskills The skills are presented in the following figures These skills can begained from self-paced/instructor-led training sessions and from workexperience These prerequisites are highlighted in the figures and are outlined onthe following pages The participant should have a working knowledge of:

■ Commonly used networking terms, numbering schemes, and topologies

■ The Open System Interconnection (OSI) reference model

■ Operation and configuration of a Cisco router

■ TCP/IP stack and configuration of IP addresses

■ Distance-vector routing protocol (RIP, IGRP) operation and configuration

■ Static and default route usage, implementation, and configuration

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• Verifying router

configurations with available tools like show and debug commands

• Displaying and interpreting a

Cisco routing table

• Enabling an IP standard and

extended access list

• Enabling a WAN serial

connection

• Configuring Frame Relay

PVCs on interfaces and subinterfaces

Prerequisites

The participant should also have a working knowledge of:

■ Contents and interpretation of a Cisco routing table

■ Traffic filtering with standard and extended access lists

■ Verifying router configuration using show and debug command output

■ WAN serial interface configuration using HDLC

■ WAN serial interface configuration using Frame Relay PVCs

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Prerequisites

Successful completion of:

• Internetworking Technologies Multimedia (ITM)

• Plus

One of the following combinations:

• Introduction to Cisco Router Configuration (ICRC) and Cisco LAN Switch Configuration (CLSC)

• Cisco Router and LAN Switches (CRLS)

• Interconnecting Cisco Network Devices (ICND)

The participant should already possess certain knowledge and skills gained in astructured learning environment These skills can be gained from completing theInternetworking Technology Multimedia (ITM) CD-ROM plus a combination ofinstructor-led training sessions These courses are highlighted in the figure andare outlined below:

■ Introduction to Cisco Router Configuration (ICRC) contains routerconfiguration basics and Cisco LAN Switch Configuration (CLSC) containsLAN switch configuration basics

■ Cisco Router and LAN Switches (CRLS) contains router and LAN switchconfiguration basics

■ Interconnecting Cisco Network Devices (ICND) contains router and LANswitch configuration basics

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Participant Role

This section discusses your responsibilities as a student

Student role

• Meet prerequisites

• Introduce yourself

• Ask/answer questions Participant Role

To take full advantage of the information presented in this course, you shouldmeet the prerequisites for this class

Introduce yourself to the instructor and other students who will be working withyou during the five days of this course

You are encouraged to ask any questions relevant to the course materials

If you have pertinent questions concerning other Cisco features and products notcovered in this course, please bring these topics up during breaks or after classand the instructor will try to answer the questions or direct you to an appropriateinformation source

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Welcome: Please Introduce Yourself

• Your name and work location

• Your job responsibilities

• Your internetworking experience

• Your objectives for this week

Introduce yourself, stating your name and the job function you perform at yourwork location

Briefly describe what exposure you have with installing and configuring Ciscorouters, attending Cisco classes, and how your work experience helped you meetthe prerequisites highlighted earlier

You should also state what you expect to learn from this course

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■ Recording your name on a sign-in sheet

■ The starting and anticipated ending time of each class day

■ What materials you can expect to receive during the class

■ The appropriate attire during class attendance

■ Rest room locations

■ What to do in the event of an emergency

■ Class breaks and lunch facilities

■ How to send and receive telephone, email and fax messages

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Sources of Information

This section identifies additional sources of information

To learn more about the subjects covered in this course, feel free to access thefollowing sources of information:

■ Cisco Documentation CD-ROM or www.cisco.com

■ ITM CD-ROM or www.cisco.com

■ Cisco IOS 12.0 Configuration Guide and Command Reference Guide

All of these documents can all be found at the following URL:

http://www.cisco.com

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Course Syllabus

This section discusses the week’s schedule

Course Syllabus

Building Scalable Cisco Networks Introduction

Overview of Scalable Interenetworks

Routing Principles

Extending IP Addressess

Managing Traffic and Access

Configuring IP Access List

Optimizing Routing Update Operation

Scaling IP Addresses in Your Internetwork

Implementing Scalability Features

in Your Internetwork

Scalable Routing Protocol Overview

Configuring OSPF in a Single Area

Interconnecting Multiple OSPF Areas

Configuring Enhanced IGRP

Configuring Basic Border Gateway Protocol

Implementing BGP in Scalable ISP Networks

The following schedule reflects the recommended structure for this course Thisstructure allows enough time for your instructor to present the course

information to you and for you to work through the laboratory exercises Theexact timing of the subject materials and labs depends on the pace of yourspecific class

Module 1, Scalable Internetworks

The purpose of the module is to introduce you to the training room andthe BSCN network environment This section describes the

characteristics of scalable networks and provides a review of routingfundamentals You will also be introduced to methods for extending IPaddresses, such as VLSM and route summarization

Module 1 includes the following chapters:

■ Chapter 1Building Scalable Cisco Networks Introduction

■ Chapter 2Overview of Scalable Internetworks

■ Chapter 3Routing Principles

■ Chapter 4Extending IP AddressesModule 2, Scalable Routing ProtocolsThe purpose of the module is to describe the operation andconfiguration of different, sophisticated, routing protocols

You will learn to configure OSPF, Enhanced IGRP and BGP

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■ Chapter 5Scalable Routing Protocols Overview

■ Chapter 6Configuring OSPF in a Single Area

■ Chapter 7Interconnecting Multiple OSPF Areas

■ Chapter 8Configuring Enhanced IGRP

■ Chapter 9Configuring Basic Border Gateway Protocol

■ Chapter 10Implementing BGP in Scalable ISP NetworksModule 3, Controlling Scalable Internetworks

The purpose of the module is to describe ways to control overheadtraffic, including routing updates, in a growing network environment.You will also learn how to control network access using access lists Inthis section, you will learn about redistributing routes between

different routing protocols such as RIP, IGRP, Enhanced IGRP andOSPF After a discussion on scalable IP address solutions, the modulecompletes with a comprehensive lab implementing most of thescalability features discussed throughout the course

■ Chapter 11Managing Traffic and Access

■ Chapter 12Configuring IP Access Lists

■ Chapter 13Optimizing Routing Update Operation

■ Chapter 14Scaling IP Addresses in Your Internetwork

■ Chapter 15Implementing Scalability Features in YourInternetwork

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Graphic Symbols

This section illustrates symbols that are used throughout the course

Access server

ISDN switch

Data Service Unit/

Channel Service Unit

Web Server

Graphic Symbols

Bridge Switch Router

Ethernet Fast Ethernet Serial Line

DSU/CSU

File Server Personal

computer

Modem

VLAN (Color May Vary)

Hub Network Cloud or

Broadcast Domain

Circuit Switched Line

Multi-layer switch

Network switch

These symbols are used in the graphical presentations of this course to representdevice or connection types

Note The addressing schemes and telephone numbers used in this course are reserved and not to be used in the public network They are used in this course as examples to facilitate learning When building your network, use only the addresses and telephone numbers assigned by your network designer and service provider.

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2

Overview of Scalable Internetworks

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Objectives

Upon completion of this chapter, you will

be able to perform the following tasks:

• Describe the key requirements of a scalable internetwork

• Select a Cisco IOS feature as a solution for a given internetwork requirement

Objectives

This chapter defines scalable internetworks and discusses some of the Cisco IOSfeatures that can be used to meet the needs of these networks Topics include:

■ Objectives

■ Scaling Large Internetworks

■ Characterizing Scalable Internetworks

■ Summary

■ Written Exercise: Overview of Scalable Internetworks

■ Answers to Exercise

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Scaling Large Internetworks

Core

Distribution

Access

Campus Backbone

Branch Office

WAN Backbone

Scaling Large Internetworks

Today’s internetworks need to be scalable because they are experiencingphenomenal growth, primarily due to the increasing demands for connectivity inbusiness and at home What do scalable networks look like? What are therequirements that you, as an administrator, must be aware of when managing thegrowth of your scalable internetwork?

Scalable internetworks are typically described as networks that are experiencingconstant growth They must be flexible and expandable The best-managedscalable internetworks are typically designed following a hierarchical model.This simplifies the management of the internetwork and allows for controlledgrowth without overlooking the network requirements

The graphic illustrates a three-layer hierarchical model The layers are defined asfollows:

■ Core—The core is the central internetwork for the entire enterprise and mayinclude LAN and WAN backbones The primary function of this layer is toprovide an optimized and reliable transport structure

■ Distribution—This represents the campus backbone The primary function ofthis layer is to provide access to various parts of the internetwork, as well asaccess to services

■ Access—This provides access to corporate resources for a workgroup on alocal segment

A hierarchy simplifies things such as addressing and device management Using

an addressing scheme that maps to the hierarchy reduces the chance of youneeding to redo the network addresses as a result of growth Knowing wheredevices are placed in a hierarchy enables you to configure all routers within onelayer in a consistent way because they all must perform similar tasks Routerspecialization allows the best use of the Cisco IOS features discussed in thiscourse

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Defining the Router’s Role in a

Building Backbone

Dial-In Branch

Office Workgroups Telecommuter

WAN Backbone

Defining the Router’s Role in a Hierarchy

The hierarchical model provides a physical topology for building internetworks.Because the hierarchical structure uses three distinct layers that provide uniquefunctionality, the routers placed at each layer also have unique functionality

■ Core routers provide services that optimize communication among routes atdifferent sites or in different logical groupings In addition, core routersprovide maximum availability and reliability Core routers should be able tomaintain connectivity when LAN or WAN circuits fail at this layer

■ Distribution routers control access to resources that are available at the corelayer, and must make efficient use of bandwidth In addition, a distributionrouter must address the quality of service (QoS) needs for different protocols

by implementing policy-based traffic control to isolate backbone and localenvironments

■ Access routers control traffic by localizing broadcasts and service requests tothe access media Access routers must also provide connectivity withoutcompromising network integrity For example, the routers at the access pointmust be able to detect whether a telecommuter dialing in is legitimate, withminimal authentication steps required by the telecommuter

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Scalable internetworks need to be:

• Reliable and available

Characterizing Scalable Internetworks

The key requirements inherent in scalable internetworks are in the following list.This course presents features and technologies that can be used to respond tothese requirements

■ Reliable and available—This includes being dependable and available 24hours, 7 days a week In addition, failures need to be isolated and recoverymust be nonvisible to the end user

■ Responsive—This includes managing the QoS needs for the differentprotocols being used without affecting response at the desktop For example,the internetwork must be able to respond to latency issues common forSystems Network Architecture (SNA) traffic, but still allow for the routing

of desktop traffic, such as IPX, without compromising QoS requirements

■ Efficient—Large internetworks must optimize the use of resources,especially bandwidth Reducing the amount of overhead traffic such asunnecessary broadcasts, service location, and routing updates results in anincrease in data throughput without increasing the cost of hardware or theneed for additional WAN services

■ Adaptable—This includes being able to accommodate disparate networksand interconnect independent network clusters (or islands), as well as tointegrate legacy technologies, such as those running SNA

■ Accessible but secure—This includes the ability to enable connections intothe internetwork using dedicated, dialup, and switched services whilemaintaining network integrity

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Making the Network Reliable

Making the Network Reliable and Available

The internetwork should be reliable and available at all layers, but most critically

at the core layer Core routers are reliable when they can accommodate failures

by rerouting traffic and respond quickly to changes in the network topology Theprotocols that enhance network reliability and availability that the Cisco IOSsupports are as follows:

■ Scalable protocols—Includes Open Shortest Path First (OSPF) and EnhancedIGRP (EIGRP) These protocols provide the following features:

– Reachability—Scalable networks, including those using a hierarchicaldesign, can have a large number of reachable networks or subnetworks.These networks can be subject to reachability problems due to metriclimitations of distance vector routing protocols Scalable routingprotocols such as OSPF and EIGRP use metrics that expand thereachability potential for routing updates because they use cost, ratherthan hop count, as a metric

– Fast convergence time—Scalable protocols can converge quicklybecause of the router’s ability to detect failure rapidly and because eachrouter maintains a network topology map Routers also forward networkchanges quickly to all routers in the network topology

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Making the Network

Responsive

Use routing protocols that

• Use alternate paths

• Load balance

• Use dial backup over WANs

Making the Network Responsive

■ Scalable protocols support additional features, such as:

– Alternate paths—Scalable protocols, such as EIGRP and OSPF, enable

a router to maintain a map of the entire network topology, so when afailure is detected the router can reroute traffic by looking at thenetwork topology and finding another path Enhanced IGRP is also afeasible solution because it keeps a record of alternate routes in case thepreferred route goes away

– Load balancing—Because scalable protocols have a map of the entirenetwork topology, and because of how they maintain their routingtables, they are able to transport data across multiple paths to a givenlocation simultaneously

■ Dial backup—On WAN connections, you can configure backup links whenyou need to do the following:

– Make the primary WAN connection more reliable by configuring one ormore on backup connections

– Increase availability by configuring the backup connections to be usedwhen a primary connection is experiencing congestion

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Making the Network Efficient

Optimize bandwidth utilization using

• Access lists

• Route summarization

• Incremental updates

Optimizing your network at all layers of an internetwork hierarchy is criticalbecause it can reduce potential costs in additional WAN services In this course,the focus is on optimizing your bandwidth Bandwidth optimization is normallydone by reducing the amount of update traffic over a WAN connection, withoutdropping essential routing information, to increase data traffic throughput.Cisco IOS features discussed in this course that help optimize bandwidth use are:

■ Access lists—Can be used to permit or drop (deny) protocol update traffic,data traffic, and broadcast traffic Access lists are available for IP and otherprotocols and can be tailored to meet the needs for each protocol Forexample, an access list can be defined by Transmission Control Protocol(TCP) port or by other criteria, depending on the situation

■ Reduce the number of routing table entries—You can reduce the number ofrouter processing cycles by reducing the overall number of routing entries in

a router’s routing table This can be done using the following Cisco IOSfeatures:

– Route summarization—The number of entries in a routing table can be reduced

by using route aggregation or, as it is more commonly known, route summarization Summarization of routes occurs at major network boundaries for most routing protocols Some IP routing protocols, such as OSPF and Enhanced IGRP, allow manual summarization on arbitrary boundaries within the major network Careful planning and address allocation is required for route summarization to be most effective.

– Incremental updates—Protocols such as Enhanced IGRP and OSPF make more efficient use of bandwidth than distance vector protocols by only sending topology changes rather than the entire routing table contents at fixed intervals.

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Making the Network Efficient (cont.)

■ Dial-on-demand routing (DDR)—Connections for infrequent traffic flow can

be accomplished using DDR Active links are created only after interestingtraffic is detected by the router This “only as required” service replacesdedicated circuits that are charged for even when that link is idle

■ Switched access—Packet-switched networks such as X.25 and Frame Relayoffer the advantage of providing global connectivity through a large number

of service providers with established circuits to most major cities

■ Snapshot routing—Allows peer routers to exchange full distance vectorrouting information upon initial connection, then on a predefined interval.Typically used with ISDN, this feature can reduce WAN costs when usingdistance vector protocols because routing information is exchanged at aninterval you define Between update exchanges, the routing tables for thedistance vector protocols are kept frozen

■ Compression over WANs—Several compression techniques can be used toreduce traffic that is crossing a WAN connection Cisco supports TCP/IPheader compression and data (payload) compression In addition, you canconfigure link compression, which compresses header and data information

in packets that cross point-to-point (leased lines) connections Compression

is accomplished in software by the router before the frame is placed on themedium

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Making the Network Adaptable

Network must support routable and nonroutable traffic

IP

Making the Network Adaptable

Because scalable internetworks experience change frequently, they must be able

to adapt to changes such as:

■ Mixing routable and nonroutable protocols—A network delivering bothroutable and nonroutable traffic has some unique problems Mostnonroutable protocols lack a mechanism to provide flow control and aresensitive to delays in delivery Any delays in delivery or packets arriving out

of order can result in session loss

■ Integrating “islands” of networks—Many companies are integrating islands

of networks that are typically using different protocols in their hierarchicaldesign In this case, you can add any protocols used by the network islands tothe core layer, or create a tunnel in the backbone that will connect the

network islands but not add new protocol traffic to the core backbone

■ Meeting the varying requirements for each protocol in the internetwork—When multiple protocol traffic is present, the network must be balancedbetween the special needs of each protocol

In this course, Cisco IOS features that focus on network adaptability are asfollows:

■ EIGRP—A routing protocol that supports IP, IPX, and AppleTalk traffic

■ Redistribution—You can exchange routing information between networksthat are using different routing protocols

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Making the Network Accessible but Secure

Network should support the necessary connection types

Dialup Dedicated

Packet

PSTN

Making the Network Accessible but Secure

The network should be accessible, particularly at the access layer Access routersneed to connect to a variety of WAN services, yet be secure For example, accessrouters must allow telecommuters to dial in, but be able to differentiate betweenlegitimate and hostile connection attempts Cisco IOS features discussed in thiscourse that support access are as follows:

■ Dedicated and switched WAN support:

– Dedicated access—Cisco routers can be directly connected to basictelephone service or digital services such as T1/E1 This means that youcan create a core WAN infrastructure for heavy traffic loads, then useother access services for sporadic traffic requirements

– Switched access—Cisco routers support Frame Relay, X.25, SMDS, andATM With this variety of support, you can determine which switchedservice, or combination of switched services, to use, based on cost,location, and traffic requirements

■ Exterior protocol support—Cisco IOS supports several exterior protocolsincluding Exterior Gateway Protocol (EGP) and Border Gateway Protocol(BGP) BGP, which is discussed in this course, is often used by InternetService Providers (ISPs) and by organizations that want to connect to ISPs

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Making the Network Accessible but Secure (cont.)

• Secure access to and from each remote site

• Secure access to devices within a network

Authentication Procedure

Central-Site

Making the Network Accessible but Secure (cont.)

Features discussed in this course that support network security are as follows:

■ Access lists—Access lists can be defined to prevent user traffic fromaccessing portions of the network Access lists can also assist in providingsecurity because when they block user traffic effectively, the users

themselves are being denied access to sensitive areas of the network

■ Authentication protocols—On WAN connections using PPP, you canconfigure authentication protocols such as Password Authentication Protocol(PAP) or Challenge Handshake Authentication Protocol (CHAP)

Note For a complete discussion of how Cisco supports security, and how to make your network secure using Cisco products, refer to the “Managing Cisco Network Security” course.

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Summary

• Scalable internetworks must be reliable, responsive, efficient, adaptable, and accessible

• Routers can be specialized based on their location in the internetwork

• Cisco IOS features can be used to meet the requirements of today's scalable

internetworks

Summary

Key points from this chapter include the following:

■ Scalable internetworks must meet several requirements, as listed in thegraphic Note that these requirements are more or less critical to meet at eachlayer of the three-layer hierarchy presented

■ Remember that routers should be configured based on the key functions theyneed to perform at a given layer of the hierarchy

■ The Cisco IOS provides a large number of features, but not all featuresshould be configured on a router Only those features that meet the desirednetwork requirements should be enabled on the router

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Written Exercise: Overview of Scalable Internetworks

Objective: Describe the key requirements of a scalable internetwork.

Objective: Select a Cisco IOS feature as a solution for a given internetwork requirement.

Complete the table by doing the following:

■ Assigning each network problem one of the five requirements listed below. _ Reliable and available

_ Responsive _ Efficient _ Adaptable _ Accessible but secure

■ Listing one or more Cisco IOS features that can be used to correct eachnetwork problem

Network Problem Key Requirement Cisco IOS Feature(s)

Connectivity restrictions Single paths available to all networks

Too much broadcast traffic Convergence problems with metric limitations

Competition for bandwidth Illegal access to services on the internetwork

Single WAN links available to each remote site

Expensive tariffs on WAN links that do not get much use Very large routing tables Integrate networks using legacy protocols

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Answers to Exercise

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Written Exercise: Overview of Scalable Internetworks

Connectivity restrictions Accessible but secure ■ Dedicated and switched

access technologies

■ BGP support Single paths available to all networks Reliable and available ■ Scalable protocols

■ Dial backup Too much broadcast traffic Efficient ■ Access lists

■ Scalable protocols Convergence problems with metric limitations Reliable and available ■ Scalable protocols Competition for bandwidth Efficient ■ Access lists

■ Snapshot routing

■ Compression over WANs

■ Generic Traffic Shaping Illegal access to services on the internetwork Accessible but secure ■ Access lists (not an

■ Incremental updates Integrate networks using legacy protocols Adaptable ■ Bridging mechanisms

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It includes the following topics:

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Objectives

This section lists the chapter’s objectives

Objectives

Upon completion of this chapter, you will be able to perform the following tasks:

operation

the topology, analyze the routing table, and test connectivity using accepted troubleshooting techniques

Objectives

This chapter covers a review of IP addressing and routing principles Thedifference between distance vector and link-state routing protocol behavior isexplained and an example of each is presented in a Case Study Convergenceissues surrounding the most commonly used interior routing protocols (RIP,IGRP, Enhanced IGRP and OSPF) are also presented

Upon completion of this chapter, you will be able to perform the following tasks:

■ List the major classes of IP addresses and describe the default maskassociated with each

■ Describe the requirements for subnetting a classful network

■ Compare and contrast the two major functions performed by a router

■ Describe, in detail, the functionality of each field in a routing table entry

■ Explain the convergence steps used by the following routing protocols

– Routing Information Protocol (RIP)

– Interior Gateway Routing Protocol (IGRP)

– Enhanced IGRP

– Open Shortest Path First (OSPF)

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■ Default Routing Masks

■ Creating Subnets by Extending the Mask

■ Subnetting Examples

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IP Address Overview

Boundary between network and host is determined by a 32-bit mask

Once the network portion of the address is defined, all devices on the network will share the same binary pattern in the network portion

An IP address contains two parts: a network part and a host part The boundarybetween the two parts of the IP address is defined by another 32-bit field, referred

to as a routing mask There is a bit-for-bit alignment between the IP address andthe routing mask The routing mask contains a field of all ones and a field of allzeros

The routing mask contains contiguous ones starting at the left and moving to theright The routing mask also contains a field of contiguous zeros starting at theright and moving to the left Where the contiguous ones stop indicates theboundary between the network part and the host part of the IP address Thenetwork boundary can occur at any place after the eighth bit position from theleft Once the boundary between the network part and the host part of the IPaddress is known, all devices addressed in that network will have a commonbinary pattern in the network part that identifies the device as belonging to thespecified network

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IP Address Classes

IP address and associated mask are represented in 32-bit dotted decimal

Other formats are commonly used

Decimal value in the first octet determines the Class of the address

do not use binary numbers on a daily basis and have therefore adopted otherformats to represent 32-bit IP addresses Some common formats include decimal(base 10) and hexadecimal (base 16) notation

The generally accepted method of representing IP addresses and routing masks is

to break the 32-bit field into four groups of eight bits and to represent those bit fields in a decimal format separated by decimal points Hence the reference to

eight-IP addresses and routing masks being represented in 32-bit dotted decimalnotation

Although the dotted decimal notation is commonly accepted, that notation meansnothing to the routing device because the device internally uses the 32-bit binarystring as an address identifier All routing decisions are based on the 32-bit binaryfield

IP addresses belong to ‘classes’ defined by the decimal value represented in thefirst eight bits (octet) The decimal number ranges are strictly dictated by thebinary weights of the ones and zeros patterns within the octet The class definition

is referred to the ‘First Octet Rule’ Classes A through E are defined, but onlyClass A, B and C addresses are used to identify devices connected to the Internet.The two remaining classes are used for special or testing purposes

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IP Address Default Masks

IP address Class determines the default mask

• A = 255.0.0.0

• B = 255.255.0.0

• C = 255.255.255.0

A bit for bit alignment exists between

an IP address and its associated mask

IP Address Default Masks

Once the class of address is known, the number of bits in the default routing mask

is also known By default, routing masks contain one or more octets of contiguousones that define the network part of the IP address To simplify the class

representation, the fields of contiguous ones are limited to the fields separated bythe decimal points

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Which Class of address is 201.222.5.12?

Determining IP Address Class

Default Mask

Determining IP Address Class

To determine the class of an IP address, one only has to match the decimal value

in the first octet to the accepted number ranges Once the class of address isknown, the default routing mask is also known

In the example above, because 201 is in the Class C range of addresses there is adefault 24-bit routing mask The bit position where the 24 contiguous one bits enddefines the end of the network part of the address Therefore, all of the bits thatrepresent 201.222.5 are part of the network part of the IP address As describedearlier, the routing mask will help routing devices forward traffic to the networkdefined by the binary bits in the network portion of the IP address

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Subnetting Overview

Moving the network boundary to the right creates additional subnetworks at the expense of fewer hosts on each segment

The new network mask will contain additional contiguous ones indicating by how many bits the network portion has been extended

The formula 2n, where n equals the number of extended bits, indicates the maximum number

of subnets created

Subnetting Overview

When additional contiguous ones are added to the default routing mask, the ones field is longer and that extends the definition of the network part of an IPaddress Adding bits to the network part of an address is responsible fordecreasing the number of bits in the host part Creating additional network binarypatterns is done at the expense of the number of host devices that can occupy eachnetwork segment

all-The number of bits added to the point where the default routing mask endedcreates a counting range This binary range counts ‘sub’ networks Each count is aunique binary pattern and defines a location within the master network addressspace

The remaining bits not allocated as the network part or the subnetwork part form

a counting range for hosts Host addresses are selected from these remaining bitsand must also be numerically unique from all other hosts on this network

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Extending the mask creates subnets

** Special usage

Creating Subnets

In the example above, the network part has been extended and is indicated by thenew mask of 255.255.255.224, which is three bits longer than the default mask of255.255.255.0 Once the default routing mask has been extended, in this case bythree bits, it creates a counting range to represent subnetworks

For the sample network of 201.222.5.0, the fourth octet now contains twocounting ranges; a three-bit field for counting subnetworks and a five-bit field forcounting hosts Each counting range is displayed with individual binary weightsassigned to the bit positions As is true for eight-bit binary fields discussed earlier,the least significant bit (LSB) has a binary weight of one and is always located atthe right side of the field Each count is an assignment for a subnetwork or adiscrete host

Reminder: The count of all zeros and of all ones in the host range is reserved forspecial usage The count of all zeros represents the segment identifier and thecount of all ones represents a broadcast address used to contact all hosts on thatsegment

Reminder: The count of all zeros in the subnetwork range must be explicitlyallowed The count of all ones has no special significance in the subnetwork rangeand identifies a location within the major network

Only after all of the binary counts have been performed, can the 32 binary bits bebroken into four eight-bit fields and represented as a dotted decimal value Onlyafter the binary bits have been represented in dotted decimal notation does the IPaddress have significance to the network administrator An example of the dotteddecimal notation for the counting ranges is shown on the following page

Tiêu đề	Building Scalable Cisco Networks
Trường học	Cisco Systems, Inc.
Chuyên ngành	Networking
Thể loại	Khóa học
Năm xuất bản	1999
Thành phố	San Jose

Định dạng
Số trang	719
Dung lượng	5,16 MB