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1啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊(fm.DocWikiCISCO/zslcn编译注)五．RoutingBasics路由基本原理:56五．RoutingBasics路由基本原理FromDocWikiJumpto:navigation,searchThisarticleintroducestheunderlyingconceptswidelyusedinroutingprotocols.Topicssummarizedhereincluderoutingprotocolcomponentsandalgorithms.Inaddition,theroleofroutingprotocolsisbrieflycontrastedwiththeroleofroutedornetworkprotocols.本章介绍广泛用于路由协议的基本概念。在这里概述的论题包括路由协议的组成部分和算法。另外,将路由协议的作用与被路由或网络协议的作用作一简短的对比。在第VII部分的章节路由协议s,里,会更详细地讲述特殊的路由协议s,而使用路由协议s的网络协议,是在第VI部分,网络协议s,中讨论。Contents[hide]1WhatIsRouting?2RoutingComponents2.1PathDetermination2.1.1Figure:Destination/NextHopAssociationsDeterminetheData’sOptimalPath2.2Switching2.2.1Figure:NumerousRoutersMayComeintoPlayDuringtheSwitchingProcess3RoutingAlgorithms3.1DesignGoals指导目录Contents联网技术手册指导目录Part1InternetworkingBasics互联网络联网基本原理Part2LANTechnologies局域网技术part3WANTechnologies广域网技术part4InternetProtocols互联网协议part5BridgingandSwitching桥接和交换part6Routing路由part7NetworkManagement网络管理part8Voice/DataIntegrationTechnologies声音/数据集成技术part9WirelessTechnologies无线技术part10CableAccessTechnologies有线接入技术part11Dial-upTechnology拨号技术part12SecurityTechnologies安全技术part13QualityofServiceNetworking联网服务质量part14NetworkCachingTechnologies网络缓存技术part15IBMNetworkManagementIBM网络管理part16MultiserviceAccessTechnologies多业务接入技术23.1.1Figure:SlowConvergenceandRoutingLoopsCanHinderProgress3.2AlgorithmTypes3.2.1StaticVersusDynamic3.2.2Single-PathVersusMultipath3.2.3FlatVersusHierarchical3.2.4Host-IntelligentVersusRouter-Intelligent3.2.5IntradomainVersusInterdomain3.2.6Link-StateVersusDistanceVector3.2.7RoutingMetrics4NetworkProtocols5ReviewQuestions1.WhatIsRouting?什么是路由？Routingistheactofmovinginformationacrossaninternetworkfromasourcetoadestination.Alongtheway,atleastoneintermediatenodetypicallyisencountered.Routingisoftencontrastedwithbridging,whichmightseemtoaccomplishpreciselythesamethingtothecasualobserver.TheprimarydifferencebetweenthetwoisthatbridgingoccursatLayer2(thelinklayer)oftheOSIreferencemodel,whereasroutingoccursatLayer3(thenetworklayer).Thisdistinctionprovidesroutingandbridgingwithdifferentinformationtouseintheprocessofmovinginformationfromsourcetodestination,sothetwofunctionsaccomplishtheirtasksindifferentways.路由就是一种作用,将信息从源经过互联网络移动到目的地。沿着这条通路,通常会遇到至少一个中间节点。路由通常与桥接相对照,在陌生的旁观者看来,它们精确地完成同一件事情。两者主要的不同在于,桥接发生在OSI参考模型第二层(链路层),而路由则发生在第三层(网络层)。这种不同提供给路由和桥接用不同的信息,来处理从源到目的信息移动的过程，所以这两种功能(路由和桥接)是用不同的方法来完成它们的任务的。Thetopicofroutinghasbeencoveredincomputerscienceliteratureformorethantwodecades,butroutingachievedcommercialpopularityaslateasthemid-1980s.Theprimaryreasonforthistimelagisthatnetworksinthe1970sweresimple,homogeneousenvironments.Onlyrelativelyrecentlyhaslarge-scaleinternetworkingbecomepopular.路由这个论题,出现在计算机科学文献里,已经有多于20年的时间,然而路由实现在商业上流行,则是迟至80年代中期。其原因在于70年代的网络s还是简单,同质的环境s。大规模互联网络联网的流行出现较晚。2.RoutingComponents路由的组成部分Routinginvolvestwobasicactivities:determiningoptimalroutingpathsandtransportinginformationgroups(typicallycalledpackets)throughaninternetwork.Inthecontextoftheroutingprocess,thelatteroftheseisreferredtoaspacketswitching.Althoughpacketswitchingisrelativelystraightforward,pathdeterminationcanbeverycomplex.路由涉及两种活动:确定优化路由通路s,和传输通过互联网络的信息组s(通常叫数据包s)。,在路由过程的上下文中,后一活动称为包交换。虽然包交换相对直观,通路确定却可能十分复杂。PathDetermination通路确定Routingprotocolsusemetricstoevaluatewhatpathwillbethebestforapackettotravel.Ametricisastandardofmeasurement,suchaspathbandwidth,thatisusedbyroutingalgorithmstodeterminetheoptimalpathtoadestination.Toaidtheprocessofpathdetermination,routingalgorithmsinitializeandmaintainroutingtables,whichcontainrouteinformation.Routeinformationvariesdependingontheroutingalgorithmused.路由协议使用度量来评价,这样使通路能最好地传输数据包。一次度量就是一次标准测量,如通路带宽,路由算法s使用它来确定到达一个目标的最佳路径。为了辅助路径确定的过程,路由算法s启动和维持包含路由信息的路由表s。路由信息的变化取决于所用的路由算法。3Routingalgorithmsfillroutingtableswithavarietyofinformation.Destination/nexthopassociationstellarouterthataparticulardestinationcanbereachedoptimallybysendingthepackettoaparticularrouterrepresentingthenexthoponthewaytothefinaldestination.Whenarouterreceivesanincomingpacket,itchecksthedestinationaddressandattemptstoassociatethisaddresswithanexthop.路由算法s将各种信息填入路由表s。目标/下一跳组合告诉路由器,一个具体的目标可以优化地到达,只要将数据包,沿着通向最终目标的通路,送到表示'下一跳'的具体路由器。当路由器接收到一个数据包时,它就检查目标地址,并在目标地址上结合下一跳相关地址。图5-1说明了一个例样目标/效益跳路由表。Figure:Destination/NextHopAssociationsDeterminetheData’sOptimalPathdepictsasampledestination/nexthoproutingtable.Figure:Destination/NextHopAssociationsDeterminetheData’sOptimalPathRoutingtablesalsocancontainotherinformation,suchasdataaboutthedesirabilityofapath.Routerscomparemetricstodetermineoptimalroutes,andthesemetricsdifferdependingonthedesignoftheroutingalgorithmused.Avarietyofcommonmetricswillbeintroducedanddescribedlaterinthisarticle.路由表s也可以包含其他信息,如有关所希望通路的数据。路由器s比较度量值来确定优化由路,而这些度量值则取决于所使用的路由算法的设计,而有所不同。各种通用的度量s将被介绍,并在本章后面予以说明。Routerscommunicatewithoneanotherandmaintaintheirroutingtablesthroughthetransmissionofavarietyofmessages.Theroutingupdatemessageisonesuchmessagethatgenerallyconsistsofallo