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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)四．BridgingandSwitchingBasics桥接和交换的基本原理:50四．BridgingandSwitchingBasics桥接和交换的基本原理FromDocWikiJumpto:navigation,searchThisarticleintroducesthetechnologiesemployedindeviceslooselyreferredtoasbridgesandswitches.Topicssummarizedhereincludegenerallinklayerdeviceoperations,localandremotebridging,ATMswitching,andLANswitching.ChaptersinPart5,“BridgingandSwitching,”addressspecifictechnologiesinmoredetail.本章介绍用于,不严格地称为桥和交换机的设备,的技术。综述在这里的论题包括一般链路层设备的操作，本地和远程的桥接，ATM交换，和LAN交换。在手册第5部分桥接和交换的各章中将更详细地讲述具体技术。Contents[hide]1WhatAreBridgesandSwitches?什么是网桥和交换机？2LinkLayerDeviceOverview链路层设备概述3TypesofBridges网桥的类型3.1Figure:LocalandRemoteBridgesConnectLANSegmentsinSpecificAreas图3.1本地和远程网桥在指定区连接局域网段3.2Figure:AMAC-LayerBridgeConnectstheIEEE802.3andIEEE802.5Networks图3.2MAV层网桥连接IEEE802.3和IEEE802.5网络4TypesofSwitches交换机的类型4.1ATMSwitchATM交换机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24.1.1Figure:Multi-LANNetworksCanUseanATM-BasedBackboneWhenSwitchingCells图4.1.1多局域网交换信元时可以使用基于ATM的骨干网4.2LANSwitch局域网交换机4.2.1Figure:ALANSwitchCanLink10-Mbpsand100-MbpsEthernetSegments图4.2.1局域网交换机可以连接10-Mbps和100-Mbps以太网段5ReviewQuestions复习问答6ForMoreInformation参阅更多信息1.WhatAreBridgesandSwitches?什么是桥和交换机?BridgesandswitchesaredatacommunicationsdevicesthatoperateprincipallyatLayer2oftheOSIreferencemodel.Assuch,theyarewidelyreferredtoasdatalinklayerdevices.桥和交换机都是通信设备,它们主要是在OSI参考模型的第二层上操作,所以它们被广泛地称之为数据链路层设备。Bridgesbecamecommerciallyavailableintheearly1980s.Atthetimeoftheirintroduction,bridgesconnectedandenabledpacketforwardingbetweenhomogeneousnetworks.Morerecently,bridgingbetweendifferentnetworkshasalsobeendefinedandstandardized.商业上使用桥始于1980年初。当时,桥用来在同质的网络间连接并实现数据包转发。最近,在不同网络间的桥接,已经被定义和标准化了。Severalkindsofbridginghaveprovenimportantasinternetworkingdevices.TransparentbridgingisfoundprimarilyinEthernetenvironments,whilesource-routebridgingoccursprimarilyinTokenRingenvironments.Translationalbridgingprovidestranslationbetweentheformatsandtransitprinciplesofdifferentmediatypes(usuallyEthernetandTokenRing).Finally,source-routetransparentbridgingcombinesthealgorithmsoftransparentbridgingandsource-routebridgingtoenablecommunicationinmixedEthernet/TokenRingenvironments.有几种桥接已经被证实很重要,就像互联网络联网设备那样。透明的桥接主要在以太网环境中能找到,而源路由桥接则主要发生在令牌环网的环境中。透明的桥接提供格式间的转换和不同媒体类型(通常是以太网和令牌环网)的转送规则。源路由透明桥接则是组合了上述两种桥接的算法,以实现在以太网/令牌环网的混合环境中进行通信。Today,switchingtechnologyhasemergedastheevolutionaryheirtobridging-basedinternetworkingsolutions.Switchingimplementationsnowdominateapplicationsinwhichbridgingtechnologieswereimplementedinpriornetworkdesigns.Superiorthroughputperformance,higherportdensity,lowerper-portcost,andgreaterflexibilityhavecontributedtotheemergenceofswitchesasreplacementtechnologyforbridgesandascomplementstoroutingtechnology.今天,交换技术已经显露成为基于桥接的互联网络联网解案的革命性的继承技术。交换实现现在主要的应用是在那些以前的网络设计中曾用桥接技术实现的地方。超级的吞吐性能,较高的接口密度,更低的单口代价,和更大的灵活性等,由于交换机s作为桥接的替代技术和路由技术的补充而出现了。2.LinkLayerDeviceOverview链路层设备概述Bridgingandswitchingoccuratthelinklayer,whichcontrolsdataflow,handlestransmissionerrors,providesphysical(asopposedtological)addressing,andmanagesaccesstothephysicalmedium.Bridgesprovidethesefunctionsbyusingvariouslinklayerprotocolsthatdictatespecificflowcontrol,errorhandling,addressing,andmedia-accessalgorithms.ExamplesofpopularlinklayerprotocolsincludeEthernet,TokenRing,andFDDI.桥接和交换发生在链路层。链路层控制数据流,处理传送错误,提供物理寻址,以及管理对物理介质的访问。桥提供这些功能是通过使用各种链路层协议来实现的。协议安排特殊的流量控制,错误处理,寻址,和媒体访问算法。流行的链路层协议的例子包括以太网,令牌环网,和FDDI。,Bridgesandswitchesarenotcomplicateddevices.Theyanalyzeincomingframes,makeforwardingdecisionsbasedoninformationcontainedintheframes,andforwardtheframestowardthedestination.3Insomecases,suchassource-routebridging,theentirepathtothedestinationiscontainedineachframe.Inothercases,suchastransparentbridging,framesareforwardedonehopatatimetowardthedestination.桥和交换机并非复杂设备。它们解析收入的帧,根据包含在帧中的信息,作出转发决策,并将帧向目的地转发。在某些情况下,如源路由桥接,到目的地的整个通路都包含在每个帧中。在其他情况下,如透明桥接,帧是向着目的地,一次转发一跳。Upper-layerprotocoltransparencyisaprimaryadvantageofbothbridgingandswitching.Becausebothdevicetypesoperateatthelinklayer,theyarenotrequiredtoexamineupper-layerinformation.Thismeansthattheycanrapidlyforwardtrafficrepresentinganynetworklayerprotocol.ItisnotuncommonforabridgetomoveAppleTalk,DECnet,TCP/IP,XNS,andothertrafficbetweentwoormorenetworks.对上层协议的透明性是桥接和交换的主要优点。由于两种类型的设备都操作在链路层,它们并不需要检查上层信息。这意味着它们可以迅速转发表达任何网络层协议的信流。对于一个桥而言,在两个或更多的网络间移动AppleTalk,DECnet,TCP/IP,XNS,和其它信流并不罕见。BridgesarecapableoffilteringframesbasedonanyLayer2fie