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1、BridgesandLANSwitches7/24/20211CS/ECE438-UIUC,Fall2007EthernetBackoffrevisitedAfterNcollisions,pickanumberkbetween0and2N-1Waitfork*51.2usSendframeifnoonehasstartedusingthechannel7/24/20212CS/ECE438-UIUC,Fall2007RepeatedCollisionsSupposeA,B,andCeachhaveaframetosend,causingacollisio
2、nApicksk=0,BandCpickk=1Awins,sendsframeAfterAisdone,BandCbothtrytosendagainCollisionagainIncreasecollisioncounter7/24/20213CS/ECE438-UIUC,Fall2007CaptureEffectAandBcollideApicks0,Bpicks1Awins,transmitsframeSupposeAhasanotherframetosendAandBcollideagainA’scollisioncounteris1,pickkf
3、rom0,1B’scollisioncounteris2,pickkfrom0,1,2,3AislikelytowinagainAndkeepwinning!7/24/20214CS/ECE438-UIUC,Fall2007Bridges:BuildingExtendedLAN’sTraditionalLANSharedmedium(e.g.,Ethernet)Cheap,easytoadministerSupportsbroadcasttrafficProblemScaleLANconceptLargergeographicarea(>O(1km))Mo
4、rehosts(>O(100))ButretainLAN-likefunctionalitySolutionbridges7/24/20215CS/ECE438-UIUC,Fall2007BridgesProblemLANshavephysicallimitationsEthernet–1500mSolutionConnecttwoormoreLANswithabridgeAcceptandforwardLevel2connection(noextrapacketheader)AcollectionofLANsconnectedbybridgesiscal
5、ledanextendedLAN7/24/20216CS/ECE438-UIUC,Fall2007Bridgesvs.SwitchesSwitchReceiveframeoninputportTranslateaddresstooutputportForwardframeBridgeConnectsharedmediaAllportsbidirectionalRepeatsubsetoftrafficReceiveframeononeportSendonallotherports7/24/20217CS/ECE438-UIUC,Fall2007Usesan
6、dLimitationsofBridgesBridgesextendLANconceptLimitedscalabilitytoO(1,000)hostsnottoglobalnetworksNotheterogeneoussomeuseofaddress,butnotranslationbetweenframeformats7/24/20218CS/ECE438-UIUC,Fall2007BridgeswithLoopsProblemIfthereisaloopintheextendedLAN,apacketcouldcirculateforeverSi
7、dequestion:Areloopsgoodorbad?SolutionSelectwhichbridgesshouldactivelyforwardCreateaspanningtreetoeliminateunnecessaryedgesAddsrobustnessComplicateslearning/forwarding7/24/20219CS/ECE438-UIUC,Fall2007ExampleExtendedLANwithLOOPSB9B4BB7B1B5B2AKJIHGFEDCB7/24/202110CS/ECE438-UIUC,Fall2
8、007SpanningTreeAlgorithmViewexten
