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1、LinkStateRoutingProtocolsTheinformationavailabletoadistancevectorrouterhasbeencomparedtotheinformationavailablefromaroadsign.Linkstateroutingprotocolsarelikearoadmap.Alinkstateroutercannotbefooledaseasilyintomakingbadroutingdecisions,becauseithasacompletepictureofthenetwork.Ther
2、easonisthatunliketherouting-by-rumorapproachofdistancevector,linkstateroutershavefirsthandinformationfromalltheirpeer191routers.Eachrouteroriginatesinformationaboutitself,itsdirectlyconnectedlinks,thestateofthoselinks(hencethename),andanydirectlyconnectedneighbors.Thisinformatio
3、nispassedaroundfromroutertorouter,eachroutermakingacopyofit,butneverchangingit.Theultimateobjectiveisthateveryrouterhasidenticalinformationaboutthenetwork,andeachrouterwillindependentlycalculateitsownbestpaths.191Thatis,allroutersspeakingthesameroutingprotocol.Linkstateprotocols
4、,sometimescalledshortestpathfirstordistributeddatabaseprotocols,arebuiltaroundawell-knownalgorithmfromgraphtheory,E・W.Dijkstra'sshortestpathalgorithm.Examplesoflinkstateroutingprotocolsare•OpenShortestPathFirst(OSPF)forIP•TheISO'sIntermediateSystem-to-IntermediateSystem(IS-IS)fo
5、rCLNSandIP•DEC'SDNAPhaseV•Novell'sNetWareLinkServicesProtocol(NLSP)Althoughlink-stateprotocolsarerightlyconsideredmorecomplexthandistancevectorprotocols,thebasicfunctionalityisnotcomplexatall:1.Eachrouterestablishesarelationshipadjacencywitheachofitsneighbors・2.Eachroutersendsad
6、ataunitwewillcalllinkstateadvertisements(LSAs)toeachneighbor.1101TheLSAlistseachoftherouter'slinks,andforeachlink,itidentifiesthelink,thestateofthelink,themetriccostoftheroutersinterfacetothelink,andanyneighborsthatmightbeconnectedtothelink.Eachneighborreceivinganadvertisementin
7、turnforwards(floods)theadvertisementtoitsownneighbors-1101LSAisanOSPFtemi.ThecorrespondingdataunitcreatedbyIS-ISiscalledlinkstatePDU(LSP).Butforthisgeneraldiscussion,”LSA”fitsourneeds.3・EachrouterstoresacopyofalltheLSAsithasseeninadatabase.Ifallworkswell,thedatabasesinallrouters
8、shouldbeidentical-4.Thecompletedtopologicaldata
