OSPF&ISIS(Juniper)

IS-ISandOSPFAComparativeAnatomyDaveKatz,JuniperNetworksJune12,2000OverviewProtocolHistoryNutsandBoltsScalabilityIssuesPragmaticConsiderationsConclusionsJune12,2000ProtocolHistoryJune12,2000ProtocolHistory(Disclaimer--biased,foggymemory)1987IS-IS(fromDEC)selectedbyANSIasOSIintradomainprotocol(CLNPonly)1988NSFnetdeployed,IGPbasedonearlyIS-ISdraftOSPFworkbegins,looselybasedonIS-ISmechanisms(LSprotocolsarehard!)IPextensionstoIS-ISdefinedJune12,2000ProtocolHistory1989OSPFv.1RFCpublishedProteonshipsOSPFIS-ISbecomesISOproposedstandardPublicbickeringensues--OSPFandIS-ISareblessedasequalsbyIETF,withOSPFsomewhatmoreequalPrivatecooperationimprovesbothprotocols1990Dual-modeIS-ISRFCpublishedJune12,2000ProtocolHistory1991OSPFv.2RFCpublishedCiscoshipsOSPFCiscoshipsOSI-onlyIS-IS1992CiscoshipsdualIS-IS(partofDECBrouter)LotsofOSPFdeployed,butverylittleIS-IS1993NovellpublishesNLSP(IPXIS-ISknockoff)June12,2000ProtocolHistory1994CiscoshipsNLSP(rewritingIS-ISassideeffect)LargeISPsneedanIGP;IS-ISisrecommendedduetorecentrewriteandOSPFfieldexperience(andtolesserextent,NSFCLNPmandate)1995ISPsbegindeploymentofIS-IS,Ciscoimplementationfirmsup,protocolstartstobecomepopularinnicheJune12,2000ProtocolHistory1996-1998IS-ISnichepopularitycontinuestogrow(someISPsswitchtoitfromOSPF)IS-ISbecomesbarriertoentryforroutervendorstargetinglargeISPsJuniperandothervendorsshipIS-IScapablerouters1999-2000Extensionscontinueforbothprotocols(e.g,TrafficEngineering)June12,2000NutsandBoltsJune12,2000NutsandBolts10,000footviewProtocolsarerecognizablysimilarinfunctionandmechanism(unsurprising,givencommonheritage)Linkstatealgorithms(networkmapisdistributed,eachroutercalculatesroutesindependentlybasedonthemap)TwolevelhierarchiesDesignatedRouteronLANsWidelydeployed(forsomevalueof“wide”)MultipleinteroperableimplementationsJune12,2000NutsandBolts10,000footviewOSPFisforthemostpartmore“optimized”(andthereforesignificantlymorecomplex)IS-ISwasnotdesignedfromthestartasanIProutingprotocol(andisthereforeabitclunkyinplaces)June12,2000NutsandBoltsEncapsulationOSPFrunsontopofIPTraditionalIProutingprotocolapproachAllowsvirtuallinks(ifyoulikethem)ReliesonIPfragmentationforlargeLSAsSubjecttospoofingandDoSattacks(useofauthenticationisstronglyadvised)AllowsuseofATMVCmuxencapsulation(soTCPacksfitinoneATMcell)June12,2000NutsandBoltsEncapsulationIS-ISrunsdirectlyoverL2(nexttoIP)Sortofmakessense,architecturallyPartitionrepairrequirestunneling(rarelyimplemented)MoredifficulttospooforattackMoredifficulttoimplementinsomeenvironmentsRequiresATMSNAPencapsulation,forcingtwo-cellTCPacks(butHenkSmit’sNLPIDhackfixesthis)June12,2000NutsandBoltsMediasupportBothprotocolssupportLANsandpoint-to-pointlinksinsimilarwaysIS-IShasnodirectNBMAsupport--expectsO/StopresentNBMAnetworkaseitherpseudo-LAN(badidea)orsetofpoint-to-pointlinksOSPFNBMAmodeisconfiguration-heavyandrisky(allroutersmustbeabletoreachDR;badnewsifVCfails)OSPFP2MPmodemodelsNBMAaspoint-to-pointlinks(ifO/Swon’thelp)June12,2000NutsandBoltsPacketEncodingOSPFis“efficiently”encodedPositionalfieldsHoly32-bitalignmentprovidestidypacketpictures,butnotmuchelseOnlyLSAsareextensible(notHellos,etc.)UnrecognizedLSAtypesnotflooded(thoughopaqueLSAscansuffice,ifimplementeduniversally,andIS-IS-likeencodingcanprovidegoodgranularity)June12,2000NutsandBoltsPacketEncodingIS-ISismostlyType-Length-ValueencodedNoparticularalignmentExtensiblefromthestart(unknowntypesignoredbutstillflooded)AllpackettypesareextensibleNestedTLVsprovidestructureformoregranularextension(thoughbasespecdoesnotusethem;OSPFisstartingtodoso)June12,2000NutsandBoltsAreaArchitectureBothprotocolssupporttwo-levelhierarchyofareas(toreduceSPFgraphcomplexity,andpotentiallytoallowrouteaggregation)OSPFareaboundariesfallwithinarouterInterfacesboundtoareasRoutermaybeinmanyareasRoutermustcalculateSPFperareaJune12,2000NutsandBoltsAreaArchitectureIS-ISareaboundariesfallonlinksRouterisinonlyonearea,plusperhapstheL2backbone(area)Biasedtowardlargeareas,areamigrationRequiresrouterperarea(unlessmultiplevirtualroutersareimplemented)HistoricallyprovensomewhatdifficultforuserstograspLittleornomultileveldeployment(largeflatareasworksofar)June12,2000NutsandBoltsDatabaseGranularityOSPFdatabasenodeisanLSAdvertisementLSAsaremostlynumerousandsmall(oneexternalperLSA,onesummaryperLSA)NetworkandRouterLSAscanbecomelargeLSAsgroupedintoLSUpdatesduringfloodingLSUpdatesarebuiltindividuallyateachhopSmallchangescanyieldsmallpackets(butRouter,NetworkLSAscanbelarge)June12,2000NutsandBoltsDatabaseGranularityIS-ISdatabasenodeisanLSPacketLSPsareclumpsoftopologyinformationorganizedbytheoriginatingrouterAlwaysfloodedintact,unchangedacrossallfloodinghops(soLSPMTUisanarchitecturalconstant--itmustfitacrossalllinks)SmalltopologychangesalwaysyieldentireLSPs(thoughpacketsizeturnsouttobemuchlessofanissuethanpacketcount)ImplementationscanattemptcleverpackingJune12,2000NutsandBoltsNeighborEstablishmentBothprotocolsuseperiodicmulticastHel