协议执行路由操作模块4 isis bsci30s04l02.pptx

协议执行路由操作模块4 isis bsci30s04l02.pptx

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TheIS-ISProtocolPerformingIS-ISRoutingOperations

OSIAddressesOSInetworklayeraddressingisimplementedwithNSAPaddresses.AnNSAPaddressidentifiesasystemintheOSInetwork;anaddressrepresentsanentirenode,notaninterface.VariousNSAPformatsareusedinvarioussystems,

becausedifferentprotocolsmayusedifferentrepresentationsofNSAP.NSAPaddressesareamaximumof20bytes:Higher-orderbitsidentifytheinterareastructure.Lower-orderbitsidentifysystemswithinarea.

IntegratedIS-ISNSAPAddressStructureTheCiscoimplementationofIntegratedIS-ISdistinguishesonlythefollowingthreefieldsintheNSAPaddress:Areaaddress:Variable-lengthfield(1to13octets)

composedofthehigher-orderNSAPoctets,excludingsystemID

andNSEL.SystemID:ESorISidentifierinanarea;fixedlength

ofsixoctetsinCiscoIOSsoftware.NSEL:OneoctetNSAPselector,serviceidentifier.TotallengthofNSAPisfrom8(minimum)to20octets(maximum).

TypicalNSAPAddressStructureThesimplestNSAPformatusedbymostcompaniesrunningIS-ISastheirIGPisasfollows:Areaaddress(mustbeatleast1byte)AFIsetto49Locallyadministered;thus,youcanassignyourownaddresses.AreaIDTheoctetsoftheareaaddressaftertheAFI.SystemIDCiscoroutersrequirea6-bytesystemID.NSELAlwayssetto0forarouter.

IdentifyingSystemsinIS-IS:AreaAddressTheareaaddressuniquelyidentifiestheroutingarea,andthesystemIDidentifies

eachnode.Allrouterswithinanareamustusethesameareaaddress.AnESmaybeadjacent

toarouteronlyiftheyshareacommon

areaaddress.AreaaddressisusedinLevel2routing.

IdentifyingSystemsinIS-IS:SystemIDSystemIDintheaddressusedtoidentifytheIS;itisnotjustaninterface.Ciscosupportsonlya6-bytesystemID.SystemIDisusedinLevel1routingandhastobeuniquewithin

anarea.SystemIDhastobeuniquewithinLevel2routersthatformtheroutingdomain.Generalmendation:use

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