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Chapter 10. Understanding Intermediate System-to-Intermediate System (IS-IS)

Chapter 10. Understanding Intermediate System-to-Intermediate System (IS-IS)

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IS-ISProtocolOverview

TheIS-ISroutingprotocolisoneofthreeprotocolsspecifiedby

theInternationalOrganiza-tionforStandardization(ISO)to

supportconnectionlessnetworkservices(CLNS):

ConnectionlessNetworkProtocol(CLNP)ISO84381.

SeealsoIETFRFC994.

EndSystem-to-IntermediateSystemRouting

ExchangeProtocol(ES-IS)ISO95422.SeealsoIETF

RFC995.

IntermediateSystem-to-IntermediateSystem

RoutingExchangeProtocol(IS-IS)ISO105893.See

alsoIETFRFC1142.

ISOCLNSwasmeanttoprovideconnectionlessdatagram

servicesfordatatransmissioninsteadoftheconventional

connection-orientedservices.Unlikeconnection-oriented

servicesthatrequireend-to-endcallestablishmenttoprecede

anycommunicationbetweennetworkdevices,datagram

servicesallowdatatobetransmittedinindependentchunks,

knownalsoaspackets,withouthavingtosetapredefinedpath

throughthenetworkbetweensourceanddestinationbefore

transmission.

CLNP,whichisverysimilartotheInternetProtocol(IP),is

centraltotheoperationofISOCLNS.ES-ISandIS-ISare

auxiliaryprotocolsthathelpnetworknodes(endsystemsand

routers)discovereachotherandgatherroutinginformation,

whichisusedforforwardingpackets.Forexample,theIS-IS

protocolprovidesadynamicmechanismthatallowsroutersto

gatherinformationaboutvariousreachabledestinationsina



network.Thisinformationisthenprocessedtodetermine

optimalpathsthatrouterscanuseformovingdatafromone

endofthenetworktoanother.

ISO10589specifiesIS-ISforroutingCLNPpackets,andRFC

11954providesextensionstoISO10589tosupportroutingof

IPpacketsinadditiontoCLNPpackets.Specifically,RFC1195

definesIntegrated(Dual)IS-IS,whichallowsIS-IStoobtain

andalsoexchangeCLNPandIProutinginformation

simultaneously.Despiteitsdualcapabilities,IntegratedIS-IS

canbeusedinCLNS-onlyorIP-onlyenvironments.Thischapter

andthenextfocusesonuseofIntegratedIS-ISinIP-only(pure

IP)environments.

Unlikemostroutingprotocols,whicharetypicallyencapsulated

inanetworklayerprotocol,IS-ISisitselfanetworklayer

protocolandridesoverthedatalinkalongsideCLNPandIP.

Actually,allthreeISOprotocolsthatsupportconnectionless

networking(CLNP,ES-IS,andIS-IS)areindividuallynetwork

layerprotocols.ThiscontrastswiththedesignofIP-specific

routingprotocols,suchastheOpenShortestPathFirst(OSPF)

andtheBorderGatewayProto-cols,whichultimatelyare

encapsulatedinIPandoperateatahigherlayeroftheOpen

SystemInterconnection(OSI)referencemodel.Protocoldesign

requiresassociatingaprotocoloranapplicationwithan

identifierforthecorrespondinglayerofoperationintheOSI

model.Thefollowingisalistofnetworklayerprotocol

identifiers(inbinary)forthenetworklayerproto-colsthathave

beenmentionedsofar.Thehexadecimalequivalentisprovided

inbrackets:

CLNP:10000001(0x81)

ES-IS:10000010(0x82)

IS-IS:10000011(0x83)



IP:11001100(0xCC)

TheISOnetworklayerprotocolfamilyisidentifiedatthedata

linklayerby0xFEFE.IPisidentifiedby0x0800.CLNPbyitselfis

notrelevanttopureIPenvironments,andonlyIS-ISessentially

isrequiredtosupportIProutinginsuchenvironments.

However,theoperationofIS-ISistiedintrinsicallytocertain

elementsoftheISOCLNSenvironment,suchasISO

addressing,networkserviceaccesspoints(NSAP),andtheESISprotocol.TheES-ISprotocolisdesignedtofacilitate

communicationbetweenCLNSendsystemsandrouters,andit

hasnorelevancetothecommunicationbetweenIPhostsandIP

routers.InanIPenvironment,networkdevicesuseIPassociatedmechanisms,suchasdefaultgateways,theAddress

ResolutionProto-col(ARP)forIPaddress-to-datalinkaddress

resolution,andtheInternetControlMessageProtocol(ICMP)

fornetwork-discoveryandcontrolfunctions.Discussions

regardingdetailsofCLNPandES-ISarebeyondthescopeof

thisbook,andfurtherdiscussionislimitedtoissuesof

relevancetotheoperationoftheIS-ISprotocol.



IS-ISRoutingProtocol

IS-ISisalink-stateprotocoldesignedforintradomainrouting.

Itsupportsatwo-levelroutinghierarchy:

Routingwithinareas(Level1)

Routingbetweenareas(Level2)

RoutersrunningtheIS-ISprotocolformadjacencieswithother

directlyconnectedIS-ISroutersandexchangerouting

informationcontainedinlink-statepackets(LSPs).Eachrouter

collectsLSPsintoseparateLevel1andLevel2link-state



databasesbasedontheirmodeofoperation(Level1only,Level

2only,orLevel12).TheLevel1link-statedatabaseprovidesa

viewofthelocalarea'stopology,whiletheLevel2link-state

databaseprovidesaglobalviewofinterareaconnectivity.The

shortestpathfirst(SPF)algorithm(namedafterDijkstra)isrun

separatelyoverLevel1andLevel2databasestoobtainthe

bestpathstovariousdestinationsinthenetwork.

IS-ISisoneoftwopopularInteriorGatewayProtocols(IGP)

usedonthelargeservice-providernetworksthatare

interconnectedtoformtheglobalInternet.Theotherpopular

IGPistheOpenShortestPathFirst(OSPF)protocol.TheBorder

GatewayProtocol(BGP)isusedforinter-domainrouter

betweennetworkdomains(orautonomoussystems).

AsidefromRFC1195,whichallowsIS-IStocarryIProuting

information,severalotherenhancementshavebeenproposed

forstandardizationintheIETF.Mostprominentoftheseare

MultiprotocolLabelSwitchingTrafficEngineering(MPLSTE)

relatedenhancements7.Inrecenttimes,interestintheIS-IS

protocolhassignificantlyincreased,culminatinginthe

reopeningoftheIS-ISworkinggroupintheIETF.Severalofthe

newcapabilitiesproposedinIETFalreadyhavebeen

implementedasvendor-specificenhancements,andtheeffortis

directedatstandardizationandinteroperabilityacrossdifferent

vendorrouterproducts.

ThesuccessfuladoptionandwidespreadacceptanceoftheIS-IS

protocolforIProutingisaresultofitsflexibilityforextension,

simplicity,andeaseoftroubleshooting.Troubleshootingofthe

IS-ISprotocolisdiscussedinthenextchapter.Thischapter

drillsintokeyconceptsbehindtheIS-ISprotocolandlaysthe

groundworkforChapter11,"TroubleshootingIS-IS."



IS-ISProtocolConcepts

Thegoalofthissectionistohelpyouunderstandtheoperation,

features,strengths,andlimitationsofthevariousarchitectural

conceptsunderlyingtheIS-ISprotocol.Inparticular,the

followingpointsarediscussed:

IS-ISnodes,links,andareas

IS-ISadjacencies

Level-1andLevel-2routing

IS-ISpackets

IS-ISmetrics

IS-ISauthentication

AddressingfortheCLNPprotocol



IS-ISNodes,Links,andAreas

IS-ISinheritsthefollowingISOclassificationanddefinitionof

thetwobasictypesofnet-worknodes:

Endsystems

Intermediatesystems

Endsystemsarehostsinanetworkthattypicallydonothave



extensiveroutingcapabilities.Intermediatesystemsreferto

routerswhoseprimaryfunctionistoroutepackets.

Networknodesareinterconnectedbylinks.Again,inIS-IS,only

twobasiclinkstypesareofpracticalrelevance:

Point-to-pointlinks

Broadcastlinks

Point-to-pointlinksinterconnectpairsofnodes,whilebroadcast

typelinksaremultipointandcaninterconnectmorethantwo

nodesatthesametime.Transporttechnologies,suchasserial

(T1,DS-3,andsoon)andPacket-over-SONET(PoS)links,are

inherentlypoint-to-point,whilelocal-areanetwork(LAN)media,

suchasEthernet,aretypicalbroadcast-typelinks.

Nonbroadcastmultiaccess(NBMA)transportmedia,suchas

AsynchronousTransferMode(ATM)andFrameRelay,canbe

configuredtooperateassimulatedbroadcastorpoint-to-point

links.Becausebroadcastlinksinherentlyimplyconnectednodes

arefullymeshed,NBMAmediashouldbeconfiguredas

broadcastlinksonlywhentheroutersarefullymeshedbythe

underlyingpermanentvirtualcircuits(PVC).

NonfullymeshedNBMAenvironmentsshouldusepoint-to-point

setups,whichalignwiththeunderlyingtopologyofPVC

interconnectionsandaresimplertomanageandtroubleshoot.A

networkrunningtheIS-ISroutingprotocolfrequentlyis

referredtoasanIS-ISroutingdomain.AlargeIS-ISrouting

domaincanbepartitionedintomultipleareasforthepurposeof

scalingroutingovertheentiredomain.Aroutingareacanbeof

anyarbitrarysize;thenumberofnodesthatitcontainslargely

isdefinedatthediscretionofthenetworkdesigner.Keyfactors

normallytakenintoconsiderationwhencreatingareasinclude

memoryandprocessingcapacitiesoftheroutersinvolved.The

largertheareais,thehighertheresource(memoryandCPU



capacity)needsperrouterareformaintainingtheIS-IS

databaseandcomputingroutesfastenoughtosustain

reasonableconvergencetimeswhenchangesoccurinthe

network.

AllIS-ISroutersinthedomainareassignedtoatleastoneISISarea.EachIS-ISnodehasauniquenode-basedaddress

referredtoasanetworkserviceaccesspoint(NSAP).NSAPs

arediscussedlaterinthischapter,but,fornow,allyouneedto

knowisthattheNSAPhasanareaidentifiercomponentthat

definesthenativeareaofeachnode.

Figure10-1showsthelayoutofanIS-ISdomaincarvedinto

threeareaswiththefollowingsimplearea-identifiers:49.001,

49.002,and49.003.Asdepicted,theareasareinterconnected

throughtheregionknownasthebackbone.Fromthisdiagram,

itisalsoapparentthateachnodewhollysitsinaspecificarea

andthattheareaboundariescutacrossthelinkstoother

areas.EachIS-ISareaisspecifiedinISO10589tobea

stubmeaningthatinterarearoutinginform-ationremainsonly

withinthebackbone.ArecentIETF-sponsoredenhancement6

removesthisrestriction.ThiscapabilityisavailableinCiscoIOS

SoftwareasafeatureknownasIS-ISrouteleaking.



Figure10-1.IS-ISAreas



Adjacencies

Asalink-stateprotocol,IS-ISreliesoncurrentandcomplete

knowledgeofthenetworktopologytocomputeroutes

accuratelyandoptimally.SomekeyfunctionsofroutersparticipatinginIS-ISroutinginvolvediscovering,establishing,and

maintainingroutingadjacencieswithneighborrouters.Thetype

andmannerinwhichanadjacencyisformedbetweentwo

routersdependonthetypeoflinkinterconnectingthem.This

sectionaddressesthetwotypesofIS-ISadjacencies,which

correlatewiththetwotypesoflinksdiscussedearlier.The

adjac-encytypesarelistedhere:

Adjacenciesoverpoint-to-pointlinks

Adjacenciesoverbroadcastlinks

FormationandmaintenanceofadjacenciesbetweenIS-IS



routerstakeplacethroughtheexchangeofspecialpackets,

referredtoashellos.RoutersneedtoformbothES-ISandISISadjacenciesovereitherpoint-to-pointorbroadcastlinks.

EventhoughES-ISisnotnecessaryforIProuting,IS-IS

adjacencyformationonpoint-to-pointlinksisdependentonESISadjac-encydetectiononsuchlinks.Therefore,CiscoIOS

SoftwareenablestheES-ISprotocolevenifIS-ISisenabledfor

onlyIProuting.ES-ISusesend-systemhellos(ESHs)and

intermediate-systemhellos(ISHs)forES-ISadjacencies,while

IS-ISusesintermediatesystem-to-intermediatesystemhellos

(IIHs).



ES-ISAdjacencies

InES-IS,endsystemsadvertiseESHstoroutersbyaddressing

themtotheMACaddress09-00-2B-00-00-05

(AllIntermediateSystems).Ontheotherhand,routersadvertise

ISHsto09-00-2B-00-00-04(AllEndSystems).ES-ISessentially

providesahost-discoverymech-anismintheISOCLNS

environment.Throughthismechanism,endsystemslocatethe

closestrouterthoughwhichtheycantransitdatato

nonconnectedmedia.Routers,inturn,learnaboutendsystems

intheirarea.RoutersalsodiscovereachotherthroughtheESISadjacencyprocess.Figure10-2showsISHandESH

transmissionsbetweenroutersandaworkstationonaLAN.



Figure10-2.ES-ISAdjacencies



IS-ISAdjacencies

Forrouterstoexchangeroutinginformation,theyneedto

transcendES-ISadjacenciesandformIS-ISadjacencieswith

theirneighbors.AninterestingpointtonoteisthatES-IS

adjacencyisrequiredtotriggeradvertisementofIIHsonpointto-pointlinks,whichultimatelycanleadtoIS-ISadjacencies.

TheIS-ISadjacenciesonpoint-to-pointlinksareformedand

maintainedalittledifferentlythanonbroadcastlinks.Also,IIHs

usedonpoint-to-pointlinkshaveaslightlydifferentformatthan

thoseusedonbroadcastmultipointlinks.Thefollowingarethe

threetypesofspecifiedIIHs:



Point-to-pointIIHUsedoverpoint-to-pointlinks.

Level1LANIIHUsedoverbroadcastlinksbutforLevel1

adjacencies.Advertisedto01-80-C2-00-00-14(AllL1ISs).

Level2LANIIHUsedonbroadcastlinksbutforLevel2

adjacencies.Advertisedto01-80-C2-00-00-15(AllL2ISs).

Thepoint-to-pointIIHsandLANIIHshaveslightlyvaried

informationintheirfixedheaderareas.Forexample,thepointto-pointIIHshavealocalcircuitID,whereasLANIIHshavea

LANID.Also,point-to-pointIIHsdon'thavethepriority

informationfoundinLANIIHs.IS-ISpacketformatsare

coveredlaterinthischapter,inthesectiontitled"IS-IS

Packets."Completeformatsofhellopacketsareprovidedatthe

endofthechapterinthesectiontitled"AdditionalIS-ISPacket

Information."IS-IShasatwo-levelroutinghierarchy,and,as

alludedtopreviously,thetypeofadjacencyformedbetweenISISroutersdeterminesthetypeofroutingrelationshipbetween

them(thatis,Level1,Level2,orboth).

Routinginformationisexchangedthroughtheuseoflink-state

packets(LSPs),withcontrolprovidedbysequencenumber

packets(SNP).LSPsandSNPswillbediscussedfurtherinthe

section"IS-ISLink-StateDatabase."

OnmultiaccesslinkssuchasbroadcastLANsormultipointATM

andFrameRelaylinksinwhichmorethentwonodesare

connectedtothesamelink,formingadjacenciesbetweenallof

themresultsinnx(n1)/2adjacencies,wherenisthenumber

ofconnectednodes.InIS-IS,allnodesonmultipointlinks

actuallydetecteachotherbymeansofthehellosmulticast

acrossthemedium.Eachnode,therefore,formsn1

adjacenciesonsuchmedia.Anodedeclaresaneighbortobe

adjacentifthatneighborisannouncedinthelistofdetected

neighbors.Reliablyupdatingandsynchronizingdatabaseswith



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