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Chapter 6. Understanding Enhanced Interior Gateway Routing Protocol (EIGRP)

Chapter 6. Understanding Enhanced Interior Gateway Routing Protocol (EIGRP)

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won'tscaletotheneedsofthenetwork.Someofthebiggest

scalabilityproblemsofIGRPandRIPareasfollows:

Fullperiodicroutingupdatesthatconsumebandwidth

RIPsendsoutitsentireroutingtableevery30seconds;

IGRPsendsoutitsentireroutingtableevery90seconds.

Thisconsumessignificantbandwidth.

RIPhop-countlimitationof15hopsThislimitation

makesRIPprotocolanon-scalableroutingprotocolin

today'snetworksbecausemostmedium-sizednetworks

havemorethan15routers.

NosupportofVLSManddiscontiguousnetworksThis

alsohindersthecapabilitytoscalelargenetworksforRIP

andIGRP.Becauseofthisfactor,routersummarizationis

notsupported.

SlowconvergencetimeBecauseRIPandIGRPsend

periodicroutingupdates,anetworkthatisnotavailablein

onepartofthenetworkcouldtakeminutesfortheother

partofthenetworktodiscoverthatit'snolongeravailable.

Not100percentloop-freeRIPandIGRPdonotkeep

topologytables,sothereisnomechanismforthemto

ensurea100percentloop-freeroutingtable.

BecauseoftheseshortcomingsofIGRPandRIP,Cisco

developedanenhancedversionofIGRPthatnotonlyfixedall

theproblemsofIGRPandRIPbutalsodevelopedarouting

protocolrobustenoughtoscaletotoday'snetworkgrowth.This

enhancedversioniscalledEnhancedInteriorGatewayRouting

Protocol(EIGRP).

EIGRPisneitheraclassicdistancevectorroutingprotocolnora

link-stateprotocolitisahybridofthesetwoclassesofrouting



protocol.Likeadistancevectorprotocol,EIGRPgetsitsupdate

fromitsneighbors.Likealink-stateprotocol,itkeepsa

topologytableoftheadvertisedroutesandusestheDiffusing

UpdateAlgorithm(DUAL)toselectaloop-freepath.The

convergencetimeinanetworkisthetimethatittakesforall

theroutersinthenetworktoagreeonanetworkchange.The

shortertheconvergencetimeis,thequickeraroutercanadapt

toanetworktopologychange.Unlikeatraditionaldistance

vectorprotocol,EIGRPhasfastconvergencetimeanddoesnot

sendfullperiodicroutingupdates.Unlikealink-stateprotocol,

EIGRPdoesnotknowwhattheentirenetworklookslike;it

dependsonlyonitsneighbor'sadvertisement.BecauseEIGRP

hascharacteristicsofbothdistancevectorandlink-state

protocols,CiscohasclassifiedEIGRPasanadvanceddistance

vectorroutingprotocol.

AdvantagesofEIGRPincludethefollowing:

100%loop-freeEIGRPisguaranteedtohavea100

percentloop-freeforwardingtableifallthenetworksare

containedwithinoneautonomoussystem.

EasyconfigurationConfigurationofEIGRPisextremely

easyandisthesameasIGRPandRIPatthebasiclevel.

FastconvergenceConvergencetimeforEIGRPismuch

fasterthanthatforRIPandIGRP.

IncrementalupdateInanEIGRPnetwork,norouting

updateisexchangedexceptforanetworkchange.Also,

onlythechangeisupdated,nottheentireroutingtable.

ThissavesCPUpowerandismoreefficient.

UseofmulticastaddressIGRPandRIPusethebroadcast

addressof255.255.255.255tosendtheirpackets.This

meansthateverydeviceonthesamenetworksegment



receivestheupdates.EIGRPsendsitspacketoverthe

multicastaddressof224.0.0.10,whichensuresthatonly

theEIGRP-enableddevicesreceivetheEIGRPpackets.

BetterutilizationofbandwidthEIGRPobtainsthe

bandwidthparameterfromtheinterfaceinwhichEIGRP

packetswillbesentout.Itisaparameterinwhichits

valuesareassignedtoaparticularinterface.Forexample,

bydefault,allserialinterfaceshaveabandwidthof1544

kbps;however,thisbandwidthparameterisconfigurable.

EIGRPcanuseupto50percentoftheinterfacebandwidth

tocarryEIGRPpackets.ThisensuresthatEIGRPpackets

willnotstarvetherouteddatapacketduringamajor

networkconvergenceevent.RIPandIGRPdonothavethis

feature,sopotentiallylargeamountsofRIPorIGRP

updateswouldpreventregulardatapacketsfromgoing

through.

SupportforVLSManddiscontiguousnetworksUnlike

RIPandIGRP,EIGRPsupportsVLSManddiscontiguous

networks.ThisenablesEIGRPtobeimplementedinthe

modernnetworkandlendsitselftobetternetwork

scalability.



Metrics

EIGRPandIGRPusethesameequationtocalculatetheir

metrics;however,theEIGRPmetricisobtainedbymultiplying

theIGRPmetricby256.Inotherwords:



wheretheIGRPmetricisshowninEquation6-1.

Bydefault,theKvaluesofK1andK3are0;therefore,the

EIGRPmetricsimplifiestothis:



Equation6-1IGRPMetric



K1,K2,K3,K4,K5=Constants

Defaultvalues:K1=K3=1,K2=K4=K5=0

BW=107/(minbandwidthalongpathsinkilobitspersecond)

Delay=(Sumofdelaysalongpathsinmilliseconds)/10

Load=Loadofinterface



Reli=Reliabilityoftheinterface

EIGRPisdifferentthanIGRPmetricbyafactorof256because

oftheMetricfield:IGRPusesonly24bitsinitsupdatepacket

fortheMetricfield,whereasEIGRPuses32bitsinitsupdate

packetfortheMetricfield.Thedifferenceof8bitsrequiresthe

IGRPmetrictobemultipliedby256toobtaintheEIGRPmetric.

Forexample,iftheIGRPmetrictoadestinationnetworkis

8586,theEIGRPmetricwouldbe8586x256=2,198,016.



EIGRPNeighborRelationships

UnlikeIGRP,EIGRPmustestablishneighborrelationshipsbefore

updatesaresentout.WhenanEIGRPprocessisconfiguredon

therouter,therouterbeginstoexchangeEIGRPhellopackets

overthemulticastaddressof224.0.0.10.Neighbor

relationshipsformbetweenrouterswhentheyreceiveeach

other'shellopacket.OverLANbroadcastmediasuchas

Ethernet,TokenRing,orFDDI,thehellopacketsaresentevery

5seconds.OverWANmultipointinterfaceswithabandwidthof

T1orgreater,andoverpoint-to-pointsub-interfaces,thehello

packetsarealsosentoutevery5seconds.WANmultipoint

interfaceswithabandwidthofT1orlowerareconsideredtobe

low-bandwidthinterfaces,andthehellopacketsaresentout

every60seconds.

Asidefromthehellotime,thereisalsoanotionofaholdtime.

Theholdtimetellstherouterthemaximumtimethatitwillwait

toresetaneighborifhellopacketsarenotreceived.Inother

words,iftheholdtimeexpiresbeforeahellopacketisreceived,

theneighborrela-tionshipwillbereset.Thedefaultvalueofthe

holdtimeisthreetimesthehellotime.Thismeansthatinthe

LANbroadcastmediawherethehellotimeis5seconds,the

holdtimewillbe15seconds,andtheslowWANinterfaceswith

ahellotimeof60secondswillhaveadefaultholdtimeof180

seconds.Keepinmindthatyoucanconfigurethehelloandhold

times.CertainconditionsmustbemetbeforeEIGRProuters

considerestablishinganeighborrelationship:

Thereceivingroutercomparesthesourceaddressofthe

hellopacketwiththeIPaddressoftheinterfacewherethe

packetwasreceived,toensurethattheybelongtothe

samesubnet.

ThereceivingroutercomparestheKconstantvaluesofthe



sourceroutertoitsown,tomakesurethattheymatch.

Thereceivingroutermustbewithinthesameautonomous

systemnumberasthesourcerouter.

Example6-1showstheoutputoftheshowipeigrpneighbor

commandwhentheneighborrelationshipisfullyestablished.



Example6-1showipeigrpneighborCommand

Output



Router_1#showipeigrpneighbor

IP-EIGRPneighborsforprocess1

HAddressInterfaceHoldUptimeSRTTRTOQSeq

(sec)(ms)CntNum

15.5.5.4Et01100:00:221450003

0192.168.9.5Et11000:00:23372223202

Theexplanationsoftheheadingoftheoutputareasfollows:

HThelistoftheneighborsintheorderinwhichtheyare

learned.

AddressTheIPaddressoftheneighbors.

InterfaceTheinterfacefromwhichtheneighborsare

learned.



HoldTheholdtimerfortheneighbor.Ifthistimerreaches

0,theneighborrelationshipistorndown.

UptimeThetimerthattrackshowlongthisneighborhas

beenestablished.

SRTT(SmoothRoundTripTime)Theaveragetimein

whichareliableEIGRPpacketissentandreceived.

RTO(RoundTripTimeout)Howlongtherouterwillwait

toretransmittheEIGRPreliablepacketifacknowledgment

isnotreceived.

QCountThenumberofEIGRPpacketswaitingtobesent

totheneighbor.

SequenceNumberThesequencenumberofthelast

EIGRPreliablepacketsbeingreceivedfromtheneighbor.

Thisistoensurethatpacketsreceivedfromtheneighbor

areinorder.



TheDiffusingUpdateAlgorithm

TheDiffusingUpdateAlgorithm(DUAL)isthebrainbehindthe

operationofEIGRP.Itisanalgorithmthattracksalltheroutes

advertisedfromaneighborandthenselectsaloop-freepathto

thedestination.BeforediscussingthedetailsofDUAL,youmust

understandseveraltermsandconcepts:

Feasibledistance(FD)Feasibledistanceistheminimum

metricalongthepathtoadestination.Figure6-1showsthe

feasibledistancecalculationtoreachNetwork7foreachof

RouterA'sneighbors,fromRouterA'sperspective.



Figure6-1.FeasibleDistanceCalculation



Reporteddistance(RD)Reporteddistance,sometimes

alsoknownasadvertiseddistance,isthemetrictowardthe

destination,asadvertisedbytheupstreamneighbor.In

otherwords,thereporteddistanceistheneighbor'smetric



goingtothedestination.Figure6-2showsthereported

distancecalculationtoreachNetwork7foreachofRouter

A'sneighbors.



Figure6-2.ReportedDistanceCalculation



Feasibilitycondition(FC)Thefeasibilitycondition(FC)is

aconditioninwhichthereporteddistance(RD)islessthan

thefeasibledistance(FD).Inotherwords,thefeasibility

conditionismetwhentheneighbor'smetrictoadestination

islessthanthelocalrouter'smetric.Thisconditionis

importanttoensurealoop-freepath.

EIGRPsuccessorAsuccessorisaneighborthatmetthe

feasibilitycondition(FC)andhasthelowestmetrictoward

thedestination.Asuccessorisusedasthenexthopto

forwardthepacketgoingtothedestinationnetwork.

FeasiblesuccessorAfeasiblesuccessorisaneighborthat

satisfiesthefeasibilitycondition(FC)butisnotselectedas

thesuccessor.Thefeasiblesuccessorcanbethoughtofasa



potentialbackuproutewhentheprimaryroutegoesaway.

Figure6-3illustratestheconceptsofsuccessorandfeasible

successor.



Figure6-3.ExplanationofSuccessorand

FeasibleSuccessor



RouterBischosenasthesuccessorbecauseRouterBhas

thelowestfeasibledistance(metric=121)toNetwork7

amongallofRouterA'sneighbors.Toselectafeasible

successor,RouterAseeswhichneighborhasareported

distance(RD)thatislessthanthefeasibledistanceofits

successor.Inthiscase,RouterHhasareporteddistanceof

30,whichislessthanthefeasibledistanceofitssuccessor,

whichis121.Therefore,RouterHischosenasthefeasible

successor.RouterDisneitherasuccessornorafeasible

successorbecauseitsreporteddistanceis140,whichis

largerthan121andthusdoesnotsatisfiesthefeasibility

condition.

PassiverouteApassiverouteinEIGRPindicatesthatthe



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