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OSPF Overview
8-4 Configuring OSPF
Figure 8-1 OSPF Topology
OSPF Areas
OSPFallowsnetworkstobegroupedintoareasacollectionofsubnetsthataregroupedina
logicalfashion.Theseareascommunicatewithotherareasthroughthebackbonearea.Routing
informationpassedbetweenareasisabstracted,potentiallyallowingasignificantreductionin
routingtraffic.OSPFusesfourdifferenttypesof
routes,listedinorderofpreference:
•IntraareaDestinationswithinthesamearea
•InterareaDestinationswithindifferentareas
•Type1ASEAutonomousSystemExternaldestinations
•Type2ASEAutonomousSystemExternaldestinations
BothtypesofASEroutesareroutestodestinationsexternaltoOSPF(andusuallyexternal
tothe
AS).RoutesexportedintoOSPFASEastype1ASEroutesaresupposedtobefrominterior
gatewayprotocols,suchasRIP,whoseexternalmetricsaredirectlycomparabletoOSPFmetrics.
Whenaroutingdecisionisbeingmade,OSPFwilladdtheinternalcosttotheASborder
routerto
theexternalmetric.Type2ASEsareusedforexteriorgatewayprotocolswhosemetricsarenot
comparabletoOSPFmetrics.In thiscase,onlytheexternalOSPFcostfromtheASborderrouteris
usedintheroutingdecision.
Allimportedroutesdefaulttoexternaltype2routes,and
canbeconfiguredasexternaltype1in
caseswherethemetricsarecomparable.
Toreducetheamountofroutinginformationpropagatedbetweenareas,youcanconfigure
summaryrangesonAreaBorderRouters(ABRs).IntraareaLinkStateAdvertisements(LSAs)
Area 1
OSPF Backbone
Area 2
ABR
ABR
ABR
ASBR
Internet
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