User`s guide

OSPF Overview

8-4 Configuring OSPF

Figure 8-1 OSPF Topology

OSPF Areas

OSPFallowsnetworkstobegroupedintoareas—acollectionofsubnetsthataregroupedina

logicalfashion.Theseareascommunicatewithotherareasthroughthebackbonearea.Routing

informationpassedbetweenareasisabstracted,potentiallyallowingasignificantreductionin

routingtraffic.OSPFusesfourdifferenttypesof

routes,listedinorderofpreference:

•Intra‐area—Destinationswithinthesamearea

•Inter‐area—Destinationswithindifferentareas

•Type1ASE—AutonomousSystemExternaldestinations

•Type2ASE—AutonomousSystemExternaldestinations

BothtypesofASEroutesareroutestodestinationsexternaltoOSPF(andusuallyexternal

tothe

AS).RoutesexportedintoOSPFASEastype1ASEroutesaresupposedtobefrominterior

gatewayprotocols,suchasRIP,whoseexternalmetricsaredirectlycomparabletoOSPFmetrics.

Whenaroutingdecisionisbeingmade,OSPFwilladdtheinternalcosttotheASborder

routerto

theexternalmetric.Type2ASEsareusedforexteriorgatewayprotocolswhosemetricsarenot

comparabletoOSPFmetrics.In thiscase,onlytheexternalOSPFcostfromtheASborderrouteris

usedintheroutingdecision.

Allimportedroutesdefaulttoexternaltype2routes,and

canbeconfiguredasexternaltype1in

caseswherethemetricsarecomparable.

Toreducetheamountofroutinginformationpropagatedbetweenareas,youcanconfigure

summaryrangesonAreaBorderRouters(ABRs).Intra‐areaLinkStateAdvertisements(LSAs)

Area 1

OSPF Backbone

Area 2

ABR

ASBR

Internet