OSPF Configuration Task List 937Configuring area IDs on PEs can differentiate VPNs. Sites in the same VPN areconsidered as directly connected. PE routers then exchange OSPF routinginformation like on a dedicated line, thus network management and OSPFoperation efficiency are improved.n For configuration of this feature, refer to “BGP Configuration” on page 825 and“MPLS Basics Configuration” on page 1311.OSPF sham linkAn OSPF sham link is a point-to-point link between two PE routers on the MPLSVPN backbone.In general, BGP peers exchange routing information on the MPLS VPN backboneusing the BGP extended community attribute. OSPF running on a PE at the otherend utilizes this information to originate a type3 summary LSA as an inter-arearoute between the PE and CE.If a router connects to a PE router in the same area and establishes an internalroute (backdoor route) for a special destination, in this case, since an OSPFintra-area route has a higher priority than a backbone route, VPN traffic willalways travel on the backdoor route rather than the backbone route. To avoid this,an unnumbered sham link can be configured between PE routers, connecting therouter to another PE router via an intra-area route with low cost.n For sham link configuration, refer to “BGP Configuration” on page 825 and “MPLSBasics Configuration” on page 1311.Related RFCs ■ RFC 1765:OSPF Database Overflow■ RFC 2328: OSPF Version 2■ RFC 3101: OSPF Not-So-Stubby Area (NSSA) Option■ RFC 3137: OSPF Stub Router Advertisement■ RFC 3630: Traffic Engineering Extensions to OSPF Version 2OSPF ConfigurationTask ListComplete the following tasks to configure OSPF:Task Description“Configuring OSPF Basic Functions” on page 939 Required“Configuring OSPF Area Parameters” on page 940 Optional“Configuring OSPF NetworkTypes” on page 941“Configuring the OSPFNetwork Type for anInterface” on page 941Optional“Configuring an NBMANeighbor” on page 942Optional“Configuring a Router Priorityfor an OSPF Interface” onpage 942Optional
