101VPNOSPF supports multi-instance, which can run on PEs in VPN networks.In BGP MPLS VPN networks, multiple sites in the same VPN can use OSPF as the internal routingprotocol, but they are treated as different ASs. An OSPF route learned by a site will be forwardedto another site as an external route, which leads to heavy OSPF routing traffic and managementissues.Configuring area IDs on PEs can differentiate VPNs. Sites in the same VPN are considered asdirectly connected. PE routers can exchange OSPF routing information on a dedicated line, sonetwork management and OSPF operation efficiency are improved.OSPF sham linkAn OSPF sham link is a point-to-point link between two PE routers on the MPLS VPN backbone.In general, BGP peers exchange routing information on the MPLS VPN backbone using the BGPextended community attribute. OSPF running on a PE at the other end utilizes this information tooriginate a Type-3 summary LSA as an inter-area route between the PE and CE.If a router connects to a PE router in the same area and establishes an internal route (backdoorroute) to a destination, in this case, because an OSPF intra-area route has a higher priority than abackbone route, VPN traffic will always travel on the backdoor route rather than the backboneroute. To avoid this, an unnumbered sham link can be configured between PE routers, connectingthe router to another PE router via an intra-area route with a lower cost.For OSPF sham link configuration, see MPLS TE in the MPLS Configuration Guide.Protocols and standards• 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 2• RFC 4811: OSPF Out-of-Band LSDB Resynchronization• RFC 4812: OSPF Restart Signaling• RFC 4813: OSPF Link-Local SignalingOSPF configuration tasksAn OSPF routing domain has different types of routers, such as intra-area routers, ABR, andASBR.
