# Understanding Usage of BGP/MPLS VPNs to Connect OSPF Domains

Before reading this section, we recommend you be thoroughly familiar with OSPF. For detailed information about that protocol, see JunosE IP, IPv6, and IGP Configuration Guide.

You can use BGP/MPLS VPNs to connect OSPF domains without creating OSPF adjacencies between the domains. The BGP/MPLS VPN backbone acts as either an OSPF backbone (area 0) or an OSPF area above the backbone.

In this topology, OSPF is the routing protocol between the CE router and the PE router. This OSPF link can be configured in area 0 or any other OSPF area. However, if the customer site has any connections to area 0, then at least one OSPF router configured on a CE router must have an area 0 link to a PE site. In this case, the BGP/MPLS VPN acts as if it is in an area above the OSPF backbone area. When the PE-CE link is in a nonbackbone area, the BGP/MPLS VPN acts as an OSPF backbone.

In either case, the OSPF router configured as a PE router in the BGP/MPLS VPN is always treated as an area border router (ABR) and functions as an area 0 router so that it can distribute interarea routes to the CE router. The BGP/MPLS VPN distributes both interarea and intra-area routes between PE routers as interarea, type 3 summary routes.

## Distributing OSPF Routes from CE Router to PE Router

You configure OSPF in the VRF associated with the VPN and associate the interface connected to the CE router with the VRF. OSPF routes can then propagate from a CE router to a PE router when an OSPF adjacency has formed between the two routers. OSPF adds routes to the VRF’s forwarding table at the PE router side with routes learned from the CE router.

## Distributing Routes Between PE Routers

The OSPF routes in the VRF forwarding table are OSPF IPv4 routes, but BGP/MPLS VPNs distribute VPN-IPv4 routes by means of MP-BGP. You must configure the VRF to redistribute the OSPF routes into MP-BGP. MP-BGP converts each imported OSPF route to a VPN-IPv4 route, applies export policy to the route, and then propagates the route to a remote PE site by means of the MPLS/VPN backbone. At the destination PE router, MP-BGP places each route in the appropriate VRF forwarding table based on the import policy for each VRF and the route target associated with the route.