Juniper JUNOSE, Configuration Manual

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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.
Preserving OSPF Routing Information Across the MPLS/VPN Backbone
MP-BGP attaches two new extended community attributes to the routes redistributed
from OSPF:
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OSPF domain identifier extended community attribute
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OSPF route type extended community attribute
MP-BGP uses these attributes and the MED to preserve OSPF routing information
across the BGP/MPLS VPN backbone.
OSPF Domain Identifier Attribute
The OSPF domain identifier attribute uniquely identifies the OSPF domain from
which a route was redistributed into MP-BGP.
You must configure an OSPF domain ID for the VRFs on the PE router with the
domain-id command. All VRFs that belong to a given OSPF domain must be
configured with the same domain ID. If not configured, the domain ID defaults to
zero. If you configure a value of zero, MP-BGP does not attach an OSPF domain
identifier attribute.
If the OSPF domain ID for the destination PE router differs from the originating PE
router, MP-BGP redistributes the route into OSPF as an OSPF type 5 external route.
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OSPF and BGP/MPLS VPNs
JUNOSe 11.0.x BGP and MPLS Configuration Guide