# Preservation of OSPF Routing Information Across the MPLS/VPN Backbone Overview

MP-BGP attaches two new extended community attributes to the routes redistributed from OSPF:

- OSPF domain identifier extended community attribute
- 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.

## OSPF Route Type Attribute

The route type attribute carries the OSPF area ID and LSA type, as indicated in Table 93:

Table 93: Route Types and Route Origins

Type of Route | Origin of Route
--- | ---
1 – intra-area route | Type 1 LSA
2 – intra-area route | Type 2 LSA
3 – interarea summary route | Type 3 LSA
5 – external route (area ID = 0) | Type 5 LSA
7 – external route (area ID = 0) | Type 7 LSA

MP-BGP uses the route type conveyed by this extended community attribute to determine the best OSPF route when it installs the routes in the VRF forwarding table on the destination PE router.