OSPFv2 as PE-CE routing protocol is supported for IPv4 VPNs. OSPFv3 is not supported as a PE-CE routing protocol.
Using OSPF as a CE to PE routing protocol allows OSPF that is currently running as the IGP routing protocol to migrate to an IP-VPN backbone without changing the IGP routing protocol, introducing BGP as the CE-PE or relying on static routes for the distribution of routes into the service providers IP-VPN. The following features are supported:
Advertisement/redistribution of BGP-VPN routes as summary (type 3) LSAs flooded to CE neighbors of the VPRN OSPF instance. This occurs if the OSPF route type (in the OSPF route type BGP extended community attribute carried with the VPN route) is not external (or NSSA) and the locally configured domain-id matches the domain-id carried in the OSPF domain ID BGP extended community attribute carried with the VPN route.
OSPF sham links. A sham link is a logical PE-to-PE unnumbered point-to-point interface that essentially rides over the PE-to-PE transport tunnel. A sham link can be associated with any area and can therefore appear as an intra-area link to CE routers attached to different PEs in the VPN.