When OSPF sites have a backdoor connection, they will by default prefer that link over the MPLS VPN link. Because of the redistribution that occurs, the VPN routes will be seen as inter-area (if OSPF process numbers match on PEs), E1 or E2 routes. As you probably know by now, inter-area and external routes are less preferred than intra-area routes in OSPF. No amount of administrative distance-altering or interface cost- changing can affect this decision making. Here we look briefly at a feature designed to allow VPN routes to look like intra-area routes, giving us the ability to prefer them over the VPN connection by adjusting interface costs.
Above topology show the how OSPF Sham-Link configured .
Below Link give you complete Document for same .
http://chetanress.blogspot.com/2010/08/mpls-traffic-engineering-with-ospf-sham.html
Regards
Chetan Kumar Ress