Configuring OSPF as PE-CE protocol enables us to distinguish between the “real external routes” and intra network routes between the sites that are stretched across VPN.  But the problem arises when VPN sites are in the same area and have a backdoor connection. With OSPF as PE-CE protocol redistribution, CE routers end up getting inter-area routes(assuming the VRFs on the PE devices that connect the CE sites, are configured with the same OSPF domain id) that actually belong to the same area and just happen to be multihomed to the backbone.