MPLS - multiarea OSPF in the core
When using OSPF as the routing protocol in the core of the MPLS network, you can deploy it as a multi-area OSFP topology. There is no requirement to keep a flat OSPF domain. Indeed, the only requirement is that the underlay routing be deployed such that the routers within the MPLS domain can reach each other. MPLS doesn’t “care” if this is via a single or multiple OSPF areas. In fact, MPLS networks maintained by ISPs will be quite large, and as such will require a multi-area OPSF implementation, so it is quite common.