OSPF - MPLS SuperBackbone
In an MPLS topology where OSPF is used between the CE and PE routers, each remote site is configured to have its own OSPF instance. However, in such a case, even if multiple CEs are configured with independent OSPF instances, the MPLS network functions as a SuperBackbone. The following diagram illustrates this:
Both of the customer sites are using OSPF area 0, normally it’s impossible to have two backbone areas unless you connect them to each other with a virtual link. When we use MPLS L3 VPN, the service provider network is seen by OSPF as the SuperBackbone: This allows us to use area 0 on multiple sites without using virtual links, the SuperBackbone connects everything together. More about this is detailed in RFC 4577 linked below.
Links:
https://networklessons.com/mpls/mpls-layer-3-vpn-pe-ce-ospf#Verification