After the P router removes the RD, which routing protocol does the PE use to advertise routes to the CE without redistribution or static routes?

After the P (Provider) router removes the Route Distinguisher (RD), the PE (Provider Edge) router uses BGP IPv4 to advertise routes to the CE (Customer Edge) router. This is because the PE router acts as a BGP speaker to the CE router in many MPLS VPN scenarios, exchanging standard IPv4 routes. The RD is a VPNv4 concept used within the MPLS backbone (MP-BGP) to distinguish overlapping IP addresses from different VPNs. Once the traffic exits the MPLS VPN cloud at the PE, the RD is no longer relevant for the CE. MP-BGP is used between PE routers to carry VPNv4 routes, not directly between PE and CE for standard IPv4. OSPF and IS-IS are interior gateway protocols (IGPs) and are not typically used for advertising routes from a PE to a CE without redistribution, especially when BGP is the native protocol for route exchange in this context.