This document will show when customer run OSPF with PE in that case the routes advertised from one CE to another CE travelling MPLS backbone should be IA(Inter Area) routes but in some cases the routes could be external. MPLS backbone is working as trasparent to the customer, so for customer every route should be inter area route. But may be a day you get a call from your customer which is getting all its routes as external routes instead of inter area routes. I have tried to explain how the external route constraint can be removed by taking a simple precaution.