The advantages of a Border Gateway Protocol (BGP)-free core are very obvious: As the core routers of your network do not run BGP, they do not carry Internet routing and are thus not affected by instabilities outside of your network. You can find more information about this particular design in my SearchTelecom.com article, "Scale your backbone with core MPLS, BGP on the edge article."
A BGP-free core requires end-to-end MPLS transport between network edges, introducing yet another technology in the carrier's network. MPLS is a given for service providers that offer MPLS-based VPN services, but it might not be used in small providers' networks. In smaller networks, carriers might prefer to run BGP on all core routers to avoid introducing MPLS in their network core.
One would expect that most ISPs run with a BGP-free core today, but an informal survey I did a while ago indicates that almost half of the respondents still use native transport of Internet traffic and run BGP on all core routers (read the caveats associated with that survey).
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