This can be done if you consider that you don't have to advertise all routes on CE-PE (CE being ISP that will be interconnected and PE being carrier who provide interconnection to that ISP).CE should advertise only internal routes to that ISP ( loopback , internal links etc).External routes (IE Internet routes , ISP Customer routes) will be exchanged between ISP IBGP neighbors . Search for Carrier Supporting Carrier feature on CCO .2547bis also explain this type of interconnection.
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