Nowadays, the success of MPLS is mostly due to the increasing demand for BGP/MPLS VPNs. Even though the need for interdomain LSPs is growing, no ISP today proposes the dynamic establishment of LSPs across AS boundaries. In this paper, we investigate the complexity of establishing end-to-end interdomain LSPs with QoS guarantees, based on the BGP routes locally available at a router. We explain the main issues of relying on BGP for the computation of interdomain constrained paths. To illustrate our point, we compare two LSP establishment techniques. Our benchmark technique is centralized and assumes the complete knowledge of the intradomain topologies. The second path computation technique isdecentralized and relies on the BGP routes locally available by each router. Our simulations confirm that the difficulty in designing BGP-based interdomain LSP establishment techniques lies within the trade-off between the scalability of the computation technique and the quality of the path found in terms of the considered metrics.