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Multipath interdomain routing has been proposed to enable flexible traffic engineering for transit Autonomos Systems (ASes). Yet, there is a lack of solutions providing maximal path diversity and backwards compatibility at the same time. The BGP-XM (Border Gateway Protocol-eXtended Multipath) extension presented in this paper is a complete and flexible approach to solve many of the limitations of previous BGP multipath solutions. ASes can benefit from multipath capabilities starting with a single upgraded router, and without any coordination with other ASes. BGP-XM defines an algorithm to merge into regular BGP updates information from paths which may even traverse different ASes. This algorithm can be combined with different multipath selection algorithms, such as the K-BESTRO (K-Best Route Optimizer) tunable selection algorithm proposed in this paper. A stability analysis and stable policy guidelines are provided. The performance evaluation of BGP-XM, running over an Internet-like topology, shows that high path diversity can be achieved even for limited deployments of the multipath mechanism. Further results for large-scale deployments reveal that the extension is suitable for large deployment since it shows a low impact in the AS path length and in the routing table size.