There are two reasonable ways of building a layer-2 leaf-and-spine fabric: use VXLAN (the direction almost everyone in the industry is taking at the moment), or routing-on-layer-2 technology like TRILL or SPB.

Only a few vendors decided to use SPB to build data center fabrics, and Roger Lapuh from Avaya was kind enough to explain how you can do that during the Leaf-and-Spine Fabric Architectures webinar (for more details on SPB, see also SPB Deep Dive video from the same webinar).