BGP was designed for AS-level routing, but that doesn't mean it can't be adapted to work at a smaller scale. iBGP is for in-network routing (as compliment to eB

BGP was designed for AS-level routing, but that doesn't mean it can't be adapted to work at a smaller scale. iBGP is for in-network routing (as compliment to eBGP), although I couldn't say whether they are using it directly or a variant. When you talk about 'slower than a spanning tree', that's presumably for route stabilization. But that's also largely irrelevant - routes would change infrequently enough that extremely fast stabilization isn't especially important (and on a LAN, BGB would still stabilize quite quickly). In comparison, BGP lets you add all sorts of routes to your internal network, and spread load among them without hotspotting. It can handle routes of varying sizes, loops and parallel paths, etc. Spanning trees on the other hand will pick one path from point A to point B, which won't necessarily be the best general solution for a network topology. Say you want to add a 'skip list' of rack-to-rack connections, double up others, and use two parallel DC-level tree networks because you're hitting backbone throughput limits with just one. BGP will be much better able to capitalize on this crazy mesh of connections then simpler protocols like building spanning trees.