Hi Ron,
Great question and very interesting topic for getting feedback from operator community on this forum.
We do see a lot of customers enquiring about deploying more scale out metro networks using CLOS and Fat Tree architectures. The underlying assumptions for going for CLOS or fat-trees topology is that alternative paths are available in case of failure of a link or a device. Given these are dense topologies, routers would detect failures locally and route packets via alternate paths. These alternate paths are ECMP / BGP multipath routes. (I have seen that BGP is popular protocol over CLOS /fat - tree. Personally, I have not seen much IGPs in CLOS / Fat-tree topologies. )
Anyways, great question. I am sure we will hear some real feedback from community. 🙂