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See http://akanda.io/tip-of-the-week/tip-5-gre-and-vxlan-encapsulation-methods/

While GRE and VXLAN seem like equivalent encapsulation methods there is a subtle difference which can impact your deployment if you’re utilizing equal-cost multi-path (ECMP) routing.  VXLAN utilizes UDP, so nearly all routers properly distribute traffic to the next hop by hashing over the 5 tuple that include the UDP source and destination ports.  

GRE tunneling is a bit different since IP packets do not contain the information necessary to construct a 5-tuple hash.  How the router selects the path for a GRE packet is largely dependent on the hardware as some routers will use the GRE tunnel key to help create the hash.  If you intend to utilize ECMP in the underlay with GRE, it is important to select hardware that supports both otherwise you might not get the performance and utilization you expect.