The answer is "Layer 2 Gateway Services". The physical switches have to implement the OVSDB and related APIs that communicate with NSX Controller so that they can provide the mapping of the segment in virtual network and VLAN in physical network. For more information in details, you can check out the following URLs.
http://networkheresy.com/2013/08/15/network-virtualization-gets-physical/
https://www.youtube.com/watch?v=QDOlggwyrVA&feature=c4-overview-vl&list=PLdYldEmmLm2lz5Bd0bzGCDKSULL52ytJT
Ivan's NSX Architecture Introduction, it is very good.
http://demo.ipspace.net/get/NSXArch
