BFD is a pretty universal method to detect whether a certain connectivity/link is online. The NSX Edges establish a BFD session between themselves, which can detect a failure of the primary edge quicker than the previous method used.
Below is a copy/paste from a BFD explanation:
The Bidirectional Forwarding Detection (BFD) protocol is a simple mechanism that detects failures in a network and works in a wide variety of network environments and topologies. In BFD operation, switches exchanges BFD hello packets at a specified interval and detect a neighbor failure if they do not receive a reply after a specified interval. The BFD failure detection timers support shorter time limits than the static route failure detection mechanisms, so they can provide faster detection of failures.
Does this answer your question?
What about bfd upstream ? i.e. between DLR and edge (ecmp) and edge (ecmp) and the physical router ?
If 6.4 has bfd support, it should be possible to use it ?