This is from the ONTAP 5.2.1 documentation on CD. It appears to be a hidden option;
Prior to ONTAP 5.0, the file ID field of a file handle was in the native byte order of the processor; as all our machines have processors running in little-endian mode, that meant it was little-endian. We added that option, as a visible option, in some release whose number I no longer remember, in order to keep some NFS clients from hashing most file handles into the same hash bucket, and thus having to search through lots of file handles when looking up the "rnode" for a file given a file handle.
In ONTAP 5.0, we changed the file handle format to include an indication of the volume on which the file exists; there's a bit in the file handle indicating whether it's an "old-style" or a "new-style" file handle - NFS requests that contain an "old-style" file handle will return "old-style" file handles, and requests that contain "new-style" file handles will return "new-style" file handles. Mount requests return "new-style" file handles.
Given that we'd changed the file handle format anyway, we figured we might as well make the file ID field big-endian. Once we'd done that, the only reason for that option is to let machines that had been running with that option, and that had handed out "old-style" file handles with big-endian file IDs, continue to handle those file handles correctly.
Given that
... If you turn the big_endianize_fileid option On, all NFS clients that have mounted directories from the filer must unmount and remount them; otherwise, they get "stale file handle" errors on all references to files already opened on the filer until they unmount and remount all directories.
and that, on a 5.x system, the mount daemon hands out "new-style" file handles, there is no difference between turning the option on and having all clients remount file systems, and leaving the option off and having all clients remount file systems; both of them cause all clients to have "new-style" file handles, which means they have file handles with big-endian file IDS.
Therefore, we made the option hidden.
It appears, however, that we didn't fix the documentation, and it still says there's a reason why you'd want to turn that option on; I'll file a bug on that.