The user was performing 'Network Link Interruption Validation Test'. After the user restored the Network connectivity access, all NFS clients except for one Red Had Enterprise Linux client could resume NFS access to the Unity NFS Shares.
Network traces taken concurrently between the Unity and the Red Had Enterprise Linux client show that the Red Had Enterprise Linux client was attempting to mount the Unity NFS share using NFSv4.2.
NFS V4 Call (Reply In 13) EXCHANGE_ID NFS V4 Reply (Call In 12) Status: NFS4ERR_MINOR_VERS_MISMATCH
The traces captured the NFSv4 client issuing an EXCHANGE_ID request with a minor version of "2" which receives a Unity response of NFS4ERR_MINOR_VERS_MISMATCH (10021) indicating that this minor version is not supported on the Unity.
After NFS4ERR_MINOR_VERS_MISMATCH (10021) response, the onus is on the NFSv4 client to back off the minor version from "2," to either "1," or to "0" (or to use NFSv3).
A 'Reboot' is recommended of the NFSv4 host. The host then attempts to negotiate with a supported minor version.