Version 2 and Version 3 Negotiation

NFS servers might be supporting clients that are not using the NFS version 3 software. So, part of the initiation procedure includes negotiation of the protocol level. If both the client and the server can support version 3, that version is used. If either the client or the server can only support version 2, that version is selected.

You can override the values that are determined by the negotiation by using the -vers option with the mount command. See the mount_nfs(1M) man page. Under most circumstances, you should not have to specify the version level, as the best level is selected by default.