The NFS initiation process includes negotiating the protocol levels for servers and clients. If you do not specify the version level, then the best level is selected by default. For example, if both the client and the server can support version 3, then version 3 is used. If the client or the server can only support version 2, then version 2 is used.
Starting in the Solaris 10 release, you can set the keywords NFS_CLIENT_VERSMIN, NFS_CLIENT_VERSMAX, NFS_SERVER_VERSMIN, NFS_SERVER_VERSMAX in the /etc/default/nfs file. Your specified minimum and maximum values for the server and the client would replace the default values for these keywords. For both the client and the server the default minimum value is 2 and the default maximum value is 4. See Keywords for the /etc/default/nfs File. To find the version supported by the server, the NFS client begins with the setting for NFS_CLIENT_VERSMAX and continues to try each version until reaching the version setting for NFS_CLIENT_VERSMIN. As soon as the supported version is found, the process terminates. For example, if NFS_CLIENT_VERSMAX=4 and NFS_CLIENT_VERSMIN=2, then the client attempts version 4 first, then version 3, and finally version 2. If NFS_CLIENT_VERSMIN and NFS_CLIENT_VERSMAX are set to the same value, then the client always uses this version and does not attempt any other version. If the server does not offer this version, the mount fails.
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.
For procedural information, refer to Setting Up NFS Services.