During initiation, the transport protocol is also negotiated. By default, the first connection-oriented transport that is supported on both the client and the server is selected. If this selection does not succeed, the first available connectionless transport protocol is used. The transport protocols that are supported on a system are listed in /etc/netconfig. TCP is the connection-oriented transport protocol that is supported by the release. UDP is the connectionless transport protocol.
When both the NFS protocol version and the transport protocol are determined by negotiation, the NFS protocol version is given precedence over the transport protocol. The NFS version 3 protocol that uses UDP is given higher precedence than the NFS version 2 protocol that is using TCP. You can manually select both the NFS protocol version and the transport protocol with the mount command. See the mount_nfs(1M) man page. Under most conditions, allow the negotiation to select the best options.