Crossing file systems means the NFS client can also potentially be a server, and remote and local mounted file systems can be freely mixed. This leads to some problems when a client travels down the directory tree of a remote file system and reaches the mount point on the server for another remote file system. Allowing the server to follow the second remote mount would require loop detection, server lookup, and user revalidation. When a client does a lookup on a directory on which the server has mounted a file system, the client sees the underlying directory instead of the mounted directory.

The NFS server does not support crossing file systems in NFS protocol versions 2 (NFSv2) and 3 (NFSv3) for either local or remote file systems. In NFS protocol version 4 (NFSv4) the z/OS NFS server does support crossing local file systems, but not remote file systems. For example, if a server has a file system called /usr and mounts another local file system on /usr/src, a client can also mount /usr, but the server will only see the mounted version of /usr/src with NFSv4. In NFSv2 and NFSv3, a client could perform remote mounts that match the server’s mount points to maintain the server’s view. In this example, the client would also have to mount /usr/src in addition to /usr, even if the mounts are from the same server.