zfs send/receive on top level of filesystem heirachy

My apologies if I have missed the obvious, but I'm trying to replicate my root pool to another
disk.  My problem is that I only seem to be able to do a "| zfs receive tank2/XXX" and not a 
"| zfs receive tank2" as I would like.  I get a "cannot receive: destination 'rootzfs2' exists" for obvious
reasons.

Now I don't believe vfs.root.mountfrom="zfs:rootzfs/root" is valid either, so the only way to replicate a
root disk seems to by via rsync or some such method.

Is this correct??

thanks

Duncan