flow-based datapath. \fBovs\-openflowd\fR connects to one or more
OpenFlow controllers over TCP or SSL.
.PP
+For a more powerful alternative to \fBovs\-openflowd\fR, see
+\fBovs\-vswitchd\fR(8). Do not run both daemons at the same time.
+.PP
The mandatory \fIdatapath\fR argument argument specifies the local datapath
to relay. It takes one of the following forms:
.
.SS "OpenFlow Options"
.TP
\fB\-\-datapath\-id=\fIdpid\fR
-Sets \fIdpid\fR, which must consist of exactly 16 hexadecimal digits,
+Sets \fIdpid\fR, which must consist of exactly 16 hexadecimal digits
+and may not be all-zero,
as the datapath ID that the switch will use to identify itself to
OpenFlow controllers.
.IP