-.B active-filter \fIfilter-expression
-Specifies a packet filter to be applied to data packets to determine
-which packets are to be regarded as link activity, and therefore reset
-the idle timer, or cause the link to be brought up in demand-dialling
-mode. This option is useful in conjunction with the
-\fBidle\fR option if there are packets being sent or received
-regularly over the link (for example, routing information packets)
-which would otherwise prevent the link from ever appearing to be idle.
-The \fIfilter-expression\fR syntax is as described for tcpdump(1),
-except that qualifiers which are inappropriate for a PPP link, such as
-\fBether\fR and \fBarp\fR, are not permitted. Generally the filter
-expression should be enclosed in single-quotes to prevent whitespace
-in the expression from being interpreted by the shell. This option
-is currently only available under NetBSD, and then only
-if both the kernel and pppd were compiled with PPP_FILTER defined.
-.TP