Source: http://www.fli4l.de/en/home/news/

If pppd supports "active precompiled filters", you can use this
utiliy to generate them. This is useful for preventing the pppd
to bring an on-demand connection up by spurious traffic, such
as ntp or routing protocol packets.

usage: pfc <expressiona> > /etc/ppp/your.active.filter

(specify precompiled-active-filter=/etc/ppp/your.active.filter in the ppp
 options file)

example: ./pfc ntp and ldap