I'm trying to setup a snmp filter not monitor my border public

I've made the following filter, but it doesn't work. Can someone
point me
my mistakes?:

src int: all
dst int: all
packet: snmp (built in)
src port: all
prot: UDP
dst port: 161
src addr: host
ip addr: public int
dest addr: host
ip addr: mrtg server
-- Same setup for snmp-trap -- except port no ;-)

I also tried to put src addr and dest addr to any address without it working either? Any ideas?? Also, when I unload ipflt it works.