Alert only when n number of rule matches rcvd

["Jason Linden" <jlinden7 () adelphia net>]

I am trying to setup an rule that will only generate an alert if n
number of packets are received in n number of seconds.  IE like everyone
else we receive a large number of false positive 'ICMP Destination
Unreachable' alerts.  I would like to configure snort to only generate
an alert if say 30 of these packets are rcvd in 30 seconds.  Is there
any way to do this?
 
Thanks!
 
-jason