Here is the setup: Cisco ASA 5510 (Security Plus) 7.2(3) connected in a hub-and-spoke setup in IPSec site-to-site VPN with 5 sites, each one with Cisco 877 routers. The sites have HVAC equipments connected to them talking Bacnet/IP. The central site have a Bacnet/IP "router" behind the Cisco ASA 5510. Bacnet/IP communications are always on UDP/47808 (source and destination).
Everything runs smoothly for about a week. Past 1 week, obscur packet drop occurs. We are not able to understand why. The (temporary) solution is to flush the flow that the UDP communication is using with the command "clear local-host <IP address of Bacnet/IP router>". Once this is done, everything runs smoothly for another week. We do not think it would be related to VPN. Very weird.