I have an application that operates over UDP from the LAN to the internet. I am finding that randomly the Vigor 2820 is blocking the UDP transmission even though the 'session' still appears in the NAT list. The only way to clear the problem is to restart the Vigor 2820. It is as if the 2820 is blacklisting the UDP traffic from individual LAN addresses. I can have multiple examples of the application running through the same 2820 from different LAN addresses going to the same destination and not all of them are affected at the same time. The application sends periodic UDP packets of about 90bytes every 30seconds and there is a similar size response that comes back within the NAT'ed window.

Some additional information to my original post. When the problem occurs the outbound traffic from LAN to WAN is passed OK but the return response is not NAT'ed back to the source device on the LAN even though the route is still shown in the NAT list.