hi there,
i am having an issue keeping a vpn up and running reliably between 2 vigor 2800 adsl routers located in different offices.
i have noticed in the "dialing" router syslog vpn log section has the following error message-

Initiating IKE Main Mode to 91......
Nat-Traversal; using RFC 3947, no NAT detected.

then the router procedes to re-establish the vpn connection.
this happens 2 or 3 times a day at seemingly random times.
Internet access is not affected from either router during the breakdown of the vpn - so its not the microfilters?
very occasionally the vpn line has to be dropped via its control interface and redialed.

the vpn had been working fine for about 2 years, dropping once every 100 hours or so.
both routers have the same firmware 2.8.1_RC3
any help will be much appreciated.
thanks
george