I have an old NetVanta 3448 that has a VPN connection to a satellite facility. The tunnel has been stable for a long time, until recently. The tunnel will visually look like it's ok, but it's not. Facility A will lose internet connectivity, and Facility B will lose connectivity to Facility A, but still have internet connectivity. Facility B is on a different subnet, has a router, etc. The two facilities connect via fiber over this VPN. What seems to always fix it is letting the tunnel tear down and rebuild, even though it looks like its fine and other VPN's on the same router are functioning fine.