I have a client with a site-to-site VPN between a 2811 w/security software and an ASA 5505. The VPN was configured about 1 year and has been working no problem. About a week or 2 ago, the VPN will drop after 20 minutes wether traffic is on the VPN or not. The router and the ASA both show the VPN as up, but no traffic will pass. If they shutdown and re-enable the public port on the router, the tunnel will come back up. I tried setting the SA lifetimes to 10 minutes, still drops after 20 minutes. I updated the IOS on both to the latest version and rebooted, no change.
