If a VPN connection drops due to an inactivity timeout or for any other reason, a reboot may be required to restore the VPN connection on the router.
In DD-WRT and Tomato firmware there is a feature that allows your router to monitor the active connection and will reboot on its own in order to immediately restore connectivity in case a connection drop is ever detected.
Keep Alive with DD-WRT Firmware:
Go to the Administration > Keep Alive page and enable WDS/Connection Watchdog. Enter 300 for the Interval and 188.8.131.52 as the IP Address.
Click Apply Settings and then navigate to the Administration > Management page and click Reboot Router.
Keep Alive with TomatoUSB Firmware:
Go to VPN Tunneling > OpenVPN Client > Client 1 > Advanced tab section and in the Custom Configuration box, add a separate line at the end of any other content already in the field with the following:
keepalive 10 60
This feature works by pinging Google DNS or checking for connection activity on servers to make sure the router is up and running with a full Internet connection.