This article describes how the high availability works with the Vigor3300 routers:
After enabling High Availability on both 3300 and setup the LAN IP and virtual IP addresses, the Slave unit will download and sync the settings from the Master unit automatically. It implies that the slave and master device must use exactly the same settings except for the LAN management IP address.
When the master is up, the WAN connections on the slave will not come up.
All the connections destined to the High Availability virtual IP address will be directed to the master device when it is up. If the master device is down (being powered off, having power failure, freezing or having LAN connectivity problems), the connection will be directed to the slave device automatically.