The ASUS RT-N66U is a breeze to set up for dual WAN (where one LAN port can indeed be specified as a second WAN port) but the functionality is more limiting than I expected. Following is what I found on trying the two available modes: Load Balancing and Failover:
Failover mode - The router sticks with the primary WAN even when throughput has dropped to almost zero because it hasn't actually disconnected. I have to physically pull the plug on the primary WAN to get the router to failover to the secondary WAN - which it then near-instantly does.
Load balancing mode - To my untrained mind not true load balancing as ISPs and therefore networks are different so packets cannot be split and then recombined at the other end. All packets for a given connection therefore follow one route - WAN1 or WAN2, not a blend of the two. The load balancing ratio setting appears to be used to allocate connected sessions, not allocate actual load. To explain: In this sequence, imagine 2 web sessions opening, followed by a torrent download and finally a internet radio stream. With a ratio of 1:1 these will get shared equally between WANs in the sequence they were created; WAN1 ends up with one web session and a torrent download and WAN 2 with a web session and a radio stream. With 3:1 the first three get sent to WAN1 and the last one to WAN2 - so the low bitrate radio has a whole pipe to itself whilst two web sessions compete with a torrent.
It is possible to peg specific devices to a specific WAN but this is no help to me and my scenario; WAN1 degrading badly between the same times every day so me wanting to switch everything to WAN2.
Conclusion - To force switchover when WAN1 degrades use failover mode all the time (forcing everything to WAN1) and pull out the WAN1 cable from the router when the service does degrade - but leave the WAN1 modem powered up. Switching back then takes about 5 seconds on reconnecting the WAN1 cable. Another way is to put the WAN1 modem on a mains timer to switch it off just before the time of day when service degrades and back on at the time of day when this has typically passed - but don't then join a one hour VOIP call just before the timer kicks in!
Edited by TheVoyager, 15 March 2016 - 08:57 AM.