My client has a 2820Vn with two ISP connections: WAN1 is 7Mbps ADSL and WAN2 is connected to Openreach 50Mbps FTTC modem. (This must be such a common setup but I cannot find any definitive guide to making it work happily.)

If I use Auto Weight in the WAN General Setup and have both connections Always On everyone gets internet access but the traffic graph shows that traffic is evenly split between WAN1&2 even though WAN2 is so much faster.

If I make WAN1 Active only on WAN2 fail, then all traffic goes over WAN2 and the speed is good but WAN1 connection is dropped and subsequently most users cannot browse the Internet. I know they are still connected because I can log into their computers remotely and the VPN (on WAN2) works fine but DNS or Routing just stops working for them. Q.1 Why is that?

To get around this, I scoured this forum and found a tip to go to WAN >> Load-Balance Policy and tick Enable WAN2. This has worked in so far as the traffic graph now shows that WAN2 is handling nearly all the traffic but I’m wondering what will happen if the ADSL dies, will people lose DNS and routing again?

Setting up this router seems to be a very black art with so many settings and so little explanation of what each does. I think, for instance most of us would expect Auto Weight to intelligently favour the faster connection but this doesn’t seem to be the full solution.

Should I tick WAN1 in the ‘WAN >> Load-Balance policy’ and perhaps have it lower down than WAN2, say on Index 10? Is that how it works?

One other quirk I have is the the Online Status window shows WAN 1’s Mode as PPPoA but it has a link to Drop PPPoE - what’s that about?

Model Name : Vigor2820Vn
Firmware Version : 3.3.5.2_232201
Build Date/Time : Apr 7 2011 18:10:43
ADSL Firmware Version : 232201_A Hardware: Annex A

Both connections use 8.8.8.8 & 8.8.4.4

To get around this, I scoured this forum and found a tip to go to WAN >> Load-Balance Policy and tick Enable WAN2. This has worked in so far as the traffic graph now shows that WAN2 is handling nearly all the traffic but I’m wondering what will happen if the ADSL dies, will people lose DNS and routing again?

i am not sure newer firmwares ... when i was using it long time with 2connections was working great everything (load balance / fail over) even if only 1 wan was ticked
The secret was at Lan>general setup>details> & add manually some dns servers that will work for both ISP's

Both connections use 8.8.8.8 & 8.8.4.4

Then ones should be fine
But you need to tick both wans at load balance to make sure will work for both
All the above Not sure now days... but i guess must be the same

Should I tick WAN1 in the ‘WAN >> Load-Balance policy’ and perhaps have it lower down than WAN2, say on Index 10? Is that how it works?

when tried similar it did work (ticking both and moving them down) so yes it should work but again the one on top will get divert all the main traffic

see here for possible combinations
https://www.draytek.com/index.php?option=com_k2&view=itemlist&task=category&id=383:route-policy&Itemid=293&lang=en

One other quirk I have is the the Online Status window shows WAN 1’s Mode as PPPoA but it has a link to Drop PPPoE - what’s that about?

I think if you click drop PPPoE, it will just resynchronise the connection ... this is helpful if you want new line synchronization without rebooting the modem (which may effect and wan 2 in this case) ... it will get new ip address if client has dynamic one

Thanks babis3g for your thoughts on this. I finally bit the bullet and updated the firmware to 3.3.7.6_232201 but next morning people were unable to browse the internet. Then I realised that some ports were open and our SBS2008 was getting bombarded with logon attempts. That led to it having over 6000 open sessions and I think the whole router was getting bogged down - rebooting fixed it for a short while but the sessions would quickly climb back up and Internet browsing would fail again. Once I closed the ports (I use VPN to logon anyway) we finally got reliable web browsing. I suppose there's a limit on the total number of sessions the router can handle and this would explain why it was only web browsing that kept failing.
Load balancing is now working–it may just be that the router was then able to properly configure balancing or perhaps the new firmware fixed it. I have set Load Balance Mode: According to line speed with 7000/400 ADSL and 50000/10000 Fibre and all is well.

So if DNS is working, the connection speed is good, VPN is working perfectly, PING looks okay but you cannot browse web pages, check your SESSIONS in Diagnostics >> Data Flow Monitor!