I'm running a 2820 on firmware 3.3.7.6. We have WAN1 connected to an ADSL service but recently installed a leased line on WAN2.

To get everybody on the network using WAN2 I set load balance policy under firewall > general setup > default rule to WAN2.

If WAN1 is disconnected for any reason we completely lose access to the internet. Any ideas why his would be and if we can stop this from happening? I have a feeling I've missed something.

I think is because due to the firewall rule worth to try delete & set the load balance with one of the following way
http://www.draytek.com/index.php?option=com_k2&view=item&id=5181:policy-route&Itemid=293&lang=en
http://www.draytek.com/index.php?option=com_k2&view=item&id=1810:faq-article-1810&Itemid=293&lang=en
personal never needed to set firewall rule with load balance

Hi babis3g. Thanks for the swift response.

I read through the documents and reconfigured the router. I still lose internet when WAN1 is disconnected. I'm not even able to ping an external IP address (tried 8.8.8.8).

Could there be anything else?

try at lan > general setup to add your preferred dns manually, on exit tick the box force router to use .. Lan 1, if there is this option
Reboot & test

I don't own this model but If not works the other i can think ... have updated correctly the unit? for correct functionality

IMPORTANT NOTE: Do not upgrade directly from 3.3.5 (and earlier) to 3.3.7.5.
Due to differences in the Web UI and functionality the router MUST first be
upgraded to 3.3.5.2 prior to upgrading to 3.3.7.6.

Upgrade your router to Version 3.3.5.2 first, and afterward upgrade the router
to Version 3.3.7.6.

(i guess using the *rst* files extension for a clean & fresh instal)

I think you've cracked it!

I did a bit of a test using the GRC DNS Benchmark tool and found that ALL of BTs DNS server addresses are inaccessible when going over the Virgin Media Business line on WAN2. My guess is that if the DNS is unreachable the router may treat the WAN as down maybe.

I have set the DNS addresses in LAN > General Setup to those for Virgin Media Business and forced the router to use them. I cannot test to confirm as this would potentially kill off internet and also the phone system but I think we have a clear candidate for a fix.

Regardless of outcome I much appreciate your help with this.

When i had 2 internet connections (VM & TT) left both wans enabled for load balancing... for just testing the dns

Used the DNS Benchmark tool ( i think is better) and while both wans was running the dns tool showed which ones was responding good for both connections
If i remember at that times was good the level 3, google, open dns, cable & wireless & BT (194.74.65.69, 194.74.65.68 - only them 2 at that times) was good enough for both

So i picked up one of them & ignored the Virgin Media ones
(VM ones was causing same issues for me ... was not responding when was in use with load balance mode of the draytek 2830, 2850)
I think your solution to ignore VM ones
... so try one of the above mentioned 3rd party dns (or similar) & i am sure will solve your issue

In other words basically just run the dnsbench while both wans are on load balance (set them for testing) and will show you which ones are the fastest & responding for both wan
Then set them manually at lan & force router to use them
There for can set failover afterwards