According to the Draytek Vigor 2820 page:

WAN-Backup provides contingency (redunancy) in case of your primary ADSL line or ISP suffering temporary outage). Internet Traffic will be temporarily routed via the secondary Internet access. When normal services is restored to your primary ADSL line, all traffic is switch back to that.

So for redundancy the setup needs to be:

2*ADSL Connections
1*Vigor 2820
1*ADSL Modem

ADSL Connection1 connects into DSL port on Vigor 2820
ADSL Connection2 connects into ADSL Modem which connects to WAN2 on Vigor 2820

One of the switch ports on the Vigor then connects to our main switch.

Does the Vigor have a single internal (192.168.1.x) address and the redundancy happen automatically behind the scenes? That's certainly the impression from the website. Will there be any interuption to service if the primary ADSL connection dies? Is the administrator notified of any connectivity problems on *either* connection? I woudn't want the primary to drop only to find out the secondary dropped a week back

I'm about to recommend some 2820's for a number of sites but need to be confident that they work as described. My alternative is dual ADSL circuits with seperate routers and manually changing gateways, not something I want to consider!

Thanks in advance

your assumptions are correct, but WAN 2 could also be an ethernet based service (cable modem) too.

it is possible in the router configuration to do failover if one WAN link fails but not possible to generate an alert if that happens. To do that you might need to setup SMTP and monitor the router via a suitable tool within the lan or you could enable ping from the internet combined with DYNDNS and use something like pt360 to monitor the links.

Thanks for the reply, I am keen to see some redundancy in the links we use and it sounds as if this is the way forward for me. Although some form of notification would be nice to have it's far from essential, as you suggest I can monitor from the Internet anyway.