I've recently put in a Draytek 2820 with a failover ADSL.

WAN1 = ADSL2+ ISP
WAN2 = BT Wholesale ISP.

WAN2 has a Zyxel P660 router attached which is configured with a private static IP

I've got this setup working fine, failover and failback works perfectly but I'm wondering if it's possible to to port forwarding to this backup line. For instance, if the WAN1 goes down, I'd like for email to come in but down the WAN2, obviously I'll need to have MX records set but what does that mean for my port forwarding? The Zyxel and Draytek routers are on completely seperate LAN addresses so I don't know if I could double NAT it. i.e port 25 => 192.168.1.2 (Zyxel router IP) => 192.168.1.1 (Draytek WAN2 IP) => 192.168.16.1 (Draytek 2820 IP).


Interested to hear if anyone has any suggestions