Redundant WANs

[rasoftware]

My ISP is able to provide SDSL and ADSL backup. Some how they are able to assign the same Ip address to each using 1700 series router which prevents a lot of problems when the backup kicks in. I guess this is some trick they pull off internally, just wondered if anyone knows how they do it?

[ekiriakos]

Not exactly sure how they do it, but here is plausible explanation.

Some ISPs allocate static IP addresses dynamically based on the username. So I suspect if the primary link goes down and the second dials, then it gets the same ip address.

Let us know if you find out how they do it...I am asking because they may use something like ip unnumbered loopback0 with the backup command etc...not sure how it will work with NAT...