I have the following setup: a small office network behind a Cisco 2900 series router. The network consists of about 20 devices - PCs, printers and switches. I have two ISPs that I want to configure failover with. The thing is, ISP1 gives us real, static, public IP addresses, while ISP2 gives us just one such address and everything on the inside network has to be privately addressed (via DHCP).
Now, if both ISPs were giving us just one public address, failover could be configured with IP SLA to check if the primary route is still available and then a secondary static route that takes over if the primary fails. But how to do it if all the internal addresses have to change as well, from public to private? Is there some way to have a "dormant" DHCP server, which becomes active only when the primary route fails, and then turns off again when the primary route is restored? Or maybe there is some other option that is exactly for this scenario?
Thank you in advance.