Failover with two ISPs, one with public and one with private addresses

Question

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.

Answer 1

That depends on whether or not you need to public addresses to be available outside you company. If so, you are going to need to work with the ISPs to set up BGP routing with them.

If you don't need for your internal, public addresses to be seen outside your network, then you can simply NAT with the failover ISP. NAT doesn't care that addresses are public or private; there is nothing inherent in IP that distinguishes public or private addresses.

Answer 2

Putting aside the question of why you are using public addresses in your network for a moment, you would configure this the same way if you were using private addresses: you configure NAT (actually PAT) to translate your inside addresses to the one public address. In other words, it does not matter if your inside addresses are public or private -- NAT works exactly the same way.