[Porter]

Firewall and Smartdefense paper, chapter 5, starts on page 101

before we run into a misunderstanding, of course are your servers unavailable if ips a goes down, to avoid this just setup a second dns in a second offical ip range, that means your webservers will have two ip addresses entered inside dns, resolving is done via round robin, both dns are behind your fw-1, fw-1 dns proxy is configured to handle ip adresses from both ranges for one host, if isp a is down fw-1 will recognise it and will only reponse with ip-adresses from isp b

I understood your setup and needs but with your current setup it won't be possible, you have to change something, either setup the isp red as CP recommends it or do something like autonomous system