2. Advanced architecture: transparent redirection

When deploying an Internet access solution based on the use of proxies declared in client workstation browsers, we quickly come up against the problem of how to behave in the event of the loss of one of the site's Internet exit points. The browser always tries to ask the application relay to do the job. Of course, we could imagine a solution where a server would supply a PAC browser configuration file, which would be generated after checking that the application relay is working properly. So why tinker when the problem of redundant access from one network to another is already handled by dynamic routing protocols such as OSPF or BGP, or even WCCP for simple interfacing with an application relay?