Say we have the given scenario, see diagram. The building has given us two routable addresses (66.56.77.24 and 11.54.21.12), one for each of their ISP's for redundancy to the firewall in our suite. So ISP1 routed the 66.56.77.16/28 to the building's firewall, and the building puts a sub-interface on the insided interface of their firewall (Top firewall) so our suite firewall can use that routable network. Is it possible for the building firewall to route this 66.56.77.16/28 over ISP2 if ISP1 goes down? Wouldn't return traffic be a problem if ISP1 were down since ISP1 is routing that block to the building firewall? I don't ever see ISP2 being able to route incoming traffic to the building firewall for this public subnet. I know BGP is a whole different animal, assume no BGP is used.
