Looks like client is asking for off-site failover, not really 100% uptime and the OP doesn't know how to achieve it over a WAN. Esp if this is a real enterprise

Looks like client is asking for off-site failover, not really 100% uptime and the OP doesn't know how to achieve it over a WAN. Esp if this is a real enterprise customer, they want Disaster Recovery (DR). This is a solved problem, albeit not a commonly known solution. Any of F5, Radware, and other expensive boxes can do this. This can also be done through DNS or with HA-Proxy etc.