An application session is configured between two CSS switches in two different geographic locations. There is no requirement for a point-to-point link joining the sites; the only requirement is that the two sites can route to each other.
Using the CSS switches DNS capability, a higher level DNS server (in this example, the one authoritative for yourdomain.com) will be configured to use both CSS' circuit VLAN address as NS records for a sub-domain called www.yourdomain.com. Whichever CSS is referred to by the higher level DNS server will then resolve that a record request with the IP address of the DNS content rule on the CSS.
The primary site will be preferred through a combination of using an ACL on the secondary CSS, an application session between the two CSS switches, and configuring the above mentioned NS records on the higher-level DNS server.
This URl should help you:
http://www.cisco.com/en/US/products/hw/contnetw/ps792/products_configuration_example09186a00801dcd75.shtml