Cisco’s dial-on-demand routing (DDR) feature allows you to use existing telephone lines to form a wide-area network (WAN). While using existing telephone lines, you can analyze traffic patterns to determine whether the installation of leased lines is appropriate. DDR provides significant cost savings over leased lines for links that are utilized for only a few hours each day or that experience low traffic flow.


DDR over serial lines requires the use of dialing devices that support V.25bis. V.25bis is an International Telecommunication Union Telecommunication (ITU-T) Standardization Sector standard for in-band signaling to bit synchronous data communications equipment (DCE) devices. A variety of devices support V.25bis, including analog V.32 modems, ISDN terminal adapters, and inverse multiplexers. Cisco’s implementation of V.25bis supports devices that use the 1984 version of V.25bis (which requires the use of odd parity), as well as devices that use the 1988 version of V.25bis (which does not use parity).