Dial-on-Demand Routing (DDR) is a routing technique developed by Cisco that allows a user to utilize existing telephone lines, or public circuit-switched networks, to form a WAN instead of lines that are dedicated specifically to the WAN. DDR is typically implemented by users that do not need permanent, continuous links between sites on the WAN because the volume of traffic over the WAN is low and the transmissions are periodic as opposed to continuous. The connection only becomes active when data is sent to the remote site. When no data has been sent over the link for a specified amount of time, the link is disconnected.
Using DDR, a connection between sites is only established when a specific type of traffic initiates the call or when you a backup link is needed for redundancy or load sharing. DDR is used in order to save on the costs of a dedicated WAN line for organizations that do not need permanent continuous connection and as a back-up by organizations that use the dedicated line for critical applications.
For other uses of DDR, see DDR-SDRAM.