|  | 2.5 | WAN Link Options
| 2.5.7 | DDR
Dial-on-demand routing (DDR) is a technique in which a router can dynamically initiate and close circuit-switched sessions when transmitting end stations need them. When the router receives traffic destined for a remote network, a circuit is established, and the traffic is transmitted normally. The router maintains an idle timer that is reset only when interesting traffic is received. (Interesting traffic refers to traffic the router needs to route.) If the router receives no interesting traffic before the idle timer expires, however, the circuit is terminated. Likewise, if uninteresting traffic is received and no circuit exists, the router drops the traffic. When the router receives interesting traffic, it initiates a new circuit. DDR enables you to make a standard telephone connection or an ISDN connection only when required by the volume of network traffic. DDR may be less expensive than a dedicated-line or multipoint solutions. DDR means that the connection is brought up only when a specific type of traffic initiates the call or when you need a backup link. These circuit-switched calls, indicated by the broken lines in Figure are placed using ISDN networks. DDR is a substitute for dedicated lines when full-time circuit availability is not required. In addition, DDR can be used to replace point-to-point links and switched multi-access WAN services. DDR can be used to provide backup load sharing and interface backup. For example, you might have several serial lines, but you want the second serial line to be used only when the first line is very busy so that load sharing can occur. When your WAN lines are used for critical applications, you might want a DDR line configured in case the primary lines go down. In this case, the secondary line enables itself so traffic can still get across. Compared to LAN or campus-based networking, the traffic that uses DDR is typically low volume and periodic. DDR initiates a WAN call to a remote site only when there is traffic to transmit. When you configure for DDR, you must enter configuration commands that indicate what protocol packets make-up interesting traffic to initiate the call. To do this, you enter access control list statements to identify the source and destination addresses, and you choose specific protocol selection criteria for initiating the call. Then you must establish the interfaces where the DDR call initiates. This step designates a dialer group. The dialer group associates the results of the access control list specification of interesting packets to the router's interfaces for dialing a WAN call.