Dial-on-demand routing (DDR) determines whether to bring up a connection that is not already active based on interesting and uninteresting traffic coming into the router. Interesting traffic brings up a connection, and uninteresting traffic doesn't.

How does a router know which traffic is interesting and which isn't? Through preconfigured access lists and dialer lists.

It is very important to understand the need for static route entries to prevent routing updates from initiating a call and thus adding unnecessary service charges. DDR can be configured with a number of different options. For instance, an idle timer disconnects a call when no traffic has been transmitted for a predetermined period of time.

It can also be used for other valuable purposes, such as backup for a leased line or Frame Relay connection. In this case, an ISDN link may be brought up after a certain load has been reached on the main line or a preconfigured length of time has lapsed since the line became inactive.

Another DDR concept is so-called legacy DDR versus dialer profiles. You can think of legacy DDR as the configuration that applies to the physical interface, unlike dialer profiles, which use logical dialer interfaces to accomplish DDR.

To accomplish DDR configuration, you need to go through the following steps.

• Specify interesting traffic.
  
  
• Assign these parameters to an interface.
  
  
• Define the destination aspects with legacy DDR or dialer profiles.