Dial-on-demand routing (DDR) enables two Cisco routers to use a dial-up connection on an
as-needed basis and is usually used as a backup solution in case of WAN circuit failure. DDR
is used only for low-volume, periodic network connections using either a PSTN asynchronous
or ISDN link. This was designed to reduce WAN costs if you have to pay on a per-minute or
per-packet basis.
Other terms you will undoubtedly run into are Legacy DDR Spoke configuration and Legacy
DDR Hub configuration. These terms are pretty simple to understand. A spoke interface
is any interface that calls or receives calls from exactly one other router. A hub, on the other
hand, calls or receives calls from more than one other router. Both configurations are similar
in theory, except that the hub is configured to call multiple locations.
DDR works when a packet received on an interface meets the requirements of an administratordefined
access list, which defines interesting traffic. The following seven steps give a basic description
of how DDR works when an interesting packet is received in a router interface:
1. The route to the destination network is determined.
2. Interesting packets dictate a DDR call.
3. Dialer information is looked up.
4. The call is placed.
5. The connection is established.
6. Traffic is transmitted.
7. The call is terminated when no more interesting traffic is being transmitted over a link and
the idle-timeout period ends.