4000 Router Series Layer 2 - WAN Configuration Guide

Configuring DDR

Overview

Dial-on-demand routing (DDR) is a routing technology used when routers interconnect through a public

switched network, such as a PSTN or an ISDN. It can provide the dial-on-demand service in which any

two routers dial to set up a connection when data needs transferring instead of setting up a connection

before that. When the link becomes idle, DDR automatically disconnects it.

Under certain circumstances, connections between routers are instantly established whenever there is

data to be transferred, so data transfer is time-independent, bursty, and small sized. DDR is a flexible,

economical, and efficient solution for such applications. In DDR, backup mechanisms are available to

guarantee communications. In case a primary line fails, DDR switches traffic over to a secondary line to

ensure ongoing services.

Approaches to DDR

Two approaches are available to DDR: circular DDR (C-DDR) and bundle DDR (B-DDR). They are suitable

for different applications. In practice, the two parties in a call do not necessarily adopt the same

approach.

The interfaces used in DDR are defined as follows:

• Physical interface—An interface that supports DDR. Examples are serial, BRI, and asynchronous

interfaces.

• Dialer interface—A logical interface created for configuring DDR parameters. A physical interface

can inherit the DDR configurations after it is assigned to a dialer interface.

• Dialup interface—Any interface used for dialup connection. It can be a dialer interface, a physical

interface assigned to a dialer interface, or a physical interface directly configured with DDR

parameters.

C-DDR

C-DDR delivers the following features:

• A dialer interface can contain multiple physical interfaces, but a physical interface can be assigned

to only one dialer interface. A physical interface can provide only one type of dial service.

• You can assign a physical interface to a dialer interface to inherit DDR parameters by assigning the

physical interface to a dialer circular group. Alternatively, you can configure DDR parameters

directly on a physical interface.

• All the physical interfaces in a dialer circular group inherit the attributes of the same corresponding

dialer interface.

• You can associate a dialer interface with multiple call destination addresses by configuring the

dialer route command or with a single call destination address by configuring the dialer number

command.

C-DDR is powerful and has broad applications. However, it lacks flexibility and extensibility.