Overview
In the previous chapter, you learned how to configure dial-on-demand routing (DDR) on an access server. This chapter introduces the configuration of dialer profiles and rotary groups. These technologies will help you build more flexibility into your network design by introducing a more modular approach.

As seen in the last chapter, “Using ISDN and DDR to Enhance Remote Connectivity,” dialer-map statements are convenient when one physical interface is responsible for calling one destination.

The dialer-map command can also be used if your router calls multiple destinations that all use the same communication parameters. As an example, if your router performs DDR to three different destinations, and for every call the encapsulation is Point-to-Point Protocol (PPP), the authentication method is Challenge Handshake Authentication Protocol (CHAP), and the idle timeout is 300 seconds; you can configure one physical interface with all the parameters and provide it with three separate dialer-map statements.

On the other hand, what if your router is responsible for reaching three separate locations that use different communication parameters? Suppose that one location requires Password Authentication Protocol (PAP) authentication when another is doing CHAP authentication. One location might require an ISDN speed of 56 Kbps, whereas the other destinations communicate at 64 Kbps. If this is the case, specific call parameters are defined under three separate physical interfaces, each of them connected to a separate line.

The previous scenario might result in a waste of resources and money. You would have to procure a router with three WAN interfaces, and you would have to pay for three lines that might be used for only a few minutes daily.

In this case, you will need a mechanism in which physical interfaces are not locked with permanent configurations, but the mechanism assumes call parameters on an as-needed basis. When the call is finished, the same interface is freed of the previous configuration and is ready to service another calling destination.

This mechanism is called dialer interface. The dialer interface is not a physical interface; it is an entity that allows you to propagate an interface configuration to multiple interfaces. When a physical interface is being used for dialing, it inherits the parameters configured for the dialer interface.

After an interface configuration is propagated to a set of physical interfaces, those interfaces can be used to place calls by using standard DDR criteria. Using the dialer interface allows you to specify one set of dialer maps that can apply to multiple physical lines.

Dialer interfaces provide flexibility through rotary groups and dialer profiles. The following sections explain the differences between these two configurations.