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.