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146 Chapter 6: Using ISDN and DDR Technologies
Obviously, while there is no connectivity, routing updates cannot flow. If all IP traffic is defined
as interesting and the implementation in question is using a dynamic routing protocol, the
routing updates keep the link up at all times. For example, IGRP uses a 90-second update cycle.
Previous examples in this chapter placed a dialer idle-timeout 180 command on the interface.
This command states that the link should come down after 180 seconds of idle time. If IGRP
updates are sent every 90 seconds, the idle timeout countdown is reset with each update. The
very purpose of DDR has been defeated.
Step 4: Defining the Interface Encapsulation and ISDN Addressing
Parameters
ISDN installations are capable of employing HDLC or PPP encapsulation (among others). PPP
is most often used because of its rich feature set and flexibility. PPP offers the use of a single
B channel or the combination of the two B channels in a single aggregate pipe. It enables us to
decide when a connection should be dialed, when an additional channel should be brought up
and used, when to disconnect the call, and other options that are discussed in the next couple of
sections.
As discussed earlier in Chapter 5, PPP encapsulates network layer protocol information over
point-to-point links. Although it can be configured on a variety of interfaces, our focus remains
on the ISDN-capable interface. To establish communications over an ISDN link, each end of
the PPP link must first send Link Control Protocol (LCP) packets to configure and test the data
link. After the link has been established and optional facilities have been negotiated as needed,
PPP must send Network Control Protocol (NCP) packets to choose and configure one or more
network-layer protocols.
Once each of the chosen network layer protocols has been configured, datagrams from each
network layer protocol can be sent over the link. The link remains configured for
communications until explicit LCP or NCP packets close the link down, or until some external
event occurs (for example, a period of inactivity).
Functionally, PPP is simply a pathway opened for multiple protocols to share simultaneously.
The call setup is initiated by interesting traffic as defined using access lists and terminated by
an external event, such as manual clearing or idle timer expiration. Any interesting traffic that
traverses the link resets the idle timer; non-interesting traffic does not.
Configuring ISDN Addressing
ISDN addressing uses phone numbers that are exactly like the phone numbers utilized by
millions of people day in and day out. These numbers are telco-designated and locally
significant. They usually include an area code, a local exchange, and additional digits.