Cisco 1003 - 1003 Router Manual de usuario descargar pdf (Pagina 9)

Designing ISDN Internetworks 11-9

Dial-on-Demand Routing

ISDN is a circuit-switched technology. Like the analog telephone network, ISDN connections are

made only when there is a need to communicate. Cisco uses dial-on-demand routing (DDR) to

determine when a connection needs to be made between two sites. With DDR, packets are classiﬁed

as either interesting or uninteresting based on protocol-speciﬁc access lists and dialer lists. DDR

makes an ISDN connection only if the packet is interesting. Figure 11-2 shows the DDR decision

process for determining packet ﬂow of an ISDN connection.

Figure 11-2 DDR Flow Chart

In Figure 11-2, an outgoing packet is compared with the conﬁgured dialer lists. If the packet is found

to be interesting and a connection is already established for the destination, the router resets the idle

timer and sends the packet. If the packet is found to be interesting but a connection to the destination

is not active, the router checks to see if a phone number is conﬁgured for the destination. If a phone

number is conﬁgured, the router dials the destination and sends the packet.

When dealing with ISDN, it is important to consider cost structures. Typically, the initiator of an

ISDN connection incurs a cost each time a connection is made and for each minute that the

connection remains in effect. The primary goal of router conﬁguration is to minimize the number of

connections and the duration of connections without sacriﬁcing end-user connectivity.

This chapter summarizes information about DDR as it pertains to ISDN. For detailed information

about designing internetworks for use with DDR, see the chapter, “Designing DDR Internetworks.”

Outgoing

packet

Dialer list

Interesting

Phone #

configured?

Discard

Connected?

Reset idle timer

Send

Dial

Yes

No Uninteresting

S4439

Discard

Hold queue

configured?

Discard

Queue

Yes

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Cisco 1003 - 1003 Router Manual de usuario Pagina 9