5.3 ISDN BRI and DDR
5.3.1 Interesting or uninteresting traffic
Cisco implements DDR from the perspective of the incoming data to the router.

With DDR, all incoming traffic is classified as either interesting or uninteresting. As shown in Figure , if a packet is uninteresting and there is no connection established, the packet is dropped. If the packet is uninteresting, but a connection is already established to the specified destination, the packet is sent across the connection, but the idle timer is not reset. If the packet is interesting, and there is no connection on the available interface, the router attempts to establish a connection.

An idle timer is used to reset the connection if no traffic for the destination arrives within the configured timer interval. When a connection is made, all traffic goes through, unless an access list is applied to the interface. For example, if you configured your DDR link to deny Telnet traffic but allow ping traffic, a user can send a ping to bring up the connection, and then start a Telnet session on the open DDR interface.

Access routers use DDR to connect to remote routers. The access router calls the remote router only when interesting traffic arrives. Dialer lists specify what traffic is considered interesting. The BRI interface is placed in a dial group that is linked to a dialer list that specifies interesting traffic. You can use multiple dialer-list settings to designate interesting traffic that is mapped for other DDR destination routers.

Access lists can also be used to produce more granularity in defining interesting packets that initiate DDR calls. For this periodic-use environment, specify static routes so that routing updates will not initiate ISDN calls to remote routers that run up service charges from the ISDN service provider.

DDR commands map a host ID and dialer string to initiate the setup of an ISDN call for interesting traffic. The router then makes an outgoing call from its BRI through the ISDN NT1. If using an external TA, it must support V.25bis dialing. Calling details for these devices come from dialer commands.

ISDN end stations now use this static route to transmit packet traffic. When no more traffic is transmitted over the ISDN call, an idle timer starts. After the idle timeout occurs, the call disconnects.

Some global and interface tasks are required to configure ISDN on a router for DDR.

Global tasks do the following:

  • Select the switch that matches the ISDN provider's switch at the central office (CO). Note: This can also be done on the interface with Cisco IOS® versions greater than 11.3.
    This requirement is necessary because signaling specifics differ regionally and nationally, despite standards.
  • Set destination details. Indicate static routes from the router to other ISDN destinations.
  • Establish the criteria for interesting packets in the router that initiate an ISDN call to the appropriate destination.

Interface tasks do the following:

  • Select interface specifications. Specify the interface type BRI and the number for this ISDN BRI port. For PRI, the interface task description occurs later in this chapter. The interface uses an IP address and subnet mask.
  • Configure ISDN addressing, DDR dialer information, and any ISDN parameters supplied by the ISDN service provider. Indicate that the interface is part of the dialer group using the interesting packets set globally. 
  • Additional commands place the ISDN call to the appropriate destination.
  • Following interface configuration, you can define optional features: the time to wait for the ISDN carrier to respond to the call, and the seconds of idle time before the router times out and drops the call.

Following interface configuration, you can define optional features: the time to wait for the ISDN carrier to respond to the call and the seconds of idle time before the router times out and drops the call.