DDR and ISDN Usage

This topic describes the sequence of events that triggers an ISDN DDR call. ISDN is commonly configured with DDR

Using DDR with ISDN

1.

2 PRI ma^^ma 4, j -► Provider —-—*

II

1.

Packet arrives.

2.

Switch packet to DDR interface, determine if interesting.

3.

If interesting, dial DDR destination via

ISDN.

4.

Connect to remote router.

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Access routers use DDR to connect to remote routers. The access router will initiate a connection only when it detects "interesting traffic" that is bound for a remote site. Dialer lists specify interesting traffic. You can place a BRI interface in a dial group, which is linked to a dialer list that specifies interesting traffic. You can use multiple dialer list entries to identify traffic that is interesting and destined for other DDR destination routers, based on various protocols. Access lists can also be used to refine the designation of interesting packets that will initiate DDR calls.

Routing updates may cause ISDN calls to remote routers. This could dramatically increase service charges from the ISDN service provider. For this reason, it is usually best to use static and default routes to reach destination networks.

Note Some dynamic routing protocols, like Open Shortest Path First (OSPF), support features specifically designed to work over DDR connections. In addition, Cisco IOS software supports a feature called Snapshot Routing. This feature permits the use of distance-vector routing protocols over DDR links while minimizing routing and service advertisement updates, thus saving link charges. Further information on these features can be located at http //www cisco com

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 interface through the ISDN NT-1. If using an external TA, it must support V.25bis dialing. Calling details for these devices come from dialer commands.

An idle timer starts when no more interesting traffic is transmitted over the ISDN call. The timer is reset if an interesting packet is received before the Idle-Timeout value is reached. If no interesting packets are received when the Idle-Timeout expires, the call disconnects.

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