| 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.
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