Dial-on-demand routing (DDR) is a
technique in which a router can dynamically initiate and close
circuit-switched sessions when transmitting end stations need them.
When the router receives traffic destined for a remote network, a
circuit is established, and the traffic is transmitted normally. The
router maintains an idle timer that is reset only when interesting
traffic is received. (Interesting traffic refers to traffic the router
needs to route.) If the router receives no interesting traffic before
the idle timer expires, however, the circuit is terminated. Likewise,
if uninteresting traffic is received and no circuit exists, the router
drops the traffic. When the router receives interesting traffic, it
initiates a new circuit.
DDR enables you to make a standard
telephone connection or an ISDN connection only when required by the
volume of network traffic. DDR may be less expensive than a
dedicated-line or multipoint solutions. DDR means that the connection
is brought up only when a specific type of traffic initiates the call
or when you need a backup link. These circuit-switched calls,
indicated by the broken lines in Figure
are placed using ISDN networks. DDR is
a substitute for dedicated lines when full-time circuit availability
is not required. In addition, DDR can be used to replace
point-to-point links and switched multi-access WAN services.
DDR can be used to provide backup load
sharing and interface backup. For example, you might have several
serial lines, but you want the second serial line to be used only when
the first line is very busy so that load sharing can occur. When your
WAN lines are used for critical applications, you might want a DDR
line configured in case the primary lines go down. In this case, the
secondary line enables itself so traffic can still get across.
Compared to LAN or campus-based
networking, the traffic that uses DDR is typically low volume and
periodic. DDR initiates a WAN call to a remote site only when there is
traffic to transmit.
When you configure for DDR, you must
enter configuration commands that indicate what protocol packets
make-up interesting traffic to initiate the call. To do this, you
enter access control list statements to identify the source and
destination addresses, and you choose specific protocol selection
criteria for initiating the call. Then you must establish the
interfaces where the DDR call initiates. This step designates a dialer
group. The dialer group associates the results of the access control
list specification of interesting packets to the router's interfaces
for dialing a WAN call. 
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