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Chapter 10
Wide Area Networking Protocols
Dial-on-Demand Routing (DDR)
D
ial-on-demand routing (DDR) is used to allow two or more Cisco
routers to dial an ISDN dial-up connection on an as-needed basis. DDR is
only used for low-volume, periodic network connections using either a public
switched telephone network (PSTN) or ISDN. This was designed to reduce
WAN costs if you have to pay on a per-minute or per-packet basis.
DDR works when a packet received on an interface meets the require-
ments of an access list defined by an administrator, which defines interesting
traffic. The following five steps give a basic description of how DDR works
when an interesting packet is received in a router interface:
1.
Route to the destination network is determined.
2.
Interesting packets dictate a DDR call.
3.
Dialer information is looked up.
4.
Traffic is transmitted.
5.
Call is terminated when no more traffic is being transmitted over a
link and the idle-timeout period ends.
Configuring DDR
To configure legacy DDR, you need to perform three tasks:
1.
Define static routes, which define how to get to the remote networks
and what interface to use to get there.
Should we really still use ISDN?
Only if you cannot get anything else. If your only option is a 56Kbps dial-up
modem, then yes, ISDN will be better. Since it is a true 128Kbps data ser-
vice, it will provide much better performance then any modem.
However, if you can get DSL, a cable modem, Frame Relay, or even a wire-
less connection, you will be much better off.
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