Dial-on-demand routing (DDR) is useful in applications that don't require a permanent connection between two sites. This is often the case for small offices or home users who can't justify the expense of a permanent connection. Instead, communication is through some sort of dial-up terminal server using standard telephone service (either analog or ISDN). The router places a call when it has traffic to send and establishes an IP connection using PPP. When the connection is idle, the router should hang up automatically.[*]
[*] Newer services, such as xDSL and cable modems, could potentially reduce the need for dial-on-demand routing. Both of these services establish permanent connections at low cost. However, dial-up services will be with us for the foreseeable future, especially as a backup when DSL or cable service is down.
DDR is also useful for backup links; a router can establish a dial-up connection if a permanent, leased-line connection fails. It's also useful if you need to make connections to many sites through a limited number of modems or asynchronous ports.
Cisco's IOS support for dial-on-demand routing falls into two categories:
In legacy DDR, all the DDR commands are tied to a specific interface. This includes commands to set up dialer scripts, timeouts, dialer groups ("interesting" traffic), and other DDR information. Legacy DDR is supported from the earliest of IOS versions.
Dialer profiles allow you to create a single profile that contains all the DDR information necessary. The profile can then be applied across many dialer interfaces.
I'll cover both types in this chapter. But first, let's examine the basic DDR commands.