Most network designers do not implement a completely redundant network. Instead, network
designers implement partially redundant internetworks. The section, “Choosing Internetworking
Reliability Options,” later in this chapter, addresses several hypothetical networks that represent
commonly implemented points along the reliability continuum.
Switched access provides the capability to enable aWAN link on an as-needed basis via automated
router controls. Onemodel for a reliable backbone consists of dual, dedicated links and one switched
link for idle hot backup. Under normal operational conditions, you can load balance over the dual
links, but the switched link is not operational until one of the dedicated links fails.
Traditionally, WAN connections over the Public Switched Telephone Network (PSTN) have used
dedicated lines. This can be very expensive when an application requires only low-volume, periodic
connections. To reduce the need for dedicated circuits, a feature called dial-on-demand routing
(DDR) is available. Figure 2-8 illustrates a DDR connection.Identifying and Selecting Internetworking Capabilities
