| Engineering Journal 5.3.3: Dial backup You can use ISDN as a backup service for a leased-line connection between remote offices and the central office. If the primary connectivity goes down, an ISDN circuit-switched connection is established, and traffic is rerouted over ISDN. When the primary link is restored, traffic is redirected to the leased line, and the ISDN call is released. ISDN dial backup can also be configured based on traffic thresholds or limits as a dedicated primary link. If traffic load exceeds a user-defined value on the primary link, the ISDN link is activated to increase bandwidth between the two sites, as shown in the figure.