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B-5
Cisco 860 Series, Cisco 880 Series, and Cisco 890 Series Integrated Services Routers Software Configuration Guide
OL-18906-02
Appendix B Concepts
Dial Backup
Dial Backup
Dial backup provides protection against WAN downtime by allowing a user to configure a backup
modem line connection. The following can be used to bring up the dial backup feature in Cisco IOS
software:
• Backup Interface, page B-5
• Floating Static Routes, page B-5
• Dialer Watch, page B-5
Backup Interface
A backup interface is an interface that stays idle until certain circumstances occur, such as WAN
downtime, at which point it is activated. The backup interface can be a physical interface such as a Basic
Rate Interface (BRI) or an assigned backup dialer interface to be used in a dialer pool. While the primary
line is up, the backup interface is placed in standby mode. In standby mode, the backup interface is
effectively shut down until it is enabled. Any route associated with the backup interface does not appear
in the routing table.
Because the backup interface command is dependent on the router’s identifying that an interface is
physically down, it is commonly used to back up ISDN BRI connections, asynchronous lines, and leased
lines. The interfaces to such connections go down when the primary line fails, and the backup interface
quickly identifies such failures.
Floating Static Routes
Floating static routes are static routes that have an administrative distance greater than the administrative
distance of dynamic routes. Administrative distances can be configured on a static route so that the static
route is less desirable than a dynamic route. In this manner, the static route is not used when the dynamic
route is available. However, if the dynamic route is lost, the static route can take over, and the traffic can
be sent through this alternative route. If this alternative route uses a dial-on-demand routing (DDR)
interface, then that interface can be used as a backup feature.
Dialer Watch
Dialer watch is a backup feature that integrates dial backup with routing capabilities. Dialer watch
provides reliable connectivity without having to define traffic of interest to trigger outgoing calls at the
central router. Hence, dialer watch can be considered regular DDR with no requirement for traffic of
interest. By configuring a set of watched routes that define the primary interface, you can monitor and
track the status of the primary interface as watched routes are added and deleted.
When a watched route is deleted, dialer watch checks for at least one valid route for any of the IP
addresses or networks being watched. If there is no valid route, the primary line is considered down and
unusable. If there is a valid route for at least one of the watched IP networks defined and the route is
pointing to an interface other than the backup interface configured for dialer watch, the primary link is
considered up and dialer watch does not initiate the backup link.