In the Figure, interface
serial 0 (S0) is the primary interface. You can see from the
configuration displayed in the Figure that if the primary interface is
down for 40 seconds, the backup interface (BRI0) will be activated. The
secondary line will deactivate 20 seconds after the primary line is
re-enabled. The example in the Figure illustrates only the
commands to enable a backup. The interface must also be configured as
needed (for DDR, Frame Relay, X.25, and so on).
An alternative to dial backup for link
failure is a floating static route. A floating static route is a static
route that has an administrative distance greater than the
administrative distance of corresponding 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. If the dynamic route is lost,
however, the static route can take over, and traffic can be sent through
this alternative route. When this alternative route is provided by a DDR
interface, DDR is used as a backup mechanism.
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