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162 Chapter 6: Using ISDN and DDR Technologies

Figure 6-11

Dial Backup Scenario

In Example 6-25, notice the backup delay 5 60 command, which specifies that if there is a
failure, the system should wait five seconds to bring up the backup link. Once the failure has
passed, the system should wait 60 seconds to bring the backup link back down.

Implementation of these timers is an attempt to compensate for a "bouncing" interface. In other
words, the timers exist to compensate for an interface that drops momentarily and then comes
right back up.

The backup functionality available with Cisco IOS is not simply for redundancy in the case of
a failure. In situations where a WAN link approaches saturation, the ISDN service can be
initialized until the period of congestion has passed. The configuration enables the use of a load
setting both for the initialization of the link as well as for the termination of the link once the
condition is clear.

Example 6-25

Dial Backup Example Configuration

RouterA(config)#isdn switch-type basic-5ess
!
RouterA(config)#interface serial 0
RouterA(config-if)#ip address 10.14.1.1 255.255.255.0
RouterA(config-if)#encapsulation hdlc
RouterA(config-if)#backup interface BRI0
RouterA(config-if)#backup delay 5 60
RouterA(config-if)#backup load 90 5
!
RouterA(config)#interface BRI0
RouterA(config-if)#ip address 10.12.1.1 255.255.255.0
RouterA(config-if)#encapsulation ppp
RouterA(config-if)#dialer idle-timeout 180
RouterA(config-if)#dialer map ip 10.12.1.2 2145552222
RouterA(config-if)#dialer-group 1
!
RouterA(config)#dialer-list 1 protocol ip permit

BRI0

Primary

link

Secondary

link

BRI0

Frame

Relay

ISDN