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Cisco IOS Cookbook, 2nd Edition by Ian Brown, Kevin Dooley

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Using Backup Interfaces

Problem

You want to configure a router to dial only if it sees a physical failure on the primary WAN interface.

Solution

Cisco routers can watch the physical signals on an interface and trigger a backup interface if the primary link fails. The router will automatically drop the call after the primary circuit comes back up:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface Serial0/0
Router1(config-if)#backup delay 0 300
Router1(config-if)#backup interface BRI0/0
Router1(config-if)#encapsulation frame-relay
Router1(config-if)#down-when-looped
Router1(config-if)#exit
Router1(config)#interface Serial0/0.1 point-to-point
Router1(config-subif)#ip address 10.1.1.10 255.255.255.252
Router1(config-subif)#frame-relay interface-dlci 50   
Router1(config-subif)#exit
Router1(config)#interface BRI0/0
Router1(config-if)#ip address 10.1.99.55 255.255.255.0
Router1(config-if)#encapsulation ppp
Router1(config-if)#dialer idle-timeout 300
Router1(config-if)#dialer map ip 10.1.99.1 name dialhost broadcast 95551212
Router1(config-if)#dialer load-threshold 50 either
Router1(config-if)#dialer-group 1
Router1(config-if)#isdn switch-type basic-ni
Router1(config-if)#isdn spid1 800555123400 5551234
Router1(config-if)#isdn spid2 800555123500 5551235
Router1(config-if)#ppp authentication chap
Router1(config-if)#ppp multilink
Router1(config-if)#exit
Router1(config)#dialer-list 1 protocol ip permit
Router1(config)#end
Router1#

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