You want to use Cisco's dialer watch feature to trigger dial backup.
The dialer watch feature allows the router to track a particular destination IP address in its routing table. If all of the tracked IP addresses disappear from the routing table, the router automatically triggers the dial backup connection:
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. 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 map ip 10.1.1.0 name dialhost broadcast 95551212 Router1(config-if)#dialer map ip 10.2.0.0 name dialhost broadcast 95551212 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)#dialer watch-group 1 Router1(config-if)#dialer watch-disable 300 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)#username dialhost password dialpassword Router1(config)#dialer-list 1 protocol ip list 101 Router1(config)#access-list 101 deny eigrp any any Router1(config)#access-list 101 permit ip any any Router1(config)#router eigrp 55 Router1(config-router)#network 10.0.0.0 Router1(config-router)#exit Router1(config)#dialer watch-list 1 ip 10.1.1.0 255.255.255.0 Router1(config)#dialer watch-list 1 ip 10.2.0.0 255.255.0.0 Router1#
This configuration is similar to that of Recipe 13.1, but this time the router uses the dialer watch feature to trigger the dial backup. The dialer watch-group command configures the backup interface to belong to a particular group. Usually you would configure only one such group on a router, but there is nothing to prevent you from having several different watch groups, each with different dial interfaces.
The watch group in this example looks for two prefixes in the routing tables, 10.1.1.0/24 and 10.2.0.0/16. If both of these routes drop out of the routing table, the router will automatically bring up the dial interface. Note that all of the watched routes must disappear before the router will dial.
We have configured several dialer map statements for this example. The same dialer map statement that we used in Recipe 13.1 defines the basic IP routing of the interface. We have also included two watch lists, one for each of the watched IP addresses.
We used the same dialer list configuration here as we did in Recipe 13.1. This is because we still don't want routine EIGRP packets to bring up the dial interface.
After it triggers the dial backup, the dialer watch configuration will keep track of the primary interface by periodically looking for the watched IP addresses in its routing table. The dialer watch feature will consider the primary circuit active only if the watched routes exist in the routing table, and do not point through the dial interface. We have configured a delay of 300 seconds before the router drops the dial backup after deciding (based on the routing tables) that the primary interface has recovered:
Router1(config-if)#dialer watch-disable 300
In general, we still prefer to use floating static routes to trigger dial backup, as in Recipes Recipe 13.1 and Recipe 13.2. This is because they offer greater control and flexibility.
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