R1#sh run

no ip domain lookup
ip domain name lab.local
ip sla monitor 1
 type echo protocol ipIcmpEcho 12.1.1.2 source-ipaddr 12.1.1.1 ## from 12.1.1.1 to 12.1.1.2 是主线路
 timeout 100
 threshold 2
 frequency 2
 

track 1 ip route 0.0.0.0 0.0.0.0 reachability

!
interface FastEthernet0/0
 no ip address
 shutdown
 duplex auto
 speed auto
!
interface Serial0/0
 ip address 12.1.1.1 255.255.255.0
 clock rate 2000000
!
interface FastEthernet0/1
 no ip address
 shutdown
 duplex auto
 speed auto
!
interface Serial0/1
 ip address 13.1.1.1 255.255.255.0
 clock rate 2000000
!
ip route 0.0.0.0 0.0.0.0 12.1.1.2 track 1
ip route 0.0.0.0 0.0.0.0 13.1.1.2 10
正常情况下

R1#sh ip route

Gateway of last resort is 12.1.1.2 to network 0.0.0.0

     1.0.0.0/24 is subnetted, 1 subnets

C       1.1.1.0 is directly connected, Loopback0
     12.0.0.0/24 is subnetted, 1 subnets
C       12.1.1.0 is directly connected, Serial0/0
     13.0.0.0/24 is subnetted, 1 subnets
C       13.1.1.0 is directly connected, Serial0/1
S*   0.0.0.0/0 [1/0] via 12.1.1.2
当主线路ping不通时候

R1#sh ip route
 

Gateway of last resort is 13.1.1.2 to network 0.0.0.0

     1.0.0.0/24 is subnetted, 1 subnets

C       1.1.1.0 is directly connected, Loopback0
     13.0.0.0/24 is subnetted, 1 subnets
C       13.1.1.0 is directly connected, Serial0/1
S*   0.0.0.0/0 [10/0] via 13.1.1.2