Router - 

interface GigabitEthernet0/0
description Link to BT
ip address 10.10.10.10 255.255.255.252
ip nat outside
!
interface GigabitEthernet0/1
description Link to COLT
ip address 20.20.20.20 255.255.255.252
ip nat outside
!
interface GigabitEthernet0/2
description Camput_LAN
ip address 172.16.10.1 255.255.255.0
ip nat inside
!
track 1 ip sla 1 reachability
track 2 ip sla 2 reachability
!
ip sla 1
icmp-echo 8.8.8.8 source interface GigabitEthernet 0/0
timeout 500
threshold 500
frequency 2
!
ip sla 2
icmp-echo 8.8.8.8 source interface GigabitEthernet 0/1
timeout 500
threshold 500
frequency 2
!
ip sla schedule 1 start-time now life forever
ip sla schedule 2 start-time now life forever
!
ip route 0.0.0.0 0.0.0.0 10.10.10.1
ip route 0.0.0.0 0.0.0.0 20.20.20.1
!
ip nat inside source list 1 interface GigabitEthernet0/0 overload
ip nat inside source list 1 interface GigabitEthernet0/1 overload
!
access-list 1 permit 172.16.10.0 0.0.0.255
!
event manager applet BT_DOWN
event track 1 state down
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "no ip route 0.0.0.0 0.0.0.0 10.10.10.1
action 4.0 cli command "exit"
action 5.0 cli command "clear ip nat translation *"
action 6.0 cli command "end"
!
event manager applet BT_UP
event track 1 state up
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "ip route 0.0.0.0 0.0.0.0 10.10.10.1
action 4.0 cli command "exit"
action 5.0 cli command "clear ip nat translation *"
action 6.0 cli command "end"
!
event manager applet COLT_DOWN
event track 2 state down
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "no ip route 0.0.0.0 0.0.0.0 20.20.20.1
action 4.0 cli command "exit"
action 5.0 cli command "clear ip nat translation *"
action 6.0 cli command "end"
!
event manager applet COLT_UP
event track 2 state up
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "ip route 0.0.0.0 0.0.0.0 20.20.20.1
action 4.0 cli command "exit"
action 5.0 cli command "clear ip nat translation *"
action 6.0 cli command "end"