Configure IP SLA TRACKING, here is a brief config example for a ASA 5505
SETUP BOTH INTERFACES, IF YOU HAVE AN ASA 5510 OR HIGHER YOU WILL NOT NEED TO USE THE VLAN 2 OR 3 COMMAND OR NEED THE " no forward interface Vlan2 " YOU WOULD JUST ASSING THE IP AND NAME TO THE PHYSICAL ETHERNET OR GIGABIT INTERFACE ****
interface Vlan2
nameif outside
security-level 0
ip address 1.1.1.1 255.255.255.0
!
interface Vlan3
no forward interface Vlan2
nameif outside2
security-level 0
ip address 2.2.2.2 255.255.255.0
!
******** ENABLE PINGS FROM THE OUTSIDE/OUTSIDE2 FOR BOTH INTERFACES ******
!
icmp permit any outside
icmp permit any echo outside
!
icmp permit any outside2
icmp permit any echo outside2
!
SETUP THE SLA TRACKING , 8.8.8.8 IS GOOGLE DNS, THIS IS JUST USED AS AN IP ADDRESS THAT SHOULD ALWAYS BE UP ON THE INTERNET AND BALE TO PING, AS OPPOSED TO USING THE GATEWAY OR ANOTHER IP ADDRESS FROM YOU ISP EQUIPMENT, THE PARAMETERS BELOW THA ARE HOW ONG IT WILL TAKE TO SWITCH BACK TO THE MAIN OUTSIDE INTERFACE ONCE THE PRIMARY CONNECTION IS BACK UP ******
sla monitor 100
type echo protocol ipIcmpEcho 8.8.8.8 interface outside
num-packets 5
timeout 30000
frequency 30
!
sla monitor schedule 100 life forever start-time now
!
track 100 rtr 100 reachability
!
RE ENTER YOU ROUTE COMMANDS WITH THE PRIMARY OUTSIDE INTERFACE BEING THE PRIMARY ROUTE WITH THE NEW SLA TRACK ID YOU CONFIGURED, THE THE OUTSID2 INTERFACE AS A BACKUP, OBVIOUSLY THE IP ADDRESSES BELOW AND ABOVE ARE JUST EXAMPLES THE ROUTE COMMANDS SHOULD BE THE GATEWAY ADDRESSES THAT THE ISP IS PROVIDING
route outside 0.0.0.0 0.0.0.0 1.1.1.1 1 track 100
route outside2 0.0.0.0 0.0.0.0 2.2.2.2 240
HOPEFULLY THAT SHOULD WORK FOR YOU, I have set it up this way on multiple ASA 5505 models and 5512X so you should be good, let me know if you have any questions
Thanks
