Monitoring a router interface and failover when its down

[Mark Verwey]

I have a small business using a broadband connection with a 2951 router.  I have it working just fine, last week the BB circuit when down when construction workers cut the cable down the street.  They want a backup internet solution that monitors the main connection and automatically switches over to an alternate connection that maybe 4G.  I have found a solution that will work using a cradlepoint device, I configured it inline on this interface and it monitors the link going to the bb router.  It does a ping test every 30 seconds out to the internet.  If the ping fails, it makes the necessary routing changes and switches over to the 4G connection.  It continues to monitor the primary connection and when and when its gotten 3 successful ping sessions (90 seconds) it automatically changes the route table back and the 4G goes idle because traffic is now going back out the broadband interface.   

[balaji.bandi]

you need IP SLA with tracking the main gateway go down, move the route to 4G or alternative link.

here is good thread for your requirement :

https://community.cisco.com/t5/routing/4g-failover-using-ip-sla-and-track-how-to-fail-back/td-p/3002219

[Georg Pauwen]

Hello,

here is an updated version of the script mentioned in the thread referred to by Balaji (the primary and the backup cellular interfaces are probably different on the 2951 you have, so change them accordingly):

ip sla 1
icmp-echo 8.8.8.8 source-interface GigabitEthernet0
frequency 5
ip sla schedule 1 life forever start-time-now
!
track 1 ip sla 1 reachability
delay down 10 up 10
!
event manager applet PRIMARY_DOWN
event track 1 state down
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "interface GigabitEthernet0"
action 4.0 cli command "shut"
action 5.0 cli command "exit"
action 6.0 cli command "interface Cellular0/1/0"
action 7.0 cli command "no shut"
action 5.0 cli command "end"
action 6.0 cli command "clear ip nat translation *"
!
event manager applet PRIMARY_UP
event track 1 state up
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 6.0 cli command "interface Cellular0/1/0"
action 7.0 cli command "shut"
action 5.0 cli command "exit"
action 3.0 cli command "interface GigabitEthernet0"
action 4.0 cli command "no shut"
action 5.0 cli command "end"
action 6.0 cli command "clear ip nat translation *"