Dynamic Failover with IP SLA

Posted on 04. Aug, 2008 by rivansystems in Greatness habits revealed, Routing

I just stumbled across this killer post on Shawn's Blog that describes how to set up IP SLA to allow a static route to failover if pings to a specified internet host should fail. This is really useful if you have two ISPs (and thus, two default routes) where the router cannot detect a link failure. This is really common if you have a cable/DSL modem or some type of wireless connectivity. Here's the basic config, taken directly from Shawn's Blog:

ip sla 1 < The number 1 here is arbitrary, used only to identify this sla. It is otherwise knows as the operation number>

icmp-echo 4.2.2.2 < 4.2.2.2 is a DNS server that responds to pings out on the internet>

timeout 500 < This is how long to wait for a response from the ping>

frequency 3 < This is the repeat rate for the SLA>

ip sla schedule 1 start-time now life forever < This command says "start SLA 1 now and keep it running forever>

track 1 rtr 1 reachability < This comand creates the track object "1" and monitors the SLA 1>

now for the routing, we need to change the default route and associate it with the tracker

no ip route 0.0.0.0 0.0.0.0 1.1.1.1

and then put it back with the tracking

ip route 0.0.0.0 0.0.0.0 1.1.1.1 track 1

Then we need to add our secondary route

ip route 0.0.0.0 0.0.0.0 1.1.1.2 10

Now when the ping to 4.2.2.2 fails the primary route is removed and the secondary route with the higher metric becomes the default. The route will be reinstated when the connectivity is restored.

With the 12.4 and higher releases the commands have changed slightly but the "?" is your friend. If I receive requests for the syntax I will post it as well, but it is pretty easy to convert.

Here is the reference to the Cisco IP SLA documentation

http://www.cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a0080441845.html