ISP Link Failover

Hi all,

Some of you may wonder how to failover ISP Link if one fails and another still active.

For eg, You have 2 ISP link. 1 with T1, and another is DSL. You may not want to load balance between this two link since both of this link serve different SLA and  Performance. So typical design would be active/passive, whereby Primary T1 link is Active and DSL site idle and used only if T1 link fails.

We can achieve this by using IP SLA and object tracking. Remember, IP SLA command may very depends on which IOS you running.

ip sla 1                                   # Define SLA number 1
 icmp-echo 8.8.8.8 source-interface ser0//0/0   # Define which IP you wish to ping to detect if T1 is UP
 timeout 3000           # Define maximum to wait to get the reply in ms
 frequency 10           # Define how frequent you wish to send ping request

ip sla schedule 1 life forever start-time now  # Tell router to start SLA 1 now and run it forever.

ip route 8.8.8.8 255.255.255.255 ser0/0/0 # very important command, otherwise can cause link flapping.


track 1 ip sla 1 reachability     # create track 1 to track sla 1

Now the routing part. By right you would have 2 default routing.

ip route 0.0.0.0 0.0.0.0 ser0/0/0
ip route 0.0.0.0 0.0.0.0 DSL_Interface

We are now tracking T1 link, so apply track to default routing on T1 Link. And  floating static  route for Backup DSL link by using metric.

ip route 0.0.0.0 0.0.0.0 ser0/0/0 track 1
ip route 0.0.0.0 0.0.0.0 DSL_Interface 200

By doing this , routing table will only show default route for T1 link till IP SLA 1 fails get reply from 8.8.8.8 within 3000ms, and that change track 1 state to DOWN, eventually removing routing for T1 link from routing table and route for DSL link will take order.