We are having two ethernet links ( provided by service provider for WAN connectivity ). We need to configure failover routing that is if first link fails traffic should be routed via second link. As we are having connectivity via ethernet , locally router can not sence that link is down and then route traffic to second link. Only way to check that if first link is working or not working is to check IP address connectivity at second end. That means if my router is not able to contact other end of the first link , it should route that subnet to second link.This is similar to dialer-watch concept, when a particular subnet is not available in routing table it will triger ISDN.
Here is example
location A :
subnet : Network_A
ethernet(WAN_FIRST_LINK) : A.B.C.D1 255.255.255.252
ethernet (WANLINK_SECOND_LINK ) : P.Q.R.S1 255.255.255.252
location B :
ethernet (WAN_FIRST_LINK) : A.B.C.D2 255.255.255.252
ethernet (WAN_SECOND_LINK) P.Q.R.S2 255.255.255.252.
Now if location A router can not contact A.B.C.D1 then it shoud route the traffic on P.Q.R.S1
Any link on cisco.com.
This is similar to if -then -else statement in higher end programming lanuage.
Thanks in advance