Sometime ago, during my preparation for Cisco CCIE certification, I encountered a task that I had to admit made me think a little bit, even I should see the solution from the first minute. The idea, at least as I see it, is that as much as you learn for some certification you start to see only the complex and painful part of the networking and this made me skip over the simplest solution. Something like, I learn to fly to the moon but I forget how to step on earth…
Before I start please have a look to this network topology. The task was having some statement that due to the monthly cost, R1 should use only one line (Frame-Relay) to communicate to the networks behind R2 (I took in this example Loopback0: 220.127.116.11 /32) and in case that the R1’s protocol interface to Frame-Relay cloud is going, the connection to R3 should become active and traffic should flow through there. The scope was to achive some redundancy from R1 to the rest of the network. As I said before the solution was much more simplest that I start initially to think of and you can see it immediately.
Regarding the routing since this is not the main point discussed here, I just add 2 static routes on R1 to 18.104.22.168; one route through R2 and another one through R3 (with higher distance metric). Of course I put the necessary static routes and tracking on R2 and R3.
One advice if you want to try this on your own with this topology. Do not manually shutdown the main interface to enable the backup one, as it will not work. For testing you have to find a way that the main interface is down, but not administratively down. This is just not to get angry that this method is not working.