Every once in a while I have seen people working on cisco certification talking about backup interfaces configuration not working. Although configurations are usually good and should have worked just fine. But the problem usually I see is the way they test things.
Let's take a look at topology given above. Say S0/0 link is the primary interface and S0/1 is the backup interface.
The way usually try people to test backup interface configuration on R1 is - they try to shut S0/0 interface and expect S0/1 interface to trigger up. Though it should have worked ideally. But our very own IOS is intelligent enough to recognize that Administrator has manually shut down interface and S0/0 didn't went down by its own. And decides to not to trigger backup interface S0/1. So below are the different methods or right methods I should say to test your backup interface configurations.