Task Interpretation (probably part 1 of many)

Recently I had a task that told me a serial link between two routers will only be used if the Ethernet and frame-relay links between these routers went down. Detection of the failed links and convergence should happen within 10 seconds. Immediately I thought of the backup interface command, however:

R4(config)#interface e0/0
R4(config-if)#backup interface s0/1
Serial0/1 is already a backup for Serial0/0
R4(config-if)#

You can only have an interface be a backup for one interface!

What the task was really asking for was to make OSPF detect the failed neighbors within 10 seconds and thus install the direct serial link route. We do this by adjusting the hello-interval on the Ethernet and frame-relay links to 2 seconds (2 x 4 = 8 second dead interval).

Another reason it is important to think a little outside the box and try to understand what the goal of the task is.