Recently I set up backup DNS-servers, with keeping in mind that I will have some faill-over once the primary DNS-server fails 
So I had configure tank.adslweb.net and ns-ext.adslweb.net as secondary DNS-servers for several domains and the morpheus.adslweb.net as primary DNS-server. Yesterday I had some issues, because none of the systems was reachable using their FQDN. First I restart the nameserver on tank.adslweb.net and found somehow that the zone-files were expired 
After some further investigation I found that the DNS-server on morpheus died somehow, so tank couldn't check for updates.
So the lesson learned of the above is... Even if you have backup DNS-configured... they will only serve for the period mentioned in the expiry field of that zone. 
