Sorry for the delay, we had some spare days here in Germany :-)

> You don't see anything of the secondary interfaces in ifconfig. Anyway, why do you want/need to?

My plan was to let BIND 9.x Nameserver bind 2 addresses: The real one and the virtual one. This should be done on two machines.
The virtual IP address should be the service IP which will be used by customers.
Only the active one of the two machines should have the virtual IP up and thsi should change in case of failure.
But BIND cannot bind an address until the interface is configured/visible. So that's my problem.
So, if I bring down an interface with ifconfig BIND can still see the interface. If I do this with ip addr instead BIND cannot see the interface and will not use it until a complete restart (what will, unfortunately take to much time).