[CentOS] Cluster: understanding virtual IP

Hi folks,

I have set up a cluster on CentOS 5.2 using /etc/cluster/cluster.conf - and 
it works fine. It's only purpose is to switch a virtual IP between two 
routers.

Now the service is running, I can ping the virtual IP from outside - but 
this virtual IP is not bound to any interface. How does this work? Can I 
force the cluster to bind it to a certain interface?

I need this because the routing daemon (xorp) does not make use of an ip 
address that is not bound to an interface.

Any hint or help is appreciated.

Dirk