You're going to have to look at the cluster admin/config to find the virtual IP address of the cluster. Essentially, the Virtual IP address should have it's own virtual MAC address (with a short time out), and should register it with the switch gear in order to resolve properly.
Once a failure occurers, the secondary machine should update the MAC entry on the virtual IP address to the MAC address of the new server....
That's the theory behind it, it's just a matter of finding where (inside your cluster admin/config) the virtual IP is setup, which should give you information about it and the associated MAC address.
Thanks,
Sean Brown
http://www.sleepyshark.com