A floating IP address is typically used to provide redundancy in a hot standby configurations. One machine is able to take over the ip address (hence floating ip) of the other and respond to ARP requests. Usually some type of heartbeat software/hardware is also used to detect the state of the master machine and the hot standby machine. This method introduces a security risk in that gratuitous ARP must be allowed, some network routers and switches don't allow it, and those that do are usually configured not to. This is to prevent a malicious machine from responding on behalf of one of yours and take over.
The more standard method is to use a machine/software that fronts your redundant machines with one IP address and then funnels the requests to the active server(s).
To checking the multicast functioning on a Linux machine give the following command.
$ tcpdump -i eth0 'port 8080'
Here you need to replace 8080 with your system heartbeat port.