About MAC masquerade addresses
http://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip-device-service-clustering-11-4-0/8.html
A primary purpose of a MAC masquerade address is to minimize ARP communications or dropped packets as a result of a failover event. A MAC masquerade address ensures that any traffic destined for the relevant traffic group reaches an available device after failover has occurred, because the MAC masquerade address floats to the available device along with the traffic group. Without a MAC masquerade address, on failover the sending host must relearn the MAC address for the newly-active device, either by sending an ARP request for the IP address for the traffic or by relying on the gratuitous ARP from the newly-active device to refresh its stale ARP entry.
The assignment of a MAC masquerade address to a traffic group is optional. Also, there is no requirement for a MAC masquerade address to reside in the same MAC address space as that of the BIG-IP device.