Abstract: Processing multicast messages in a data processing system that includes compute nodes, each of which includes a communications controller, the communications controllers operatively coupled for data communications amongst the compute nodes, each compute node in the data processing system executing at least one logical partition, the data processing system including a hypervisor, where multicast messages are processed by receiving, by more than one of the communications controllers, a multicast message; responsive to receipt of the multicast message, signaling, by one of the communications controllers, an interrupt in the hypervisor without signaling an interrupt by any other communications controller; determining, by the hypervisor, that the message received by the interrupt signaling communications controller is a multicast message; and providing, by the hypervisor, the multicast message to at least one logical partition configured to receive multicast messages.