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Broadcasting is one of the essential communication models of MANETs. Many MANET multicast routing protocols rely heavily upon MAC layer's broadcast service for data delivery, multicast architecture construction and maintenance. However, the broadcast mechanism of the IEEE 802.11 standards cannot provide reliable broadcasting support, and therefore, satisfactory performance of the multicast protocols cannot be guaranteed. An extension to IEEE 802.11 broadcast mechanism, called round-robin acknowledge and retransmit, to address this shortcoming is proposed. Different from other mechanisms, we employ a simple and effective acknowledge mechanism, in which the lost frames are reported by neighbouring nodes in a round-robin style to avoid the notorious ack explosion problem. A MAC layer retransmission scheme is then provided for the lost frames. By varying the traffic load and group size, under the on demand multicast routing protocol (ODMRP), extensive simulations show that the proposed scheme provides a highly reliable broadcast service to the routing layer, and ODMRP's performance is improved significantly.