In cloud computing, multiple users coexist in one datacenter infrastructure and the network is always shared using VMs. Network bandwidth allocation is necessary for security and performance guarantees in the datacenter. InfiniBand (IB) is more widely applied in the construction of datacenter cluster and attracts more interest from the academic field. In this paper, we propose an IB dynamic bandwidth allocation mechanism IBShare to achieve different Weight-proportional and Min-guarantee requirements of allocation entities. The differentiated IB Congestion Control (CC) configuration is proven to offer the proportional throughput characteristic at the flow level. IBShare leverages distributed congestion detection, global congestion computation and configuration to dynamically provide predictable bandwidth division. The real IB experiment results showed IBShare can promptly adapt to the congestion variation and achieve the above two allocation demands through CC reconfiguration. IBShare improved the network utilization than reservation and its computation/configuration overhead was low.