In data center networks, micro-burst is a common traffic pattern and the packet dropping caused by it usually leads to serious performance degradation. Meanwhile, most of the current commodity switches employ on-chip shared memory, and the buffer management policies of them ensure fair sharing of memory among all ports. Among various polices, Dynamic Threshold (DT) is widely used by switch vendors. However, because DT needs to reserve a fraction of switch buffer, there is free buffer space while packets from micro-burst traffic are dropped. In this paper, we theoretically deduce the sufficient conditions for packet dropping caused by micro-burst traffic, and estimate the corresponding free buffer size. The results show that the free buffer size is very large when the number of overloaded ports is small. What's worse, to ensure fair sharing of memory among output ports, packets from micro-burst traffic may be dropped even when the traffic size is much smaller than the buffer size. In light of these results, we propose Enhanced Dynamic Threshold (EDT) policy, which can alleviate packet dropping caused by micro-burst traffic through fully utilizing the switch buffer and temporarily relaxing the fairness constraint. The simulation results show that EDT can absorb more micro-burst traffic than DT.