There are many optional paths between any server pair in modern data center networks. Traffic balancing among the paths is an important issue. Many solutions have been proposed to cope with this problem. This paper analyzes traffic balancing in data center networks with multi-tier Clos-based topology. The feature of data center networks is introduced and the traffic balancing problem is defined as minimizing the maximum potential packet loss among different equivalent links. Then the reasons of potential packet loss are summarized in data center networks and two main challenges are discussed related to traffic balancing:packet out-of-order and burst congestion. Ten existing solutions are classified into four categories:active scheduling, slice spraying, probing and adjusting, and others. The advantages of these solutions are described one by one in details, as well as their disadvantages. A comparison is made among these solutions and possible research directions are pointed out on traffic balancing in data center networks.