Abstract: Multicast routing is an important topic of both theoretical and practical interest. Several proposed multicast routing algorithms involve the designation of one or more network nodes as the "center" of the routing tree for each multicast group. The choice of this designated router (which we refer to as the "core") influences the shape of the multicast routing tree, and thus influences the performance of the routing scheme. We investigate the relationship between the choice of core and three performance measures. Specifically, we compare various methods of selecting a core with respect to their effect on the bandwidth, delay, and traffic concentration. We conclude that simple methods are adequate for widely distributed groups, but that the addition of group information can be leveraged to improve performance especially when the group is small or exhibits a high degree of locality. We also conclude that core choice can be used to control traffic concentration, in fact traffic concentration effects can be ameliorated by appropriate core choice policies.