IP multicast is not widely deployed yet over Internet. This is mainly due to the forwarding entries scalability and control explosion problems. In this paper, we propose an algorithm called STA (Scalable Tree Aggregation) which reduces the number of trees by allowing several groups to be aggregated to the same tree: the less trees, the less forwarding entries and the less control messages to maintain trees. STA performs faster aggregations than previous aggregation algorithms by evaluating fewer trees for each group, while keeping the same performance. We show the scalability and the fastness of STA by extensive simulations and we compare its performance to the previous algorithm.