Multicast tree aggregation is a technique that reduces the control overhead and the number of states induced by multicast. The main idea of this protocol is to route several groups to the same distribution tree in order to reduce the total number of multicast forwarding states. In this article, we show that this technique cannot be applied to large domains. Indeed, when the number of border routers is large, actual tree aggregation protocols are unable to find similar groups to aggregate to the same tree. However, by dividing the domain into several smaller sub-domains, we prove that it is possible to achieve important savings. A hierarchical protocol is designed to interconnect the trees of the sub-domains together. While previous protocols cannot cope with more than 25 border routers, our protocol still shows significant benefits for domains with 200 border routers.