Internet communications have traditionally been based on point-to-point connections. This technique is called unicast and allows one sender and one receiver to communicate. The Internet saw the appearance of multicast protocols in the early 90's. Multicast enables point-to-multipoint and multipoint-to-multipoint connections and is suitable for applications with several receivers. Multicast packets are transmitted from each sender to the receivers through a certain multicast tree. Multicast routing protocols are responsible for building such trees. There are different types of multicast routing protocols and each builds a specific kind of multicast tree. The main advantages of applying multicast in multiple-receiver applications are a more rational use of network resources and the minimization of processing in hosts. There are two types of multicast models: Source-Specific Multicast (SSM) and Any-Source Multicast (ASM). The former is suitable for single-source applications like Internet TV and news distribution. The latter supports multiple-source applications like videoconferencing, Distributed Interactive Simulations (DIS), multi-player gaming and interactive distance learning. The Internet multicast research community assumes that both models must co-exist. Protocol Independent Multicast - Sparse Mode (PIM-SM) is the ASM standard basis for intra-domain multicast routing in Wide-Area Networks (WANs). This protocol builds Shared Trees (STs) rooted at certain routers called Rendezvous Points (RPs). These types of trees bring some drawbacks, the most important being traffic concentration, sub-optimal forwarding of multicast packets and slow convergence when an active RP fails. Two proposals arose to alleviate these drawbacks. The first, Bidir-PIM, constructs Bi-directional Shared Trees (BSTs). However, this variant neglects the problem of having a single point of failure. The second assembles Multiple Core Trees (MCTs) and is based on two protocols, PIM-SM and Multicast Source Discovery Protocol (MSDP), used in combination with anycast. MSDP has been adapted from inter-domain multicast to enable PIM-SM to use several RPs per group. On the other side, anycast allows senders and receivers to send data and control packets to the closest RP. If a RP fails, anycast also provides an automatic backup to the next closest RP. However, this proposal is complex and is not suitable for bursty sources and the next generation Internet Protocol (IPv6). Furthermore, it suffers from high join latencies and poor multicast state scalability with the number of sources. The first purpose of this dissertation is to provide a comparison on the ìqualityî for the different sorts of multicast trees built in the ASM model by multicast routing protocols suitable to run in WANs. This study concludes that MCTs give the best performance in terms of traffic concentration and link cost of multicast trees in sparse groups and end-to-end delay. Furthermore,it is the only type of tree that does not suffer from a single point of failure, called core or RP. The main purpose of this dissertation is the design of an extension to PIM-SM that allows the building of MCTs without MSDP. The extension to the register mechanism in PIM-SM allows the construction of channels encompassing the Access Routers (ARs) and any number RPs. These channels enable sources to directly reach all the RPs, thus avoiding the exchange of data packets between RPs, which generates additional processing in the RPs and the suboptimal routing of multicast packets coming from new sources. Furthermore, these channels may improve the multicast routing state scalability with the number of sources. This approach, called Protocol Independent Multicast - Multiple Core Mode (PIM-MCM), enables PIM having the advantages of anycast RP without using MSDP. In addition, it conforms to the PIM-SM design philosophy and alleviates all the drawbacks of MSDP while minimizing the added complexity.