A multicast router may receive thousands of multicast（）every day for different groups. If a router has no knowledge about the membership status of the（）， it must broadcast all of these packets. This creates a lot of traffic and consumes（）. A better solution is to keep a list of groups in the network for which there is at least one loyal member.（）helps the multicast router create and update this list. For each group， there is one router that has the duty of distributing the（）packets destined for that group. This means that if there are three multicast routers connected to a network， their lists of groupids are mutually exclusive. A host or multicast router can have membership in a group.