In IGMP 1 implementations, hosts cannot explicitly exit a multicast group, but instead simply stop reporting interest in a given group. The router then halts retransmission of the traffic for that group upon expira tion of a time-out mechanism. An interesting dilemma, however, involves switches, which we'll discuss momentarily. Multicast-aware routers use these protocols to take the group membership information learned from an IGMP communications to dynamically build multipoint route trees, which map paths from a sender to all receivers. Support for at least one of these protocols is required in your campus routers.
Most vendors support one or two.
Each protocol has its relative strengths and weaknesses. This results in the packet reaching all subnets in the internetwork. The MBone broadcasts audio and video from events of nationwide interest, such as space shuttle transmission s. Instead, sparse-mode PIM defines a centralized rendezvous point, which links senders and receivers. Once a link is established, the rendezvous point leaves the picture and PIM optimizes the route to go directly from sender to receiver, minimizing any unneeded hops.