You have seen that in order for IP multicast forwarding to work on the local segment and beyond, switches and gateway routers need to be aware of multicast hosts interested in a specific group and where those hosts are located. Without this information, the only forwarding option is to flood multicast datagrams throughout the entire network domain. This would destroy the efficiency gains of using IP multicast.
Host group membership is a dynamic process. When a host joins a multicast group, there is no requirement to continue forwarding group packets to the segment indefinitely, nor is group membership indefinite. The only way to manage alerting the network to a multicast host location is to have multicast host group members advertise interest or membership to the network. Figure 2-6 depicts a high-level example of this requirement, known as a join.
Figure 2-6 Host Joins a Multicast Group
A Layer 3 gateway provides access to the larger network for hosts on a given subnet. The gateway is the network demarcation between Layers 2 and 3 and is the most appropriate device to manage host group membership for the larger network. Hosts forward group management information, like joins, to the network. The gateway receives these management messages and adds host segment interfaces to the local multicast table (multicast forwarding information base [FIB]). After the FIB is updated, the gateway router communicates group interest using protocol independent multicast (PIM) to the larger network domain.
It is important to note that without multicast-aware Layer 2 protocols, all hosts on a given Layer 2 segment will receive multicast packets for any groups joined by a host on that segment. For this reason, it is also logical that hosts and routers have the capability to dynamically leave a group or to prune a group from a particular segment. Figure 2-7 describes a high-level example of this process in action, known as a leave.
Figure 2-7 Host Leaves a Multicast Group
Administrators can configure the gateway router to statically process joins for specific groups using router interfaces. This alleviates the need to have a dynamic join/leave process; however, having a dynamic process simplifies the operational aspects for the administrator. In the next section, we show you the dynamic process needed to get this intelligence to the Layer 2 networks.