D-Link DGS-3308FG Product Manual - Page 56

8-port Gigabit Ethernet Switch User's Guide The states a computer will go through to join or to leave a multicast group are shown below: Figure 5-16. IGMP State Transitions Multicast Routing Algorithms Multicast routing is based on a tree concept where the multicast source is the trunk and the multicast group members are the leaves. Branches can be thought of as subnetworks. There are several algorithms that can be used to construct the multicast tree and then to prune it branches for the efficient delivery of multicast transmissions. Flooding Flooding is the simplest way to deliver multicast packets. When a multicast packet is received by a router, it checks to see if it has received this packet before. If it has not, the packet if forwarded to all ports except the one the packet was received on. Otherwise, the router drops the packet. This way, all routers on a network will receive at least on copy of the packet. Flooding generates a large number of duplicated packets and wastes network bandwidth. It also requires multicast routers to keep an entry in their table of recently received multicast packets, wasting some the router's memory. Multicast Spanning Trees Spanning Trees are constructed from a subset of links between routers and a number of these links are selectively blocked such that there is only one active link between any two routers. The blocked links then act to provide some redundant links that may become active in the future, if the currently active link fails. When a router receives a multicast packet, it floods the packet to all ports belonging to the spanning tree, except the one it was received on. This guarantees the packet will reach all routers on the network. The spanning tree does not consider group membership in forwarding decisions. 46