640 BigIron RX Series Configuration Guide53-1001986-01DVMRP overview23Initiating DVMRP multicasts on a networkOnce DVMRP is enabled on each router, a network user can begin a video conference multicastfrom the server on R1. Multicast Delivery Trees are initially formed by source-originated multicastpackets that are propagated to downstream interfaces as seen in Figure 96. When a multicastpacket is received on a DVMRP-capable router interface, the interface checks its DVMRP routingtable to determine whether the interface that received the message provides the shortest pathback to the source. If the interface does provide the shortest path, the interface forwards themulticast packet to adjacent peer DVMRP routers, except for the router interface that originated thepacket. Otherwise, the interface discards the multicast packet and sends a prune message backupstream. This process is known as reverse path forwarding.In Figure 96, the root node (R1) is forwarding multicast packets for group 229.225.0.2 that itreceives from the server to its downstream nodes, R2, R3, and R4. Router R4 is an intermediaterouter with R5 and R6 as its downstream routers. Because R5 and R6 have no downstreaminterfaces, they are leaf nodes.The receivers in this example are those workstations that are resident on routers R2, R3, and R6.Pruning a multicast treeAfter the multicast tree is constructed, pruning of the tree will occur after IP multicast packetsbegin to traverse the tree.As multicast packets reach leaf networks (subnets with no downstream interfaces), the local IGMPdatabase checks for the recently arrived IP multicast packet address. If the local database doesnot contain the address (the address has not been learned), the router prunes (removes) theaddress from the multicast tree and no longer receives multicasts until the prune age expires.In Figure 97, Router 5 is a leaf node with no group members in its local database. Consequently,Router 5 sends a prune message to its upstream router. This router will not receive any furthermulticast traffic until the prune age interval expires.
