UMN:CLI User ManualV5812G3649.3 Multicast RoutingWhen receivers join a certain group, multicast routers must deliver the multicast trafficcorresponding to the group to those receivers. To determine the appropriate forwardingpath and to replicate the multicast traffic to multiple destinations, multicast routingprotocols are needed.The multicast routing protocols establish the distribution tree by building a forwardingtable in its own way. The forwarding table contains the information of sources, groups,interfaces, and how to forward multicast packets. Note that the multicast has the differentrouting method from the unicast‘s.Reverse Path Forwarding (RPF)Routers typically forward unicast packets with the destination lookup. When unicastpackets come to interfaces, routers forward the packets to the interfaces toward thedestinations of those packets by referring to the routing table. If the routing table does notcontain the information of the destinations, the routers forward the packets to the defaultgateway.On the other hand, routers forward multicast packets based on the source of the packets.When multicast packets come to an interface, routers validate whether the interface onwhich the packets are received is directly toward the source of those packets by referringto the existing unicast routing table. This procedure is called the reverse path forwarding(RPF) check. If incoming multicast packets pass the RPF check, routers forward thepackets to the outgoing interface. If not, routers drop the packets.In the multicast routing, routers must forward packets away from the sources to preventrouting loops. Finally, the distribution tree established by RPF follows the shortest pathtree (SPT) topology.9.3.1 Multicast Routing9.3.1.1 Enabling Multicast RoutingBy default, multicast routing is disabled. To configure the V5812G to forward multicasttraffic via Layer 3 network, you need to enable multicast routing.To enable Layer 3 multicast routing, use the following command.Command Mode Descriptionip multicast-routing Global Enables multicast routing.no ip multicast-routing Disables multicast routing. (default)9.3.1.2 TTL ThresholdYou can specify a TTL threshold for multicast packets on an interface. This configurationis used on a border router which limits a multicast domain, since only the multicastpackets with a TTL value greater than a TTL specified on an interface are forwarded tooutgoing interfaces. If you intend the router to operate as a border router, the TTLthreshold must be a very high value.
