78Configuring multicast routing and forwarding(available only on the S5500-EI)OverviewIn multicast implementations, the following types of tables implement multicast routing and forwarding:• Multicast routing table of a multicast routing protocol—Each multicast routing protocol has its ownmulticast routing table, such as PIM routing table.• General multicast routing table—The multicast routing information of different multicast routingprotocols forms a general multicast routing table.• Multicast forwarding table—The multicast forwarding table guides the forwarding of multicastpackets.A multicast routing table consists of a set of (S, G) entries. Each entry indicates the routing information fordelivering multicast data from a multicast source to a multicast group. If a router supports multiplemulticast protocols, its multicast routing table includes routes generated by multiple protocols. The routerchooses the optimal route from the multicast routing table based on the configured multicast routing andforwarding policy and adds the route entry to its multicast forwarding table.The term "router" in this document refers to both routers and Layer 3 switches.The term "interface" in the multicast routing and forwarding features collectively refers to Layer 3interfaces, including VLAN interfaces and Layer 3 Ethernet interfaces. You can set an Ethernet port as aLayer 3 interface by using the port link-mode route command (see Layer 2—LAN SwitchingConfiguration Guide).RPF check mechanismA multicast routing protocol relies on the existing unicast routes, MBGP routes, or static multicast routesin creating multicast routing entries. When creating multicast routing table entries, a multicast routingprotocol uses the reverse path forwarding (RPF) check mechanism to ensure multicast data delivery alongthe correct paths. In addition, the RPF check mechanism also helps avoid data loops.RPF check processThe basis for an RPF check is as follows:• Unicast routing table—Contains the shortest path to each destination subnet.• MBGP routing table—Contains multicast routing information.
