3Com WX3000 Multicast Packet Forwarding Mechanism, Implementation of the RPF Mechanism

Figure 1-6Positions of Layer 2 multicast protocols

Source

IGMP Snooping

Receiver

Receiver

multicast packets

2)IGMP Snooping

Running on Layer 2 devices, Internet Group Management Protocol Snooping (IGMP Snooping) are multicast constraining mechanisms that manage and control multicast groups by listening to and analyzing IGMP messages exchanged between the hosts and Layer 3 multicast devices, thus effectively controlling the flooding of multicast data in a Layer 2 network.

Multicast Packet Forwarding Mechanism

In a multicast model, a multicast source sends information to the host group identified by the multicast group address in the destination address field of the IP packets. Therefore, to deliver multicast packets to receivers located in different parts of the network, multicast routers on the forwarding path usually need to forward multicast packets received on one incoming interface to multiple outgoing interfaces. Compared with a unicast model, a multicast model is more complex in the following aspects.

zIn the network, multicast packet transmission is based on the guidance of the multicast forwarding table derived from the unicast routing table or the multicast routing table specially provided for multicast.

zTo process the same multicast information from different peers received on different interfaces of the same device, every multicast packet is subject to a reverse path forwarding (RPF) check on the incoming interface. The result of the RPF check determines whether the packet will be forwarded or discarded. The RPF check mechanism is the basis for most multicast routing protocols to implement multicast forwarding.

The RPF mechanism enables multicast devices to forward multicast packets correctly based on the multicast route configuration. In addition, the RPF mechanism also helps avoid data loops caused by various reasons.

Implementation of the RPF Mechanism

Upon receiving a multicast packet that a multicast source S sends to a multicast group G, the multicast device first searches its multicast forwarding table:

1)If the corresponding (S, G) entry exists, and the interface on which the packet actually arrived is the incoming interface in the multicast forwarding table, the router forwards the packet to all the outgoing interfaces.

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