1-10
Figure 1-6 Positions of Layer 2 multicast protocols
Source
Receiver Receiver
multicast packets
IGMP Snooping
1) 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.
z In 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.
z To 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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