3com WX3000 User Manual

 

1-11 

Figure 1-6 Positions of Layer 2 multicast protocols 

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.  

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.  

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.