3com 4210 PWR 9-Port 3CR17341-91-ME User Manual

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. 

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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. 

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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:

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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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If the corresponding (S, G) entry exists, but the interface on which the packet 
actually arrived is not the incoming interface in the multicast forwarding table, 
the multicast packet is subject to an RPF check. 

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If the result of the RPF check shows that the RPF interface is the incoming 
interface of the existing (S, G) entry, this means that the (S, G) entry is 
correct but the packet arrived from a wrong path and is to be discarded. 

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If the result of the RPF check shows that the RPF interface is not the 
incoming interface of the existing (S, G) entry, this means that the (S, G) 
entry is no longer valid. The router replaces the incoming interface of the (S, 
G) entry with the interface on which the packet actually arrived and 
forwards the packet to all the outgoing interfaces. 

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If no corresponding (S, G) entry exists in the multicast forwarding table, the 
packet is also subject to an RPF check. The router creates an (S, G) entry based 
on the relevant routing information and using the RPF interface as the 
incoming interface, and installs the entry into the multicast forwarding table. 

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If the interface on which the packet actually arrived is the RPF interface, the 
RPF check is successful and the router forwards the packet to all the 
outgoing interfaces. 

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If the interface on which the packet actually arrived is not the RPF interface, 
the RPF check fails and the router discards the packet.