Multicast Packet Forwarding Mechanism - 3Com 4210 PWR Configuration Manual

Multicast Packet
Forwarding
Mechanism
Implementing the RPF
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.
In the network, multicast packet transmission is based on the guidance of the
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multicast forwarding table derived from the unicast routing table or the
multicast routing table specially provided for multicast.
To process the same multicast information from different peers received on
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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:
If the corresponding (S, G) entry exists, and the interface on which the packet
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actually arrived is the incoming interface in the multicast forwarding table, the
router forwards the packet to all the outgoing interfaces.
If the corresponding (S, G) entry exists, but the interface on which the packet
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actually arrived is not the incoming interface in the multicast forwarding table,
the multicast packet is subject to an RPF check.
If the result of the RPF check shows that the RPF interface is the incoming
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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.
If the result of the RPF check shows that the RPF interface is not the
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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.
If no corresponding (S, G) entry exists in the multicast forwarding table, the
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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.
If the interface on which the packet actually arrived is the RPF interface, the
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RPF check is successful and the router forwards the packet to all the
outgoing interfaces.
If the interface on which the packet actually arrived is not the RPF interface,
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the RPF check fails and the router discards the packet.

Multicast Packet Forwarding Mechanism

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