3Com MSR 50 Series, Configuration Manual

Page: 1095 / 2443

Multicast Packet Forwarding Mechanism 1095
Multicast Packet
Forwarding
Mechanism
In a multicast model, a multicast source sends information to a host group, which
is identified by a multicast group address in the destination address field of IP
multicast 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.
■
To ensure multicast packet transmission in the network, unicast routing tables
or multicast routing tables specially provided for multicast must be used as
guidance for multicast forwarding.
■
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.
n
For details about the RPF mechanism, refer to “RPF Mechanism” on page 1097 or
“RPF Mechanism” on page 1209 .
Multi-Instance
Multicast
Multi-instance multicast refers to multicast in virtual private networks (VPNs).
Introduction to the
Multi-Instance Concept
VPN networks need to be isolated from one another and from the public network.
As shown in
Figure 324 , VPN A and VPN B separately access the public network
through PE devices.
Figure 324 Networking diagram for VPN
VPN A
VPN A
VPN A
VPN B VPN B
Public network
P
PE 1
PE 2
PE 3
CE b3
CE a2
CE a3
CE b1
CE a1
CE b2