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only one copy of multicast to the multicast VLAN or IPv6 multicast VLAN on the Layer 2 device.
This approach avoids wasting network bandwidth and placing an extra burden on the Layer 3
device.
Multicast packet forwarding mechanism
In a multicast model, receiver hosts of a multicast group are usually located at different positions of
the network. They are identified by the same multicast group address. To deliver multicast packets to
these receivers, a multicast source encapsulates the multicast data in an IP packet with the multicast
group address as the destination address. Multicast routers on the forwarding paths usually need to
forward multicast packets that an incoming interface receives through 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, different routing tables are used to
guide multicast forwarding. These routing tables include unicast routing tables and multicast
routing tables (for example, the MBGP routing table) specially provided for multicast.
•
To process the same multicast information from different peers received on different interfaces,
the multicast device performs a reverse path forwarding (RPF) check on each multicast packet.
The result of the RPF check determines whether the packet is forwarded or discarded. The RPF
check mechanism is the basis for most multicast routing protocols to implement multicast
forwarding.
For more information about the RPF mechanism, see "
Configuring multicast routing and forwarding
Configuring IPv6 multicast routing and forwarding
."
Multicast support for VPNs
Multicast support for VPNs refers to multicast applied in virtual private networks (VPNs).
Introduction to VPN instances
VPNs must be isolated from one another and from the public network. As shown in
, VPN A
and VPN B separately access the public network through PE devices.
