H3C Technologies H3C S12500 Series Switches - Manual (Page 27)

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IGMP snooping and MLD snooping are multicast constraining mechanisms that run on Layer 2
devices. They manage and control multicast groups by listening to and analyzing IGMP or MLD
messages exchanged between the hosts and Layer 3 multicast devices, effectively controlling the
flooding of multicast data in a Layer 2 network.
• PIM snooping and IPv6 PIM snooping:
PIM snooping and IPv6 PIM snooping run on Layer 2 devices. They determine which ports are
interested in multicast data by analyzing the received IPv6 PIM messages, and add the ports to a
multicast forwarding entry to make sure that multicast data can be forwarded to only the ports that
are interested in the data.
• Multicast VLAN and IPv6 multicast VLAN:
In the traditional multicast-on-demand mode, when users in different VLANs on a Layer 2 device
need multicast information, the upstream Layer 3 device must forward a separate copy of the
multicast data to each VLAN of the Layer 2 device. When the multicast VLAN or IPv6 multicast
VLAN feature is enabled on the Layer 2 device, the Layer 3 multicast device sends only one copy
of multicast to the multicast VLAN or IPv6 multicast VLAN on the Layer 2 device. This method
avoids waste of network bandwidth and extra burden on the Layer 3 device.
Multicast packet forwarding 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 IP multicast packets. To deliver multicast packets to
receivers located at different positions of the network, multicast routers on the forwarding paths usually
need to forward multicast packets that an incoming interface receives 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 (for example, the MBGP routing table) 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 undergoes 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.
For more information about the RPF mechanism, see "Configuring multicast routing and forwarding" and
"Configuring IPv6 multicast routing and forwarding."
Multicast support for VPNs
NOTE:
Multicast support for VPNs is not available in IPv6 networks.
Multicast support for VPNs refers to multicast applied in VPNs.