3Com 4210G Series, Configuration Manual, Page: 350 / 1133

2-12
Figure 1-9
Position of Layer 2 multicast protocols
Source
Receiver Receiver
IPv4/IPv6 multicast packets
IGMP Snooping
/MLD Snooping
Multicast VLAN
/IPv6 Multicast VLAN
2) IGMP Snooping/MLD Snooping
Running on Layer 2 devices, Internet Group Management Protocol Snooping (IGMP Snooping) and
Multicast Listener Discovery Snooping (MLD Snooping) are multicast constraining mechanisms that
manage and control multicast groups by listening to and analyzing IGMP or MLD messages
exchanged between the hosts and Layer 3 multicast devices, thus effectively controlling the flooding of
multicast data in a Layer 2 network.
3) Multicast
VLAN/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 needs to forward a separate copy of the multicast
data to each VLAN of the Layer 2 device. With the multicast VLAN or IPv6 multicast VLAN feature
enabled on the Layer 2 device, the Layer 3 multicast device needs to send only one copy of multicast
to the multicast VLAN or IPv6 multicast VLAN on the Layer 2 device. This 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. 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.
z
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.
z
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.