H3C Technologies H3C S7500E Series Switches - Manual (Page 30)

1-13
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
1) 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.
2) 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.