3Com Switch 8800 Advanced Software V5 Configuration Guide

Multicast Packets Forwarding Mechanism 501
Figure 161 Positions of Layer 2 multicast protocols
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
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 feature enabled on the Layer 2 device, the Layer 3
multicast device needs to send only one copy of multicast to the multicast VLAN
on the Layer 2 device. This avoids waste of network bandwidth and extra burden
on the Layer 3 device.
Multicast Packets
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 the IP
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.
Source
Receiver Receiver
IPv4/IPv6 multicast packets
IGMP Snooping
/MLD Snooping
Multicast VLAN
/IPv6 Multicast VLAN