3Com MSR 50 Series Configuration Manual page 1094

1094 C 67: M
HAPTER ULTICAST
O
VERVIEW
on the forwarding mechanism, PIM comes in two modes - dense mode (often
referred to as PIM-DM) and sparse mode (often referred to as PIM-SM).
An inter-domain multicast routing protocol is used for delivery of multicast
■
information between two ASs. So far, mature solutions include multicast
source discovery protocol (MSDP).
For the SSM model, multicast routes are not divided into inter-domain routes and
intra-domain routes. Since receivers know the position of the multicast source,
channels established through PIM-SM are sufficient for multicast information
transport.
Layer 2 multicast protocols
Layer 2 multicast protocols include IGMP Snooping/MLD Snooping and multicast
VLAN/IPv6 multicast VLAN.
network.
Figure 323 Position of Layer 2 multicast protocols
Source
/IPv6 Multicast VLAN
Receiver
IPv4/IPv6 multicast packets
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.
Figure 323 shows where these protocols are in the
Multicast VLAN
IGMP Snooping
/MLD Snooping
Receiver