10
For the SSM model, multicast routes are not divided into intra-domain routes and inter-domain
routes. Because receivers know the positions of the multicast sources, channels established
through PIM-SM are sufficient for the transport of multicast information.
Layer 2 multicast protocols
In
, Layer 2 multicast protocols include IGMP snooping, MLD snooping, PIM snooping, IPv6
PIM snooping, multicast VLAN, and IPv6 multicast VLAN.
Figure 9 Positions of Layer 2 multicast protocols
•
IGMP snooping and MLD snooping:
IGMP snooping and MLD snooping are multicast constraining mechanisms that run on Layer 2
devices. They manage and control multicast groups by monitoring and analyzing IGMP or MLD
messages exchanged between the hosts and Layer 3 multicast devices. This effectively
controls the flooding of multicast data in Layer 2 networks.
•
PIM snooping and IPv6 PIM snooping:
PIM snooping and IPv6 PIM snooping run on Layer 2 devices. They work with IGMP snooping
or MLD snooping to analyze received PIM messages. Then, they add the ports that are
interested in specific multicast data to a PIM snooping routing entry or IPv6 PIM snooping
routing entry. In this way, multicast data can be forwarded to only the ports that are interested in
the data.
•
Multicast VLAN and IPv6 multicast VLAN:
Multicast VLAN or IPv6 multicast VLAN runs on a Layer 2 device in a multicast network where
multicast receivers for the same group exist in different VLANs. With these protocols, 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, receiver hosts of a multicast group are usually located at different areas on the
network. They are identified by the same multicast group address. To deliver multicast packets to
these receivers, a multicast source encapsulates the multicast data in an IP packet with the multicast
group address as the destination address. Multicast routers on the forwarding paths forward
multicast packets that an incoming interface receives through multiple outgoing interfaces.
Compared to a unicast model, a multicast model is more complex in the following aspects:
IPv4/IPv6 multicast packets (S1, G1)
IPv4/IPv6 multicast packets (S2, G2)
Source 1
Source 2
Receiver
IGMP Snooping
/MLD Snooping
Receiver
Receiver
Multicast VLAN
/IPv6 Multicast VLAN
PIM Snooping
/IPv6 PIM Snooping