SMC Networks TigerSwitch 10/100 SMC6724L2, Management Manual

Page: 151 / 176

M
ULTICAST
F
ILTERING
4-9
Multicast Filtering
Multicasting sends data to a group of nodes instead of a single
destination. The simplest way to implement multicasting is to
broadcast data to all nodes on the network. However, such an
approach wastes a great deal of bandwidth if the target group is
small compared to the overall broadcast domain.
Because applications such as videoconferencing and data sharing
are now widely used, efficient multicasting has become vital. A
common approach is to use a group registration protocol that
allows nodes to join or leave multicast groups. A switch or router
can then easily determine which ports contain group members and
send data out to those ports only. This procedure is called
multicast filtering.
The purpose of IP multicast filtering is to optimize a switched
network’s performance, so multicast packets will only be
forwarded to those ports containing multicast group hosts or
multicast routers/switches, instead of flooding traffic to all ports in
the subnet (VLAN).
The SMC6724L2 supports multicast filtering by passively
monitoring IGMP Query and Report messages and VMRP Probe
messages to register end-stations as multicast group members
(Layer 2), but also by actively sending GMRP Query messages to
learn the location of multicast routers/switches and member hosts
in multicast groups within each VLAN (Layer 3).
IGMP Snooping
A Layer 2 switch can passively snoop on IGMP Query and Report
packets transferred between IP multicast routers/switches and IP
multicast host groups to identify the IP multicast group members.
It simply monitors the IGMP packets passing through it, picks out
the group registration information, and configures multicast filters
accordingly. IGMP Snooping generates no additional network