Allied Telesis AT-S29 - Manual (Page 126)

Advanced Topics
126
IGMP Snooping and IP Multicast Filtering
The Internet Group Management Protocol (IGMP) runs between hosts
and their immediately neighboring multicast router/switch. The
protocol’s mechanisms allow a host to inform its local router/switch that
it wants to receive transmissions addressed to a specific multicast group.
A router, or multicast-enabled switch, can periodically ask their hosts if
they want to receive multicast traffic. If there is more than one
router/switch on the LAN performing IP multicasting, one of these
devices is elected “querier” and assumes the responsibility of querying
the LAN for group members.
Based on the group membership information learned from IGMP, a
router/switch can determine which (if any) multicast traffic needs to be
forwarded to each of its ports. At Layer-3, multicast routers use this
information, along with a multicast routing protocol, to support IP
multicasting across the Internet.
IGMP provides the final step in an IP multicast packet delivery service
since it is only concerned with forwarding multicast traffic from the local
router/switch to group members on directly attached subnetwork or
LAN segment.
This switch supports IP Multicast Filtering by:
❑ Passively snooping on the IGMP Query and IGMP Report packets
transferred between IP multicast routers and IP multicast host
groups to learn IP Multicast group members.
❑ Actively sending IGMP Query messages to solicit IP Multicast
group members.
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 to all ports in the subnet (VLAN).
The AT-8324SX switch, with I P multicast filtering capability, not only
passively monitors IGMP Query and Report messages; it can also actively
send IGMP Query messages to learn locations of multicast
routers/switches and member hosts in multicast groups within each
VLAN.
However, note that IGMP neither alters nor routes any IP multicast
packets. Since IGMP is not concerned with the delivery of IP multicast
packets across subnetworks, an external IP multicast router is needed if
IP multicast packets have to be routed across different subnetworks.