LevelOne GTL-2691 - Manual (Page 512)

C HAPTER 15 | Multicast Filtering
IGMP Protocol
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This switch can use Internet Group Management Protocol (IGMP) to filter
multicast traffic. IGMP Snooping can be used to passively monitor or
“snoop” on exchanges between attached hosts and an IGMP-enabled
device, most commonly a multicast router. In this way, the switch can
discover the ports that want to join a multicast group, and set its filters
accordingly.
If there is no multicast router attached to the local subnet, multicast traffic
and query messages may not be received by the switch. In this case IGMP
Query can be used to actively ask the attached hosts if they want to
receive a specific multicast service. IGMP Query thereby identifies the ports
containing hosts requesting to join the service and sends data out to those
ports only. It then propagates the service request up to any neighboring
multicast switch/router to ensure that it will continue to receive the
multicast service.
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).
This switch not only supports IP multicast filtering by passively monitoring
IGMP query, report messages and multicast routing probe messages to
register end-stations as multicast group members, but also supports the
Protocol Independent Multicasting (PIM) routing protocol required to
forward multicast traffic to other subnets ( page 1553 ).
You can also configure a single network-wide multicast VLAN shared by
hosts residing in other standard or private VLAN groups, preserving
security and data isolation "Multicast VLAN Registration" on page 557 .
IGMP P ROTOCOL
The Internet Group Management Protocol (IGMP) runs between hosts and
their immediately adjacent multicast router/switch. IGMP is a multicast
host registration protocol that allows any host to inform its local router 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” (at Layer 3) and assumes the role of querying the LAN for group
members. It then propagates the service requests on to any adjacent
multicast switch/router to ensure that it will continue to receive the
multicast service. 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 such as
Protocol Independent Multicasting (PIM), to support IP multicasting across
the Internet. Note that IGMP neither alters nor routes IP multicast packets.
A multicast routing protocol must be used to deliver IP multicast packets