Web Management Guide-R05

Chapter 14 | Multicast Filtering
Layer 2 IGMP (Snooping and Query for IPv4)
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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 (Layer 2) 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).
Layer 2 IGMP (Snooping and Query for IPv4)
IGMP Snooping and Query – If multicast routing is not supported on other switches
in your network, you can use IGMP Snooping and IGMP Query (page 420) to
monitor IGMP service requests passing between multicast clients and servers, and
dynamically configure the switch ports which need to forward multicast traffic.
IGMP Snooping conserves bandwidth on network segments where no node has
expressed interest in receiving a specific multicast service. For switches that do not
support multicast routing, or where multicast routing is already enabled on other
switches in the local network segment, IGMP Snooping is the only service required
to support multicast filtering.
When using IGMPv3 snooping, service requests from IGMP Version 1, 2 or 3 hosts
are all forwarded to the upstream router as IGMPv3 reports. The primary
enhancement provided by IGMPv3 snooping is in keeping track of information
about the specific multicast sources which downstream IGMPv3 hosts have
requested or refused. The switch maintains information about both multicast
groups and channels, where a group indicates a multicast flow for which the hosts
have not requested a specific source (the only option for IGMPv1 and v2 hosts
unless statically configured on the switch), and a channel indicates a flow for which
the hosts have requested service from a specific source. For IGMPv1/v2 hosts, the
source address of a channel is always null (indicating that any source is acceptable),
but for IGMPv3 hosts, it may include a specific address when requested.
Only IGMPv3 hosts can request service from a specific multicast source. When
downstream hosts request service from a specific source for a multicast service,
these sources are all placed in the Include list, and traffic is forwarded to the hosts
from each of these sources. IGMPv3 hosts may also request that service be