Web Management Guide-R05

Chapter 16 | Multicast Filtering
Layer 2 IGMP (Snooping and Query for IPv4)
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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 570) 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
forwarded from any source except for those specified. In this case, traffic is filtered
from sources in the Exclude list, and forwarded from all other available sources.
Note: When the switch is configured to use IGMPv3 snooping, the snooping
version may be downgraded to version 2 or version 1, depending on the version of
the IGMP query packets detected on each VLAN.
Note: IGMP snooping will not function unless a multicast router port is enabled on
the switch. This can accomplished in one of two ways. A static router port can be
manually configured (see “Specifying Static Interfaces for a Multicast Router” on
page 574). Using this method, the router port is never timed out, and will continue
to function until explicitly removed. The other method relies on the switch to
dynamically create multicast routing ports whenever multicast routing protocol
packets or IGMP query packets are detected on a port.
Note: A maximum of up to 1024 multicast entries can be maintained for IGMP
snooping. Once the table is full, no new entries are learned. Any subsequent
multicast traffic not found in the table is dropped if unregistered-flooding is
disabled (default behavior) and no router port is configured in the attached VLAN,
or flooded throughout the VLAN if unregistered-flooding is enabled (see
“Configuring IGMP Snooping and Query Parameters” on page 570).